Relationship of Physical Function to Single Muscle Fiber Contractility in Older Adults: Effects of Resistance Training With and Without Caloric Restriction

Relationship of Physical Function to Single Muscle Fiber Contractility in Older Adults: Effects... Abstract Background Previous studies support beneficial effects of both resistance exercise training (RT) and caloric restriction (CR) on skeletal muscle strength and physical performance. The goal of this study was to determine the effects of adding CR to RT on single-muscle fiber contractility responses to RT in older overweight and obese adults. Methods We analyzed contractile properties in 1,253 single myofiber from muscle biopsies of the vastus lateralis, as well as physical performance and thigh muscle volume, in 31 older (65–80 years), overweight or obese (body mass index = 27–35 kg/m2) men (n = 19) and women (n = 12) who were randomly assigned to a standardized, progressive RT intervention with CR (RT+CR; n = 15) or without CR (RT; n = 16) for 5 months. Results Both interventions evoked an increase in force normalized to cross-sectional area (CSA), in type-I and type-II fibers and knee extensor quality. However, these improvements were not different between intervention groups. In the RT group, changes in total thigh fat volume inversely correlated with changes in type-II fiber force (r = −.691; p = .019). Within the RT+CR group, changes in gait speed correlated positively with changes in type-I fiber CSA (r = .561; p = .030). In addition, increases in type-I normalized fiber force were related to decreases in thigh intermuscular fat volume (r = −0.539; p = .038). Conclusion Single muscle fiber force and knee extensor quality improve with RT and RT+CR; however, CR does not enhance improvements in single muscle fiber contractility or whole muscle in response to RT in older overweight and obese men and women. Aging, Caloric restriction, Muscle fiber, Resistance training Age-associated loss of skeletal muscle strength and power leads to increased risk of impaired mobility and disability in advanced age (1). Single fibers from nonhuman primates and humans are slower and weaker (2–4), show improvements in contractile properties (5), or no changes (2) with aging. Currently, exercise training is the only intervention known to consistently improve muscle mass, strength, and power, and overall physical function in older adults (6–9). Yet, the few studies that examined single-fiber contractile responses to resistance training (RT) in older adults are inconsistent. Most of these studies show an overall improvement in single-fiber force and power with RT (10–14), though some data suggest the oldest adults exhibit a limited response at the myofiber level (15,16). Aging is also characterized by changes in body composition including muscle mass loss and accumulation of adipose tissue (myosteatosis), which can further exacerbate functional decline with age (17). We recently showed that, compared to older adults with a normal body mass index (18.5–24.9 kg/m2), those who are obese (body mass index > 30 kg/m2) have greater intramyocellular lipid content which correlated with impaired single-fiber contractile function (18). Thus, improvements in single-fiber contractile force and power with RT may be blunted in older adults with obesity, and intervening on myosteatosis by adding caloric restriction (CR) to RT may result in further improvements in myofiber contractility for the older obese population. In fact, we previously reported that RT improved physical function and body composition associated with reduced thigh fat in obese older adults, while those with higher initial adiposity experienced less improvement (19). Previous studies show that CR without malnutrition results in a dramatic change in the human skeletal muscle transcriptional profile that resembles that reported for younger individuals (20). CR also attenuates age-dependent progressive functional decline in various organs, peripheral nerve damage, loss of muscle mass, abnormalities in the electron transport chain, and the onset of age-related diseases such as cancer, diabetes, and Alzheimer’s disease (21). Although evidence suggests that CR has multiple beneficial effects on skeletal muscle function in rodents (22–24), whether it improves single-fiber muscle function in older adults with obesity is unknown. Measurements of single-muscle fiber function provide a unique approach to begin to address whether CR further enhances myofiber contractility induced by RT in a fiber-type specific manner (18). This study tested whether adding CR to RT further enhances improvements in contractile properties of fast- and slow-myofibers from obese older adults. We also examined the relationships between physical function and single myofiber contractility before and after these interventions. Materials and Methods Participant Characteristics and Study Design We analyzed single myofiber contractile properties recorded ex vivo from muscle biopsies of the vastus lateralis in a subset of 31 participants (19 men, 12 women) from the larger Improving Muscle for Functional Independence Trial (IM FIT) (19). The IM FIT study (clinicaltrials.gov; NCT01049698) was a randomized controlled trial designed to determine whether CR enhances improvements in body composition and physical function in response to RT in older (65–80 years) overweight and obese (body mass index = 27–35 kg/m2) men and women. All participants, including those from this ancillary study, were randomly assigned equally to a standardized, progressive RT intervention with CR (RT+CR; n = 15) or without CR (RT; n = 16) for 5 months. The study was approved by the Wake Forest School of Medicine Institutional Review Board and all participants provided written informed consent to participate. All participants in the study underwent 5 months of RT 3 days/week on weight-stack resistance machines (Cybex International and Nautilus) at the Wake Forest University Clinical Research Center exercise facility as previously described (19). Two exercise interventionists supervised the training sessions and ensured that participants adjusted the equipment appropriately and performed the exercises safely. Participants performed an initial 5-minute warmup by walking or cycling at a slow pace followed by light stretching and concluded each session with a 5-minute cool down and light stretching. The protocol involved a gradual progression of weight and repetitions during the first month to allow familiarization with the equipment, minimize muscle soreness, and reduce injury potential. The maximal weight that a person could lift with the correct form in a single repetition (1RM) was used to prescribe intensity. The training goal was to complete three sets of 10 repetitions for each exercise at 70% 1RM for that specific exercise. Participants rested ~1 minute between sets. Resistance was increased when a participant was able to complete 10 repetitions on the third set for two consecutive sessions. Strength testing was repeated every 4 weeks, and training loads were adjusted to be consistent with the 70% 1RM goal. The exercises performed included: Leg Extension, Leg Curl, Leg Press, Seated Row, Chest Press, Biceps Curl, Triceps Extension, and Latissimus Pulldown. Participants assigned to RT only were instructed to follow a eucaloric diet, whereas those assigned to RT+CR underwent a dietary weight-loss intervention designed to elicit moderate weight loss (5%–10%) as described (19). This intervention incorporated meal replacements, nutrition education, and dietary behavior modification advice via weekly meetings with the study’s registered dietitian that took place either before or after one of their exercise sessions. Each participant was assigned a daily caloric intake to follow, which was derived from subtracting 600 kcal from his or her estimated daily energy needs for weight maintenance. A maximum of two meal replacements per day (shakes and bars; Slim-Fast Inc.) that contained ~220 kcal with 7–10 g protein, 33–46 g carbohydrates, 1.5–5 g fat, and 2–5 g fiber were provided to participants for breakfast and lunch. Dinner and snack options were recommended by the registered dietitian in accordance with each participant’s daily caloric goals and tailored to allow for individual preferences for various food items. Participants were asked to keep a diet log of all foods consumed, and the logs were monitored weekly by the registered dietitian to verify compliance with the weight-loss intervention. Specific inclusion and exclusion criteria, procedures for vastus lateralis muscle biopsy, and methods to measure physical function and thigh composition (via computed tomography) have been described previously (19,25). Single-muscle fiber experimental setup and solutions were described before (2) and a brief description is included below. Data from a different group of IM FIT participants have been included in previous publications (18,26). Single-Fiber Physiology Tests and Experimental Protocols All measurements were conducted at 15°C, and temperature was continuously monitored by a thermocouple inserted into the experimental chamber. Although this is not a physiological temperature, most previous studies have been recorded in these conditions. All functional data were collected and analyzed using a personal computer and a data acquisition board (Model 600A Digital controller, Aurora Scientific, Aurora, Ontario). A slack test was used to determine the unloaded shortening velocity (Vo). In this procedure, permeabilized fibers were transferred to activating solution (pCa: 4.5) and once peak force was attained (monitored by real-time digital oscilloscope), subjected to a rapid slack step (≤20% of fiber length [FL] within 1 ms). The procedure was repeated at different slack lengths, and the times required for tension redevelopment were plotted versus the corresponding slack distances. A straight line was fit by least-squares linear regression, and the slope of the regression line, normalized to FL, defined Vo. It should be noticed that myofiber permeabilization bypasses the physiological excitation-contraction mechanism and muscle fiber activation evoked by nerve stimulation. After the slack test, the force–velocity relationship was generated by performing a series of isotonic contractions of the muscle fiber. Briefly, the muscle fiber was placed in activating solution (pCa: 4.5) and after reaching peak force, subjected to a series of three isotonic steps varying from 3% to 80% of Po. After the last step, the fiber was rapidly (<1 ms) slackened by 20% of its length, which zeroed the force transducer, providing a baseline for force measurement. Step duration was less than ~100 ms. Shortening velocity and force were obtained as averages over the final half of each step. Velocity was calculated as the slope of the position recorded over the same time period. Five to six series of three isotonic contractions were used to establish a force–velocity relationship. The Hill equation was fitted to the data using an iterative nonlinear curve-fitting procedure to draw the force–power relationship. The following parameters were used to describe the hyperbolic fit to the data: Vmax (the velocity extrapolated to a force of zero), Po (average force obtained during the trial), and a/Po (a parameter describing the curvature or shape of the force–velocity relationship). Vmax was normalized to FL. a/Po is a dimensionless parameter. Peak power was calculated from these three parameters, and expressed as W/L fiber. In all contractions, Ca2+-activated force was measured using the transducer zero signal as a baseline. Forces were normalized to the fiber’s cross-sectional area (CSA) to obtain specific force. Quality control was performed as described (2). Fibers were excluded from analysis if force declined more than 5% or if they broke or showed partial myofibrillar tearing at any observation timepoint in the experimental protocol. Experiments were excluded from analysis if compliance, defined as displaced axis-intercept of the slack test plot, exceeded 5% of FL and if the r2 of the force–velocity regression was less than .98. Assessment of Fiber Myosin Heavy-chain Isoform At the end of each functional experiment, the single-fiber segment was removed from the test apparatus and stored in 20 µL of sodium dodecyl sulfate sample buffer (containing 62.5 mM Tris pH 6.8, 2% sodium dodecyl sulfate, 10% glycerol, 5% betamercaptoethanol, and 0.001% bromophenol blue) at −80°C. Later, fibers were denatured for 5 minutes at 95°C. To determine the myosin heavy-chain composition of the fiber segment, a sample of the fiber solute, equivalent to that tested for contraction, was run on a sodium dodecyl sulfate–polyacrylamide gel electrophoresis system that consisted of a 6% separating gel and a 4% stacking gel (acrylamide:bisacrylamide = 37:1). The gels contained 30% glycerol to improve separation of myosin heavy-chain isoforms. Electrophoresis was carried out at as described (2,18). Six percent of all fast fibers were hybrid (IIa/IIx), being IIa the predominant myosin heavy-chain isoform in 96% of the cases. Due to the small percent, hybrid muscle fibers were not included in the analysis. Statistical Analysis All analyses were performed using SAS 9.4 (Cary, NC). Baseline measures and changes in body mass, thigh composition, physical function, and type-I and type-II CSA and muscle fiber contractile function were described as mean and standard deviations by intervention groups. Statistical analyses were based on average values taken for each participant. Student t test was used to evaluate changes within groups and compare between groups measures at baseline and changes. Analysis of covariance was used to compare group differences for CSA and muscle fibers contractile function adjusted for gender and baseline measures. We calculated Spearman’s correlation coefficients of baseline measures and changes among type-I and type-II CSA and muscle fibers contractile function, and body composition and physical performance, overall and by gender. Spearman correlations were used because data failed tests of normality. Despite the large number of muscle fibers analyzed, we did not adjust for multiple comparisons due to the relatively low sample size, as well as the exploratory nature of these analyses. Results Participant Characteristics, Physical Function, Thigh Composition, and Single-fiber Contraction Properties at Baseline and Changes With Intervention Table 1 shows unadjusted values at baseline, and changes relative to baseline within groups. Body weight and total thigh fat volume decreased more in the RT+CR than RT group. Relative to baseline, both interventions increased absolute and normalized knee extension strength, but differences between groups were not significant (Table 1). Although participants were highly functional at baseline, the total short physical performance battery (SPPB) score significantly improved with both interventions, with no group difference in the magnitude of improvement (Table 1). Of the individual SPPB sub-scores, chair rise time significantly improved with both RT and RT+CR (no difference between groups), 4-m usual gait speed improved with RT+CR but not RT alone (no difference between groups), and balance scores did not change with either intervention (however, these scores were close to near perfect at baseline). RT decreased type-I, but not type-II, muscle fiber CSA; however, the difference between interventions was not significant. Adding CR to RT did not enhance these improvements in single-fiber contractile properties. Significant changes in type-I fiber CSA, force, normalized force, and type-II fiber force and power induced by RT exhibited no significant change or a reduction in significance when CR was combined with RT. Table 1. Physical Function and Muscle Strength, Thigh Composition, and Unadjusted Single-fiber CSA, Force, Velocity, and Power at Baseline and Changes With Intervention   RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413    RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413  Note: Change relative to baseline within groups: *p < .05; **p < .01. Muscle quality was calculated as the ratio of knee extensor peak torque to lean mass assessed by using dual-energy x-ray absorptiometry (in Nm/kg leg mass) as reported (19). BMI = Body mass index; CR = Caloric restriction; CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length; RT = Resistance training; SPPB = Short physical performance battery. View Large Table 1. Physical Function and Muscle Strength, Thigh Composition, and Unadjusted Single-fiber CSA, Force, Velocity, and Power at Baseline and Changes With Intervention   RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413    RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413  Note: Change relative to baseline within groups: *p < .05; **p < .01. Muscle quality was calculated as the ratio of knee extensor peak torque to lean mass assessed by using dual-energy x-ray absorptiometry (in Nm/kg leg mass) as reported (19). BMI = Body mass index; CR = Caloric restriction; CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length; RT = Resistance training; SPPB = Short physical performance battery. View Large Both Interventions Increased Single-fiber Contractile Properties Both interventions evoked an increase in normalized force in type-I fibers. However, these improvements in single-fiber force were not different between intervention groups (Table 1). In addition, there were no significant differences in single-fiber CSA and contractile properties between intervention groups, after adjusting for baseline measures and gender (Table 2). Table 2. Single-fiber CSA, Force, Velocity, and Power Values Adjusted for Baseline Measure and Gender   Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517    Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517  Note: CSA = Cross-sectional area; FL = Fiber length. View Large Table 2. Single-fiber CSA, Force, Velocity, and Power Values Adjusted for Baseline Measure and Gender   Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517    Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517  Note: CSA = Cross-sectional area; FL = Fiber length. View Large Baseline Correlations Between Physical Function, Thigh Composition, and Single-fiber Contractile Properties in All Participants Data in Table 3 show that all of the baseline type- I, and most of the type-II contractile properties, exhibit inverse correlations with SPPB. However, gait speed at baseline was positively correlated with most of Type-I contractile properties and Type-II fiber power (ie, faster gait associated with better contractile function). Thigh muscle volume correlated positively with type-I fiber and type-II fiber force (Table 3). Table 3. Baseline Correlations Between Physical Function, Thigh Composition, and Single-fiber CSA, Force, Velocity, and Power   CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339    CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339  Note: Values are r (top) and p (bottom). CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length. View Large Table 3. Baseline Correlations Between Physical Function, Thigh Composition, and Single-fiber CSA, Force, Velocity, and Power   CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339    CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339  Note: Values are r (top) and p (bottom). CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length. View Large Baseline Correlations by Gender In females, there was a significant positive correlation between baseline gait speed and type-I fiber power (r = .665; p = .026), while leg extension strength correlated positively with type-I fiber CSA (r = .629; p = .028). Leg extension strength also correlated positively with type-II fiber force (r = .709; p = .022) and velocity (r = .636, p = .048). Male 400-m walk time correlated negatively with type-I fiber power (r = −.560; p = .016). Correlations Between Changes in Type-I and Type-II Fiber Contractile Properties and RT- or RT Plus CR-induced Changes in Physical Function and Thigh Composition We next examined whether physical function and thigh composition responses to RT and RT+CR correlated with changes in myofiber contractile properties with the interventions. We found that RT-induced changes in leg extension strength correlated positively with type-I, but not type-II, fiber power (r = .657; p = .020; Figure 1A), while changes in total thigh fat volume negatively correlated with changes in type-II fiber force with RT (r = −.691; p = .019; Figure 1B). Within the RT+CR group, changes in gait speed correlated positively with changes in type-I fiber CSA (r = .561; p = .030; Figure 1C), indicating that improvements in gait speed with RT+CR are associated with a smaller decrease in type-I fiber CSA. In addition, changes in thigh muscle volume directly correlated with changes in type-I fiber CSA (r = .657; p = .008) with RT+CR, and increases in type-I normalized fiber force were inversely correlated with changes in thigh intermuscular fat volume (r = −.539; p = .038; Figure 1D). No significant correlations were found between changes in function and thigh composition and changes in type-II fiber properties with RT+CR. Figure 1. View largeDownload slide Correlations between changes in fiber contractile properties and changes in physical function and thigh composition. Changes in leg extension strength correlated positively with type-I, but not type-II, fiber power with RT (A). Changes in total thigh fat volume inversely correlated with changes in type-II fiber force with RT (B). Changes in gait speed correlated positively with changes in type-I fiber CSA with RT+CR (C). Increases in type-I normalized fiber force were inversely correlated with changes in thigh intermuscular fat volume (D). CR = Caloric restriction; CSA = Cross-sectional area; RT = Resistance training. Figure 1. View largeDownload slide Correlations between changes in fiber contractile properties and changes in physical function and thigh composition. Changes in leg extension strength correlated positively with type-I, but not type-II, fiber power with RT (A). Changes in total thigh fat volume inversely correlated with changes in type-II fiber force with RT (B). Changes in gait speed correlated positively with changes in type-I fiber CSA with RT+CR (C). Increases in type-I normalized fiber force were inversely correlated with changes in thigh intermuscular fat volume (D). CR = Caloric restriction; CSA = Cross-sectional area; RT = Resistance training. Discussion This work analyzed for the first time the effects of adding CR to RT on single muscle fiber contractile properties in older men and women with obesity. Since adiposity is associated with disability risk (27,28), we tested the hypothesis that CR-evoked fat loss further enhances RT-induced improvements in myofiber contractile properties. We used a randomized design to analyze the effects of both interventions on myofiber contractile properties, both within and between groups. The analyses showed that both interventions similarly increased type-I and type-II fiber force since there were no differences between groups. We also found cross-sectional associations of both physical function and thigh composition with myofiber contractility. Type-I fiber force, velocity, and power positively correlated with gait speed, while fiber-I force correlated with muscle volume. Type-II fiber power correlated with gait speed, and type-II fiber CSA, force and power correlated with leg press. Type-II fiber CSA, force and power correlated with knee extension strength, while type-II fiber CSA and force correlated with thigh muscle volume. The analysis by gender also showed baseline correlations between type-I and type-II fibers and physical function (data not shown). We previously showed that greater intramyocellular lipids are associated with slower myofiber contraction, force, and power development in obese older adults, providing a rationale for our hypothesis that a decrease in intramyocellular lipids would contribute to effects of RT on myofiber contractility (18) Here, we found that thigh fat volume decreased with both interventions, changes in thigh fat volume with RT+CR directly correlated with changes in type-I fiber CSA, and increases in type-I fiber force with RT+CR correlated negatively with changes in thigh intermuscular fat volume, which may account for the increase in knee extension strength despite the associated decrease in thigh muscle volume. However, contrary to our expectations, a further decrease in thigh fat induced by CR did not augment improved single muscle fiber contractility with RT. These results are also consistent with the physical function data recorded in this subset and the entire cohort (19). The force generation capacity of myofibers declines with aging in humans and nonhuman primates (2), but RT benefits human muscle function (13,15,16), while CR has been shown to prevent declines in muscle force with aging in rodents (22). RT improves single-fiber contractile force in nonobese (12,13,29), and obese older adults (30). An important question then is how RT improves single muscle contractile properties in older adults. We previously showed that RT enhanced skeletal muscle innervation in another subset of IM FIT participants (26). Improved knee extensor strength in participants of the same study was associated with changes in muscle-specific miRNAs in muscle and plasma (31). We also previously reported that TNNT1 alternative splicing (AS) 1, AS2 and the AS1/AS2 ratio are potential quantitative biomarkers of skeletal muscle adaptation to RT in older adults, and that their profile reflects enhanced single-fiber muscle force in the absence of significant increases in fiber cross-sectional area (30). This finding could be associated with changes in single-fiber calcium sensitivity (32). Additionally, RT in young and old subjects can change the velocity of the myosin molecule tested by the in vitro motility assay (33). Much less is known about how CR improves muscle function. Different CR regimens applied to rodents proved to be beneficial on whole muscle contraction properties (22,23). Proposed mechanisms mediating these effects include improved excitation-contraction coupling (24,34) and mitochondrial proton leak (35). The role of CR, IGF signaling, sirtuins, and protein intake in maintaining muscle mass and metabolism across ages in animals and humans has been amply discussed in the literature (21), but their role in specific contractile properties of skeletal muscle fiber is unknown. The reason for the lack of a significant difference in single-fiber contractility or physical function improvements between groups, despite the more pronounced loss in thigh fat in RT+CR than RT alone, is uncertain. The relationship between the amount of muscle fat, strength and mobility is complex. Since intramuscular fat content may be inversely related to daily physical activity in older adults, the correlation in Figure 1D may be a function of individual differences in physical activity that affects muscle fat content (36). Whether RT and CR influence different muscle fat pools or RT benefits muscle function in older adults through mechanisms other than controlling myosteatosis needs to be examined. CR is thought to stimulate basal autophagy (37–39), which is reduced in aging (40), whereas RT activates signaling pathways (mTOR) (41) that are linked to inhibition of autophagy (42). Perhaps the lack of an additive effect is related to these antagonistic actions. In summary, we found that both RT and RT+CR improved myofiber contractility, and that improvements in myofiber force in type-1 fibers in the RT+CR group correlated with decreases in thigh intermuscular fat. However, CR did not further improve the benefits of RT on myofiber contractile properties in older adults with obesity. Whether the amount and/or duration of CR was insufficient to counter chronic deleterious effects of myostatosis on muscle function is unknown. This concept should be tested with more prolonged or intense CR regimens. Conclusion and Limitations To our knowledge, this is the first study to investigate the effect of CR and RT in obese older adults on myofiber contractility properties and physical function. RT is a powerful intervention to improve muscle function in this population and addition of moderate CR did not contribute further benefits. Despite the smaller change in thigh muscle volume, the RT+CR group showed comparable gains in strength. Since single fiber properties do not appear to account for this effect, an explanation could be that CR+RT led to improved central activation of the muscle, probably through the reported enhanced skeletal muscle innervation (26). Future studies including such measure are needed. Adding a control age-matched untreated group, and examining intramyocellular lipid content quantity and composition, could further enhance our understanding of the mechanisms by which obesity is associated with declines in muscle function with aging. Funding This work was supported by the National Institute on Aging at the National Institutes of Health grants R01AG013934 and R01AG057013 to O.D., R01AG020583 to B.N. and the Wake Forest Claude D. 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Relationship of Physical Function to Single Muscle Fiber Contractility in Older Adults: Effects of Resistance Training With and Without Caloric Restriction

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© The Author(s) 2018. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.
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Abstract

Abstract Background Previous studies support beneficial effects of both resistance exercise training (RT) and caloric restriction (CR) on skeletal muscle strength and physical performance. The goal of this study was to determine the effects of adding CR to RT on single-muscle fiber contractility responses to RT in older overweight and obese adults. Methods We analyzed contractile properties in 1,253 single myofiber from muscle biopsies of the vastus lateralis, as well as physical performance and thigh muscle volume, in 31 older (65–80 years), overweight or obese (body mass index = 27–35 kg/m2) men (n = 19) and women (n = 12) who were randomly assigned to a standardized, progressive RT intervention with CR (RT+CR; n = 15) or without CR (RT; n = 16) for 5 months. Results Both interventions evoked an increase in force normalized to cross-sectional area (CSA), in type-I and type-II fibers and knee extensor quality. However, these improvements were not different between intervention groups. In the RT group, changes in total thigh fat volume inversely correlated with changes in type-II fiber force (r = −.691; p = .019). Within the RT+CR group, changes in gait speed correlated positively with changes in type-I fiber CSA (r = .561; p = .030). In addition, increases in type-I normalized fiber force were related to decreases in thigh intermuscular fat volume (r = −0.539; p = .038). Conclusion Single muscle fiber force and knee extensor quality improve with RT and RT+CR; however, CR does not enhance improvements in single muscle fiber contractility or whole muscle in response to RT in older overweight and obese men and women. Aging, Caloric restriction, Muscle fiber, Resistance training Age-associated loss of skeletal muscle strength and power leads to increased risk of impaired mobility and disability in advanced age (1). Single fibers from nonhuman primates and humans are slower and weaker (2–4), show improvements in contractile properties (5), or no changes (2) with aging. Currently, exercise training is the only intervention known to consistently improve muscle mass, strength, and power, and overall physical function in older adults (6–9). Yet, the few studies that examined single-fiber contractile responses to resistance training (RT) in older adults are inconsistent. Most of these studies show an overall improvement in single-fiber force and power with RT (10–14), though some data suggest the oldest adults exhibit a limited response at the myofiber level (15,16). Aging is also characterized by changes in body composition including muscle mass loss and accumulation of adipose tissue (myosteatosis), which can further exacerbate functional decline with age (17). We recently showed that, compared to older adults with a normal body mass index (18.5–24.9 kg/m2), those who are obese (body mass index > 30 kg/m2) have greater intramyocellular lipid content which correlated with impaired single-fiber contractile function (18). Thus, improvements in single-fiber contractile force and power with RT may be blunted in older adults with obesity, and intervening on myosteatosis by adding caloric restriction (CR) to RT may result in further improvements in myofiber contractility for the older obese population. In fact, we previously reported that RT improved physical function and body composition associated with reduced thigh fat in obese older adults, while those with higher initial adiposity experienced less improvement (19). Previous studies show that CR without malnutrition results in a dramatic change in the human skeletal muscle transcriptional profile that resembles that reported for younger individuals (20). CR also attenuates age-dependent progressive functional decline in various organs, peripheral nerve damage, loss of muscle mass, abnormalities in the electron transport chain, and the onset of age-related diseases such as cancer, diabetes, and Alzheimer’s disease (21). Although evidence suggests that CR has multiple beneficial effects on skeletal muscle function in rodents (22–24), whether it improves single-fiber muscle function in older adults with obesity is unknown. Measurements of single-muscle fiber function provide a unique approach to begin to address whether CR further enhances myofiber contractility induced by RT in a fiber-type specific manner (18). This study tested whether adding CR to RT further enhances improvements in contractile properties of fast- and slow-myofibers from obese older adults. We also examined the relationships between physical function and single myofiber contractility before and after these interventions. Materials and Methods Participant Characteristics and Study Design We analyzed single myofiber contractile properties recorded ex vivo from muscle biopsies of the vastus lateralis in a subset of 31 participants (19 men, 12 women) from the larger Improving Muscle for Functional Independence Trial (IM FIT) (19). The IM FIT study (clinicaltrials.gov; NCT01049698) was a randomized controlled trial designed to determine whether CR enhances improvements in body composition and physical function in response to RT in older (65–80 years) overweight and obese (body mass index = 27–35 kg/m2) men and women. All participants, including those from this ancillary study, were randomly assigned equally to a standardized, progressive RT intervention with CR (RT+CR; n = 15) or without CR (RT; n = 16) for 5 months. The study was approved by the Wake Forest School of Medicine Institutional Review Board and all participants provided written informed consent to participate. All participants in the study underwent 5 months of RT 3 days/week on weight-stack resistance machines (Cybex International and Nautilus) at the Wake Forest University Clinical Research Center exercise facility as previously described (19). Two exercise interventionists supervised the training sessions and ensured that participants adjusted the equipment appropriately and performed the exercises safely. Participants performed an initial 5-minute warmup by walking or cycling at a slow pace followed by light stretching and concluded each session with a 5-minute cool down and light stretching. The protocol involved a gradual progression of weight and repetitions during the first month to allow familiarization with the equipment, minimize muscle soreness, and reduce injury potential. The maximal weight that a person could lift with the correct form in a single repetition (1RM) was used to prescribe intensity. The training goal was to complete three sets of 10 repetitions for each exercise at 70% 1RM for that specific exercise. Participants rested ~1 minute between sets. Resistance was increased when a participant was able to complete 10 repetitions on the third set for two consecutive sessions. Strength testing was repeated every 4 weeks, and training loads were adjusted to be consistent with the 70% 1RM goal. The exercises performed included: Leg Extension, Leg Curl, Leg Press, Seated Row, Chest Press, Biceps Curl, Triceps Extension, and Latissimus Pulldown. Participants assigned to RT only were instructed to follow a eucaloric diet, whereas those assigned to RT+CR underwent a dietary weight-loss intervention designed to elicit moderate weight loss (5%–10%) as described (19). This intervention incorporated meal replacements, nutrition education, and dietary behavior modification advice via weekly meetings with the study’s registered dietitian that took place either before or after one of their exercise sessions. Each participant was assigned a daily caloric intake to follow, which was derived from subtracting 600 kcal from his or her estimated daily energy needs for weight maintenance. A maximum of two meal replacements per day (shakes and bars; Slim-Fast Inc.) that contained ~220 kcal with 7–10 g protein, 33–46 g carbohydrates, 1.5–5 g fat, and 2–5 g fiber were provided to participants for breakfast and lunch. Dinner and snack options were recommended by the registered dietitian in accordance with each participant’s daily caloric goals and tailored to allow for individual preferences for various food items. Participants were asked to keep a diet log of all foods consumed, and the logs were monitored weekly by the registered dietitian to verify compliance with the weight-loss intervention. Specific inclusion and exclusion criteria, procedures for vastus lateralis muscle biopsy, and methods to measure physical function and thigh composition (via computed tomography) have been described previously (19,25). Single-muscle fiber experimental setup and solutions were described before (2) and a brief description is included below. Data from a different group of IM FIT participants have been included in previous publications (18,26). Single-Fiber Physiology Tests and Experimental Protocols All measurements were conducted at 15°C, and temperature was continuously monitored by a thermocouple inserted into the experimental chamber. Although this is not a physiological temperature, most previous studies have been recorded in these conditions. All functional data were collected and analyzed using a personal computer and a data acquisition board (Model 600A Digital controller, Aurora Scientific, Aurora, Ontario). A slack test was used to determine the unloaded shortening velocity (Vo). In this procedure, permeabilized fibers were transferred to activating solution (pCa: 4.5) and once peak force was attained (monitored by real-time digital oscilloscope), subjected to a rapid slack step (≤20% of fiber length [FL] within 1 ms). The procedure was repeated at different slack lengths, and the times required for tension redevelopment were plotted versus the corresponding slack distances. A straight line was fit by least-squares linear regression, and the slope of the regression line, normalized to FL, defined Vo. It should be noticed that myofiber permeabilization bypasses the physiological excitation-contraction mechanism and muscle fiber activation evoked by nerve stimulation. After the slack test, the force–velocity relationship was generated by performing a series of isotonic contractions of the muscle fiber. Briefly, the muscle fiber was placed in activating solution (pCa: 4.5) and after reaching peak force, subjected to a series of three isotonic steps varying from 3% to 80% of Po. After the last step, the fiber was rapidly (<1 ms) slackened by 20% of its length, which zeroed the force transducer, providing a baseline for force measurement. Step duration was less than ~100 ms. Shortening velocity and force were obtained as averages over the final half of each step. Velocity was calculated as the slope of the position recorded over the same time period. Five to six series of three isotonic contractions were used to establish a force–velocity relationship. The Hill equation was fitted to the data using an iterative nonlinear curve-fitting procedure to draw the force–power relationship. The following parameters were used to describe the hyperbolic fit to the data: Vmax (the velocity extrapolated to a force of zero), Po (average force obtained during the trial), and a/Po (a parameter describing the curvature or shape of the force–velocity relationship). Vmax was normalized to FL. a/Po is a dimensionless parameter. Peak power was calculated from these three parameters, and expressed as W/L fiber. In all contractions, Ca2+-activated force was measured using the transducer zero signal as a baseline. Forces were normalized to the fiber’s cross-sectional area (CSA) to obtain specific force. Quality control was performed as described (2). Fibers were excluded from analysis if force declined more than 5% or if they broke or showed partial myofibrillar tearing at any observation timepoint in the experimental protocol. Experiments were excluded from analysis if compliance, defined as displaced axis-intercept of the slack test plot, exceeded 5% of FL and if the r2 of the force–velocity regression was less than .98. Assessment of Fiber Myosin Heavy-chain Isoform At the end of each functional experiment, the single-fiber segment was removed from the test apparatus and stored in 20 µL of sodium dodecyl sulfate sample buffer (containing 62.5 mM Tris pH 6.8, 2% sodium dodecyl sulfate, 10% glycerol, 5% betamercaptoethanol, and 0.001% bromophenol blue) at −80°C. Later, fibers were denatured for 5 minutes at 95°C. To determine the myosin heavy-chain composition of the fiber segment, a sample of the fiber solute, equivalent to that tested for contraction, was run on a sodium dodecyl sulfate–polyacrylamide gel electrophoresis system that consisted of a 6% separating gel and a 4% stacking gel (acrylamide:bisacrylamide = 37:1). The gels contained 30% glycerol to improve separation of myosin heavy-chain isoforms. Electrophoresis was carried out at as described (2,18). Six percent of all fast fibers were hybrid (IIa/IIx), being IIa the predominant myosin heavy-chain isoform in 96% of the cases. Due to the small percent, hybrid muscle fibers were not included in the analysis. Statistical Analysis All analyses were performed using SAS 9.4 (Cary, NC). Baseline measures and changes in body mass, thigh composition, physical function, and type-I and type-II CSA and muscle fiber contractile function were described as mean and standard deviations by intervention groups. Statistical analyses were based on average values taken for each participant. Student t test was used to evaluate changes within groups and compare between groups measures at baseline and changes. Analysis of covariance was used to compare group differences for CSA and muscle fibers contractile function adjusted for gender and baseline measures. We calculated Spearman’s correlation coefficients of baseline measures and changes among type-I and type-II CSA and muscle fibers contractile function, and body composition and physical performance, overall and by gender. Spearman correlations were used because data failed tests of normality. Despite the large number of muscle fibers analyzed, we did not adjust for multiple comparisons due to the relatively low sample size, as well as the exploratory nature of these analyses. Results Participant Characteristics, Physical Function, Thigh Composition, and Single-fiber Contraction Properties at Baseline and Changes With Intervention Table 1 shows unadjusted values at baseline, and changes relative to baseline within groups. Body weight and total thigh fat volume decreased more in the RT+CR than RT group. Relative to baseline, both interventions increased absolute and normalized knee extension strength, but differences between groups were not significant (Table 1). Although participants were highly functional at baseline, the total short physical performance battery (SPPB) score significantly improved with both interventions, with no group difference in the magnitude of improvement (Table 1). Of the individual SPPB sub-scores, chair rise time significantly improved with both RT and RT+CR (no difference between groups), 4-m usual gait speed improved with RT+CR but not RT alone (no difference between groups), and balance scores did not change with either intervention (however, these scores were close to near perfect at baseline). RT decreased type-I, but not type-II, muscle fiber CSA; however, the difference between interventions was not significant. Adding CR to RT did not enhance these improvements in single-fiber contractile properties. Significant changes in type-I fiber CSA, force, normalized force, and type-II fiber force and power induced by RT exhibited no significant change or a reduction in significance when CR was combined with RT. Table 1. Physical Function and Muscle Strength, Thigh Composition, and Unadjusted Single-fiber CSA, Force, Velocity, and Power at Baseline and Changes With Intervention   RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413    RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413  Note: Change relative to baseline within groups: *p < .05; **p < .01. Muscle quality was calculated as the ratio of knee extensor peak torque to lean mass assessed by using dual-energy x-ray absorptiometry (in Nm/kg leg mass) as reported (19). BMI = Body mass index; CR = Caloric restriction; CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length; RT = Resistance training; SPPB = Short physical performance battery. View Large Table 1. Physical Function and Muscle Strength, Thigh Composition, and Unadjusted Single-fiber CSA, Force, Velocity, and Power at Baseline and Changes With Intervention   RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413    RT  RT+CR    Baseline (n = 14–16)  Change Relative to Baseline (n = 13–16)  Baseline (n = 15)  Change Relative to Baseline (n = 13–15)  p Value Between Groups  Weight, kg  88.1 (14.5)  −0.0 (2.1)  84.8 (9.5)  −5.7 (3.9)**  <.0001  BMI, kg/m2  29.7 (2.2)  −0.1 (0.8)  29.7 (1.8)  −2.0 (1.4)**  <.0001  400-m walk time, s  297.7 (50.4)  −7.3 (30.2)  299.0 (39.2)  −15.7 (25.2)*  .42  Leg Press, W  183.2 (89.7)  6.9 (60.6)  153.1 (53.3)  5.5 (37.2)  .94  Knee extensor strength, Nm  135.3 (47.6)  15.2 (18.6)*  120.5 (37.8)  11.2 (17.5)*  .56  Knee extensor quality, Nm/kg  16.3 (3.17)  3.84 (1.6)*  15.9 (2.93)  3.79 (1.5)**  .83  SPPB (0–12)  11.2 (1.0)  0.6 (0.6)**  10.8 (1.0)  0.7 (0.9)**  .54  Usual Gait Speed (m/s)  1.18 (0.15)  0.10 (0.18)  1.11 (0.12)  0.11 (0.15)*  .78  Chair Rise Time (secs)  11.2 (2.7)  −2.1 (1.9)**  12.1 (3.0)  −2.0 (2.8)*  .92  Balance Score  3.9 (0.2)  0.0 (0.2)  3.9 (0.2)  0.1 (0.3)  .94  CT mean muscle attenuation, Hounsfield units  44.6 (4.2)  0.8 (1.8)  45.5 (3.9)  0.9 (1.2)*  .93  Total thigh volume, cm3  1,469.2 (181.5)  −4.7 (46.1)  1,416.5 (164.7)  −85.4 (77.6)**  .0017  Thigh fat volume, cm3  684.5 (230.6)  −4.7 (54.7)  680.2 (208.8)  −89.5 (55.8)**  .0002  Thigh muscle volume, cm3  688.7 (163.2)  24.1 (35.5)*  672.9 (139.3)  0.7 (20.8)  .0360  Intermuscular fat volume, cm3  31.4 (14.6)  0.1 (5.7)  27.4 (9.1)  −3.3 (6.0)  .1285  TYPE I  CSA, µm2  7,937 (1,816)  −738.9 (1,341)*  7,415 (1,645)  −247.4 (2,151)  .4481  Force, mN  0.7 (0.2)  0.1 (0.1)**  0.8 (0.3)  0.1 (0.2)  .5715  Normalized force, kN/m2  95.3 (19.2)  26.1 (15.9)**  107.7 (26.9)  16.3 (28.7)*  .2576  Vo, FL/s  1.0 (0.7)  0.0 (0.3)  1.4 (1.3)  −0.1 (1.6)  .7042  Vmax, FL/s  2.0 (2.5)  −0.2 (1.2)  1.3 (1.4)  0.3 (1.8)  .3565  Power, µN*FL/s  14.8 (5.7)  3.4 (6.3)  17.8 (10.1)  4.6 (13.6)  .7672  Normalized power, W/liter  2.1 (0.7)  0.5 (1.2)  2.6 (1.4)  0.5 (1.9)  .9812  TYPE II  CSA, µm2  7,591.7 (1,922)  610.0 (1,576)  7,394.8 (2,283)  −607.3 (2,339)  .1440  Force, mN  0.8 (0.3)  0.2 (0.2)**  0.8 (0.3)  0.1 (0.3)  .1528  Normalized force, kN/m2  102.1 (26.5)  18.8 (24.6)  103.9 (24.3)  16.7 (30.1)  .8531  Vo, FL/s  2.7 (1.3)  1.2 (2.2)  3.1 (1.4)  0.5 (2.0)  .4434  Vmax, FL/s  1.7 (0.7)  0.3 (0.8)  1.8 (0.7)  −0.0 (0.9)  .4004  Power, µN*FL/s  40.7 (17.3)  19.4 (25.2)*  41.2 (17.3)  5.5 (13.1)  .1192  Normalized power, W/liter  5.3 (1.6)  1.7 (2.8)  5.6 (1.7)  1.2 (1.9)*  .5413  Note: Change relative to baseline within groups: *p < .05; **p < .01. Muscle quality was calculated as the ratio of knee extensor peak torque to lean mass assessed by using dual-energy x-ray absorptiometry (in Nm/kg leg mass) as reported (19). BMI = Body mass index; CR = Caloric restriction; CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length; RT = Resistance training; SPPB = Short physical performance battery. View Large Both Interventions Increased Single-fiber Contractile Properties Both interventions evoked an increase in normalized force in type-I fibers. However, these improvements in single-fiber force were not different between intervention groups (Table 1). In addition, there were no significant differences in single-fiber CSA and contractile properties between intervention groups, after adjusting for baseline measures and gender (Table 2). Table 2. Single-fiber CSA, Force, Velocity, and Power Values Adjusted for Baseline Measure and Gender   Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517    Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517  Note: CSA = Cross-sectional area; FL = Fiber length. View Large Table 2. Single-fiber CSA, Force, Velocity, and Power Values Adjusted for Baseline Measure and Gender   Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517    Baseline  Post-Intervention  Outcome  Mean (SD)  RT Mean (SE)  RT+CR Mean (SE)  RT+CR – RT Difference Mean (95% CI)  p Value Between Groups  Type I muscle  CSA, µm2  7,685 (1,727)  7,084 (376)  7,260 (377)  176 (−911, 1263)  .7423  Force, mN  0.7 (0.2)  0.9 (0.0)  0.9 (0.0)  −0.0 (−0.1, 0.1)  .9243  Normalized force, kN/m2  101.3 (23.7)  121.7 (4.4)  121.4 (4.5)  −0.3 (−13.3, 12.8)  .9647  Shortening velocity, FL/s  1.6 (2.0)  1.4 (0.3)  1.8 (0.3)  0.4 (−0.5, 1.2)  .3895  Power, µN*FL/s  16.3 (8.2)  17.3 (2.6)  21.3 (2.4)  4.0 (−3.2, 11.3)  .2614  Normalized power, W/liter  2.4 (1.1)  2.4 (0.4)  3.0 (0.3)  0.5 (−0.5, 1.5)  .2945  Type II muscle  CSA, µm2  7,493 (2,076)  7,484 (488)  6,665 (438)  −819 (−2,169, 531)  .2210  Force, mN  0.8 (0.3)  0.9 (0.1)  0.8 (0.1)  −0.1 (−0.3, 0.1)  .2652  Normalized force, kN/m2  103.0 (25.0)  120.1 (4.5)  118.3 (4.1)  −1.8 (−14.3, 10.7)  .7663  Shortening velocity, FL/s  1.8 (0.7)  2.1 (0.2)  1.8 (0.1)  −0.2 (−0.7, 0.2)  .3158  Power, µN*FL/s  41.0 (17.0)  55.1 (6.4)  44.6 (5.3)  −10.5 (−27.5, 6.5)  .2130  Normalized power, W/liter  5.5 (1.6)  7.2 (0.6)  6.8 (0.5)  −0.5 (−2.2, 1.2)  .5517  Note: CSA = Cross-sectional area; FL = Fiber length. View Large Baseline Correlations Between Physical Function, Thigh Composition, and Single-fiber Contractile Properties in All Participants Data in Table 3 show that all of the baseline type- I, and most of the type-II contractile properties, exhibit inverse correlations with SPPB. However, gait speed at baseline was positively correlated with most of Type-I contractile properties and Type-II fiber power (ie, faster gait associated with better contractile function). Thigh muscle volume correlated positively with type-I fiber and type-II fiber force (Table 3). Table 3. Baseline Correlations Between Physical Function, Thigh Composition, and Single-fiber CSA, Force, Velocity, and Power   CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339    CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339  Note: Values are r (top) and p (bottom). CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length. View Large Table 3. Baseline Correlations Between Physical Function, Thigh Composition, and Single-fiber CSA, Force, Velocity, and Power   CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339    CSA (µm2)  Force (mN)  Normalized force (kN/m2)  Vo (FL/s)  Vmax (FL/s)  Power (µN*FL/s)  Normalized power (W/liter)  Type-I fiber  400-m walk time  .15196  −.13884  −.36378  −.06630  −.24877  −.36700  −.25665  .4228  .4644  .0481  .7278  .1932  .0502  .1790  SPPB (0–12)  −.04635  −.32253  −.38853  −.21467  −.33191  −.27154  −.29714  .8044  .0768  .0308  .2462  .0732  .1466  .1108  Usual gait speed (m/s)  .27305  .44449  .24107  .51190  .17270  .36394  .39276  .1372  .0122  .1914  .0032  .3614  .0480  .0318  LegPress, W  .28854  .29963  .24095  .22059  .19846  .34954  .19713  .1154  .1015  .1916  .2331  .2931  .0583  .2964  Biodex (Knee extension strength)  .15263  .21184  .22894  .10317  .10928  .08974  .05983  .4473  .2888  .2507  .6086  .5874  .6562  .7669  CT mean muscle attenuation, Hounsfield units  −.12125  .18710  .17063  .18220  −.07094  .11182  .17438  .5233  .3222  .3673  .3352  .7146  .5636  .3656  Total thigh volume, cm3  −.19911  −.03715  .16396  .10345  −.05517  .00788  .06256  .2915  .8455  .3866  .5864  .7762  .9676  .7471  Thigh fat volume, cm3  −.27608  −.14794  .10033  .07987  −.04581  .08473  .12069  .1397  .4353  .5978  .6748  .8134  .6621  .5329  Thigh muscle volume, cm3  .24449  .37709  .24316  .15328  .03103  .08670  .07882  .1929  .0400  .1954  .4187  .8730  .6547  .6844  Intermuscular fat volume, cm3  .06563  −.10701  −.17197  −.14661  .04039  −.25468  −.33103  .7304  .5736  .3635  .4395  .8352  .1824  .0794  Type-II fiber  400-m walk time  −.17539  −.21653  −.17490  −.25668  .04926  −.33498  −.19376  .3628  .2592  .3642  .1789  .8034  .0814  .3232  SPPB (0–12)  .09563  −.11806  −.24203  −.31523  .22471  −.18956  −.27965  .6152  .5344  .1975  .0897  .2412  .3247  .1418  Usual gait speed (m/s)  .31261  .30836  .14670  .21949  −.10312  .42607  .28932  .0926  .0973  .4392  .2439  .5945  .0212  .1279  Leg Press, Nm  .48259  .36756  .05763  .10880  .04089  .34265  −.04089  .0069  .0457  .7623  .5671  .8332  .0688  .8332  Biodex (Knee extension strength)  .39194  .47314  .18864  .37668  .12547  .40855  .17402  .0432  .0127  .3460  .0528  .5414  .0383  .3952  CT mean muscle attenuation, Hounsfield units  .41232  .29015  −.15271  −.01133  .08265  .04215  −.29119  .0262  .1268  .4290  .9535  .6759  .8314  .1327  Total thigh volume, cm3  −.12463  −.08424  −.02512  .10493  .09141  −.13027  .03448  .5195  .6640  .8971  .5880  .6437  .5088  .8617  Thigh fat volume, cm3  −.31478  −.24433  .01872  −.00493  .08812  −.18993  .10947  .0963  .2015  .9232  .9798  .6557  .3330  .5792  Thigh muscle volume, cm3  .51872  .38227  −.06207  .23990  .01533  .29776  −.08812  .0039  .0407  .7491  .2100  .9383  .1238  .6557  Intermuscular fat volume, cm3  −.18916  −.11429  −.08374  −.06010  .08320  −.15107  .01642  .3257  .5550  .6658  .7568  .6738  .4429  .9339  Note: Values are r (top) and p (bottom). CSA = Cross-sectional area; CT = Computed tomography; FL = Fiber length. View Large Baseline Correlations by Gender In females, there was a significant positive correlation between baseline gait speed and type-I fiber power (r = .665; p = .026), while leg extension strength correlated positively with type-I fiber CSA (r = .629; p = .028). Leg extension strength also correlated positively with type-II fiber force (r = .709; p = .022) and velocity (r = .636, p = .048). Male 400-m walk time correlated negatively with type-I fiber power (r = −.560; p = .016). Correlations Between Changes in Type-I and Type-II Fiber Contractile Properties and RT- or RT Plus CR-induced Changes in Physical Function and Thigh Composition We next examined whether physical function and thigh composition responses to RT and RT+CR correlated with changes in myofiber contractile properties with the interventions. We found that RT-induced changes in leg extension strength correlated positively with type-I, but not type-II, fiber power (r = .657; p = .020; Figure 1A), while changes in total thigh fat volume negatively correlated with changes in type-II fiber force with RT (r = −.691; p = .019; Figure 1B). Within the RT+CR group, changes in gait speed correlated positively with changes in type-I fiber CSA (r = .561; p = .030; Figure 1C), indicating that improvements in gait speed with RT+CR are associated with a smaller decrease in type-I fiber CSA. In addition, changes in thigh muscle volume directly correlated with changes in type-I fiber CSA (r = .657; p = .008) with RT+CR, and increases in type-I normalized fiber force were inversely correlated with changes in thigh intermuscular fat volume (r = −.539; p = .038; Figure 1D). No significant correlations were found between changes in function and thigh composition and changes in type-II fiber properties with RT+CR. Figure 1. View largeDownload slide Correlations between changes in fiber contractile properties and changes in physical function and thigh composition. Changes in leg extension strength correlated positively with type-I, but not type-II, fiber power with RT (A). Changes in total thigh fat volume inversely correlated with changes in type-II fiber force with RT (B). Changes in gait speed correlated positively with changes in type-I fiber CSA with RT+CR (C). Increases in type-I normalized fiber force were inversely correlated with changes in thigh intermuscular fat volume (D). CR = Caloric restriction; CSA = Cross-sectional area; RT = Resistance training. Figure 1. View largeDownload slide Correlations between changes in fiber contractile properties and changes in physical function and thigh composition. Changes in leg extension strength correlated positively with type-I, but not type-II, fiber power with RT (A). Changes in total thigh fat volume inversely correlated with changes in type-II fiber force with RT (B). Changes in gait speed correlated positively with changes in type-I fiber CSA with RT+CR (C). Increases in type-I normalized fiber force were inversely correlated with changes in thigh intermuscular fat volume (D). CR = Caloric restriction; CSA = Cross-sectional area; RT = Resistance training. Discussion This work analyzed for the first time the effects of adding CR to RT on single muscle fiber contractile properties in older men and women with obesity. Since adiposity is associated with disability risk (27,28), we tested the hypothesis that CR-evoked fat loss further enhances RT-induced improvements in myofiber contractile properties. We used a randomized design to analyze the effects of both interventions on myofiber contractile properties, both within and between groups. The analyses showed that both interventions similarly increased type-I and type-II fiber force since there were no differences between groups. We also found cross-sectional associations of both physical function and thigh composition with myofiber contractility. Type-I fiber force, velocity, and power positively correlated with gait speed, while fiber-I force correlated with muscle volume. Type-II fiber power correlated with gait speed, and type-II fiber CSA, force and power correlated with leg press. Type-II fiber CSA, force and power correlated with knee extension strength, while type-II fiber CSA and force correlated with thigh muscle volume. The analysis by gender also showed baseline correlations between type-I and type-II fibers and physical function (data not shown). We previously showed that greater intramyocellular lipids are associated with slower myofiber contraction, force, and power development in obese older adults, providing a rationale for our hypothesis that a decrease in intramyocellular lipids would contribute to effects of RT on myofiber contractility (18) Here, we found that thigh fat volume decreased with both interventions, changes in thigh fat volume with RT+CR directly correlated with changes in type-I fiber CSA, and increases in type-I fiber force with RT+CR correlated negatively with changes in thigh intermuscular fat volume, which may account for the increase in knee extension strength despite the associated decrease in thigh muscle volume. However, contrary to our expectations, a further decrease in thigh fat induced by CR did not augment improved single muscle fiber contractility with RT. These results are also consistent with the physical function data recorded in this subset and the entire cohort (19). The force generation capacity of myofibers declines with aging in humans and nonhuman primates (2), but RT benefits human muscle function (13,15,16), while CR has been shown to prevent declines in muscle force with aging in rodents (22). RT improves single-fiber contractile force in nonobese (12,13,29), and obese older adults (30). An important question then is how RT improves single muscle contractile properties in older adults. We previously showed that RT enhanced skeletal muscle innervation in another subset of IM FIT participants (26). Improved knee extensor strength in participants of the same study was associated with changes in muscle-specific miRNAs in muscle and plasma (31). We also previously reported that TNNT1 alternative splicing (AS) 1, AS2 and the AS1/AS2 ratio are potential quantitative biomarkers of skeletal muscle adaptation to RT in older adults, and that their profile reflects enhanced single-fiber muscle force in the absence of significant increases in fiber cross-sectional area (30). This finding could be associated with changes in single-fiber calcium sensitivity (32). Additionally, RT in young and old subjects can change the velocity of the myosin molecule tested by the in vitro motility assay (33). Much less is known about how CR improves muscle function. Different CR regimens applied to rodents proved to be beneficial on whole muscle contraction properties (22,23). Proposed mechanisms mediating these effects include improved excitation-contraction coupling (24,34) and mitochondrial proton leak (35). The role of CR, IGF signaling, sirtuins, and protein intake in maintaining muscle mass and metabolism across ages in animals and humans has been amply discussed in the literature (21), but their role in specific contractile properties of skeletal muscle fiber is unknown. The reason for the lack of a significant difference in single-fiber contractility or physical function improvements between groups, despite the more pronounced loss in thigh fat in RT+CR than RT alone, is uncertain. The relationship between the amount of muscle fat, strength and mobility is complex. Since intramuscular fat content may be inversely related to daily physical activity in older adults, the correlation in Figure 1D may be a function of individual differences in physical activity that affects muscle fat content (36). Whether RT and CR influence different muscle fat pools or RT benefits muscle function in older adults through mechanisms other than controlling myosteatosis needs to be examined. CR is thought to stimulate basal autophagy (37–39), which is reduced in aging (40), whereas RT activates signaling pathways (mTOR) (41) that are linked to inhibition of autophagy (42). Perhaps the lack of an additive effect is related to these antagonistic actions. In summary, we found that both RT and RT+CR improved myofiber contractility, and that improvements in myofiber force in type-1 fibers in the RT+CR group correlated with decreases in thigh intermuscular fat. However, CR did not further improve the benefits of RT on myofiber contractile properties in older adults with obesity. Whether the amount and/or duration of CR was insufficient to counter chronic deleterious effects of myostatosis on muscle function is unknown. This concept should be tested with more prolonged or intense CR regimens. Conclusion and Limitations To our knowledge, this is the first study to investigate the effect of CR and RT in obese older adults on myofiber contractility properties and physical function. RT is a powerful intervention to improve muscle function in this population and addition of moderate CR did not contribute further benefits. Despite the smaller change in thigh muscle volume, the RT+CR group showed comparable gains in strength. Since single fiber properties do not appear to account for this effect, an explanation could be that CR+RT led to improved central activation of the muscle, probably through the reported enhanced skeletal muscle innervation (26). Future studies including such measure are needed. Adding a control age-matched untreated group, and examining intramyocellular lipid content quantity and composition, could further enhance our understanding of the mechanisms by which obesity is associated with declines in muscle function with aging. Funding This work was supported by the National Institute on Aging at the National Institutes of Health grants R01AG013934 and R01AG057013 to O.D., R01AG020583 to B.N. and the Wake Forest Claude D. Pepper Older Americans Independence Center (P30-AG21332). Conflict of Interest Z.-M.W., X.L., M.L.M., S.J.C., B.N., and O.D. have no conflicts of interest to declare. A.P.M. serves on the editorial board of the Journal of Gerontology, Medical Sciences. References 1. Clark BC, Manini TM. Functional consequences of sarcopenia and dynapenia in the elderly. Curr Opin Clin Nutr Metab Care . 2010; 13: 271– 276. doi: 10.1097/MCO.0b013e328337819e Google Scholar CrossRef Search ADS PubMed  2. Choi SJ, Shively CA, Register TC, et al.   Force Generation capacity of single muscle fibers of vastus lateralis declines with age and correlates with physical function in African green Vervet monkeys. J Gerontol Biol Sci . 2012; 68( 3): 258– 267. doi: 10.1093/gerona/gls143 Google Scholar CrossRef Search ADS   3. Canepari M, Pellegrino MA, D’Antona G, Bottinelli R. Single muscle fiber properties in aging and disuse. 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The Journals of Gerontology Series A: Biomedical Sciences and Medical SciencesOxford University Press

Published: Mar 13, 2018

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