A Comparison of Self-report Indices of Major Mobility Disability to Failure on the 400-m Walk Test: The LIFE Study

A Comparison of Self-report Indices of Major Mobility Disability to Failure on the 400-m Walk... Abstract Background The objective assessment of major mobility disability (objective MMD) by a 400-m walk test (400 MWT) is important but not always practical. Previous research on the relationship between self-reported MMD (SR MMD) and objective MMD is sparse and limited to cross-sectional data. Methods We evaluated agreement between SR MMD and objective MMD using longitudinal data from the Lifestyle Interventions for Elders (LIFE) study. The SR MMD indices were defined based on having a lot of difficulty or inability to walk a quarter of a mile (SR-1/4MILE), walk several blocks (SR-BLOCKS), and climb one flight of stairs (SR-STAIRS). Results Using objective MMD as the gold standard, SR-1/4MILE and SR-BLOCKS had relatively low sensitivity (around 0.4) and high specificity (around 0.9) for prevalence. Their overall sensitivity and specificity for cumulative incident objective MMD were approximately 0.6 and 0.8, respectively. While the annual probability of staying MMD free was similar for objective MMD, SR-1/4MILE, and SR-BLOCKS (90% for all), the probability of recovering from SR MMD was higher (50%) than that of objective MMD (22%). The development of objective MMD (439 events), SR-1/4MILE (356 events), and SR-BLOCKS (379 events) had a similar trajectory over time with substantially overlapping survival curves. SR-STAIRS generally did not agree well with objective MMD. Incorporating SR-STAIRS with either SR-1/4MILE or SR-BLOCKS did not significantly improve the agreement between SR MMD and objective MMD. Conclusions Simple SR-1/4MILE and SR-BLOCKS are reasonable candidates to define MMD if the primary outcome of interest is incident MMD. Agreement, Cumulative incidence, Sensitivity, Specificity, Transition probability Preserving the ability to walk 400 m is an excellent proxy for mobility and the loss of this capacity has been used to define major mobility disability (MMD) in geriatric research (1–3). However, the assessment of this outcome requires a walking track or other suitable venue, in-person contact, and supervision. This is neither feasible nor pragmatic for use in large implementation trials or clinical practice. Previous research on the relationship between self-reported and objective assessments of MMD is sparse and limited to cross-sectional data (4,5). The Lifestyle Interventions for Elders (LIFE) study collected concurrent longitudinal data on both self-reported measures of MMD and objective failure to complete the 400-m walk (3). This provided us with a unique opportunity to comprehensively evaluate agreement between subjective and objective assessments of MMD across a period of 36 months. MMD in the LIFE study was defined as failure to complete a 400-m walk test (400 MWT) in ≤15 minutes without assistance. We refer to this as objective MMD and used it as the gold standard to compare with several indices of self-reported MMD (SR MMD). The SR MMD was defined based on self-reported ability to walk a quarter mile, walk several blocks, or climb one flight of stairs. These self-report indices implicitly address the ability to remain mobile in the community, which is what the 400 MWT is meant to address. In the literature, researchers have frequently used walking-based and stair climb-based self-report items separately (6–10) as well as jointly (11–16) to define mobility disability. Therefore, we examined the three items separately and we created composite SR MMD indices as well for ease of comparison with previous work. The current analysis examined several scientific questions related to SR MMD. First, what is the sensitivity and specificity of the SR MMD indices using objective MMD as the gold standard? Second, how would the pattern of change over time for the SR MMD indices compare with objective MMD? Finally, how would the cumulative incidence rates across time for the SR MMD indices compare with objective MMD? Methods Data Source The LIFE study is a multicenter, single-blind randomized trial that compared the effects of a physical activity intervention to a health education control program on (objective) MMD. It was designed to target adults aged 70–89 years who were sedentary, were at risk for mobility disability, and yet were able to walk 400 m in ≤15 minutes without sitting, using a walker, or needing the help of another person. A total of 1,635 participants were randomized to either physical activity or health education. Details regarding the study have been published elsewhere (3,17,18). Briefly, the physical activity intervention involved moderate intensity walking, with a goal of 150 minutes/wk, accompanied by strength, flexibility, and balance training (17). The intervention included both center-based visits and home-based activity for the duration of the study. Participants in the health education group attended workshops on topics in health education other than physical activity that were relevant to older adults. The study protocol was approved by the institutional review boards at all participating sites. Written informed consent was obtained from all study participants. Measures Objective MMD The 400 MWT was assessed every 6 months at clinic visits. This test is a modified version of a walking test originally described by Newman and colleagues (19). Participants were instructed to walk at their usual pace for 400 m (10 laps of a 20-m course defined by two cones). The maximum time allowed for the test was 15 minutes without sitting and without the help of a walker or another person; participants were allowed to stop and stand to rest for up to 1 minute and could use a cane, but they were not allowed to lean against any object to support their weight. When MMD could not be objectively measured because of the inability of the participant to come to the clinic and absence of a suitable walking course at the participant’s home, institution, or hospital, the outcome was adjudicated based on objective inability to walk 4 m in ≤10 seconds, or self-, proxy-, or medical record-reported inability to walk across a room. It would not be feasible for a participant who met any of these latter criteria to complete the 400 MWT within 15 minutes. For the purpose of this analysis, we excluded objective MMD events determined on the basis of self- or proxy-reported responses. Self-reported MMD (SR MMD) The assessment of SR MMD, which was completed at baseline, months 6, 12, 24, 36, and study closeout visits, was achieved using three questions from a disability questionnaire (8): (i) “During the past month, how much difficulty have you had walking a quarter of a mile, which is about 3 or 4 blocks, because of your health?” (SR-1/4MILE), (ii) “During the past month, how much difficulty have you had walking several blocks because of your health?” (SR-BLOCKS), and (iii) “During the past month, how much difficulty have you had climbing one flight of stairs because of your health?” (SR-STAIRS). The possible responses were “no difficulty”, “a little difficulty”, “some difficulty”, “a lot of difficulty”, “unable to do”, “did not do for other reasons”, or “don’t know/refused”. The responses “did not do for other reasons” or “don’t know/refused” were excluded from analyses. For each question, the two response categories “a lot of difficulty” and “unable to do” were combined to define participants as having SR MMD (13,15). Additionally, we created two composite SR MMD indices, SR-1/4MILE-STAIRS and SR-BLOCKS-STAIRS. Specifically, SR-1/4MILE-STAIRS was defined based on development of either SR-1/4MILES or SR-STAIRS disability. The other combination, SR-BLOCKS-STAIRS, was defined in a similar fashion. Statistical Analyses Data on the 400 MWT performed at assessment visits occurring at months 6, 12, 24, 36, and study closeout visits were included in all analyses to align comparisons of self-report and objective measures collected at the same assessment visit. Using objective MMD as the gold standard, sensitivity and specificity for each SR MMD index were assessed in two different scenarios. First, we computed sensitivity and specificity for the prevalence of each SR MMD index against that of objective MMD. That is, sensitivity and specificity were computed for each follow-up visit separately using contingency tables. These served as cross-sectional examinations of the performance of the SR MMD indices. The average cross-sectional sensitivity and specificity throughout the course of the study were calculated using generalized estimating equations (GEE) to account for the correlations among repeated measures. An independence working correlation structure was chosen for all SR MMD indices based on QIC values. The 95% confidence intervals (CI) were reported. Second, we evaluated the sensitivity and specificity for the incident SR MMD compared to the incident objective MMD at each follow-up visit and overall. Specifically, MMD events occurring at prior visits were carried forward into the analyses of subsequent visits so that cumulative incident events were examined. For example, participants who developed MMD at month 6 were treated analytically as incident events thereafter regardless of whether they recovered from MMD subsequently. We note that this approach of evaluating cumulative incidence has the limitation of ignoring the timing of the MMD events that may have occurred at different time points. Next, to describe the pattern of change over time for SR MMD indices and the objective MMD, we used data from annual visits up to month 36 to fit first-order Markov transition models. Annual probabilities that participants transition in and out of a MMD state were estimated and presented in transition matrices. Finally, the onset of SR MMD and objective MMD were graphically compared using Kaplan–Meier curves and the incident rates were estimated using Poisson regression models. All analyses were pooled across treatment groups and were conducted using SAS 9.4 (Cary, NC). Results Although all participants were able to complete the 400 MWT successfully at baseline, 111 (7.4%), 137 (9.1%), and 72 (4.6%) reported having a lot of difficulty or inability to “walk a quarter mile”, “walk several blocks”, and “climb a flight of stairs” at baseline, respectively (Supplementary Table 2). Next, for each SR MMD index, we assessed its agreement with objective MMD among the participants who were free of SR MMD at baseline. Table 1 summarizes the cross-sectional sensitivity and specificity for the prevalence of SR MMD using the prevalence of objective MMD assessed at the same follow-ups as a gold standard. It indicates that the sensitivity for SR-1/4MILE was relatively low, ranging from 0.41 for month 6 to 0.48 for month 24, with an average of 0.40 (95% CI: 0.35–0.45) across the course of the study; the specificity was very high, ranging from 0.95 at month 6 to 0.93 at month 24, with an average of 0.93 (95% CI: 0.92–0.94). Similar patterns were observed for SR-BLOCKS. The overall cross-sectional sensitivity and specificity were 0.42 (95% CI: 0.37–0.47) and 0.92 (95% CI: 0.91–0.93) for SR-1/4MILE and SR-BLOCKS, respectively. Using SR-STAIRS alone to define MMD yielded very low sensitivity (an average of 0.2) and markedly high specificity (an average of 0.96). The inclusion of this item in the composite definitions of SR MMD, that is, SR-1/4MILE-STAIRS and MMD SR-BLOCKS-STAIRS, led to marginal change in sensitivity and specificity. Table 1. Sensitivity and Specificity (95% confidence intervals) for Prevalence of SR MMD Using Objective MMD as the Gold Standard: The LIFE Study   6 mo  12 mo  24 mo  Overalla  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.40 (0.30, 0.49)  0.48 (0.40, 0.56)  0.45 (0.40, 0.51)  Specificity  0.95 (0.94, 0.97)  0.93 (0.92, 0.95)  0.93 (0.91, 0.94)  0.94 (0.93, 0.95)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.44 (0.34, 0.54)  0.43 (0.35, 0.51)  0.46 (0.41, 0.51)  Specificity  0.95 (0.93, 0.96)  0.92 (0.91, 0.94)  0.92 (0.90, 0.93)  0.92 (0.91, 0.93)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.15 (0.08, 0.21)  0.20 (0.14, 0.25)  0.20 (0.17, 0.25)  Specificity  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.96)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.43 (0.33, 0.53)  0.52 (0.44, 0.61)  0.51 (0.46, 0.57)  Specificity  0.93 (0.91, 0.94)  0.91 (0.89, 0.93)  0.91 (0.89, 0.93)  0.92 (0.90, 0.93)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.46 (0.36, 0.56)  0.48 (0.40, 0.57)  0.50 (0.45, 0.56)  Specificity  0.92 (0.91, 0.94)  0.90 (0.89, 0.92)  0.90 (0.88, 0.92)  0.90 (0.89, 0.91)    6 mo  12 mo  24 mo  Overalla  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.40 (0.30, 0.49)  0.48 (0.40, 0.56)  0.45 (0.40, 0.51)  Specificity  0.95 (0.94, 0.97)  0.93 (0.92, 0.95)  0.93 (0.91, 0.94)  0.94 (0.93, 0.95)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.44 (0.34, 0.54)  0.43 (0.35, 0.51)  0.46 (0.41, 0.51)  Specificity  0.95 (0.93, 0.96)  0.92 (0.91, 0.94)  0.92 (0.90, 0.93)  0.92 (0.91, 0.93)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.15 (0.08, 0.21)  0.20 (0.14, 0.25)  0.20 (0.17, 0.25)  Specificity  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.96)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.43 (0.33, 0.53)  0.52 (0.44, 0.61)  0.51 (0.46, 0.57)  Specificity  0.93 (0.91, 0.94)  0.91 (0.89, 0.93)  0.91 (0.89, 0.93)  0.92 (0.90, 0.93)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.46 (0.36, 0.56)  0.48 (0.40, 0.57)  0.50 (0.45, 0.56)  Specificity  0.92 (0.91, 0.94)  0.90 (0.89, 0.92)  0.90 (0.88, 0.92)  0.90 (0.89, 0.91)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. aOverall sensitivity and specificity were calculated using generalized estimating equations. View Large In contrast to the relatively small increase in cross-sectional sensitivity over time, the sensitivity for the cumulative incident events increased steadily over time, reaching 0.60 for SR-1/4MILE and SR-BLOCKS at the end of the study (Table 2). The corresponding specificity for the cumulative incident events decreased somewhat compared to the average cross-sectional specificity, valued at around 0.85 and 0.82 for SR-1/4MILE and SR-BLOCKS, respectively. For SR-STAIRS, the sensitivity remained relatively low (0.32) and the specificity remained relatively high (0.91). Similar to what we observed for prevalence, the addition of SR-STAIRS did not substantially affect the sensitivity and specificity for the cumulative incident events for the two composite definitions of SR MMD. Table 2. Sensitivity and Specificity (95% confidence intervals) for Cumulative Incident Eventsa of SR MMD Using Objective MMD as the Gold Standard: The LIFE Study   6 mo  12 mo  24 mo  Overall  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.49 (0.41, 0.57)  0.56 (0.50, 0.63)  0.60 (0.55, 0.65)  Specificity  0.95 (0.94, 0.97)  0.91 (0.89, 0.93)  0.86 (0.84, 0.88)  0.85 (0.82, 0.87)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.45 (0.37, 0.54)  0.53 (0.47, 0.60)  0.61 (0.56, 0.66)  Specificity  0.95 (0.93, 0.96)  0.88 (0.87, 0.90)  0.84 (0.82, 0.86)  0.82 (0.80, 0.84)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.18 (0.13, 0.24)  0.26 (0.21, 0.31)  0.32 (0.27, 0.37)  Specificity  0.96 (0.95, 0.97)  0.94 (0.92, 0.95)  0.91 (0.90, 0.93)  0.91 (0.89, 0.92)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.54 (0.45, 0.63)  0.59 (0.52, 0.66)  0.64 (0.59, 0.70)  Specificity  0.93 (0.91, 0.94)  0.88 (0.86, 0.90)  0.82 (0.80, 0.85)  0.81 (0.78, 0.83)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.48 (0.39, 0.56)  0.56 (0.49, 0.63)  0.65 (0.59, 0.70)  Specificity  0.92 (0.91, 0.94)  0.85 (0.83, 0.87)  0.80 (0.77, 0.82)  0.78 (0.75, 0.80)    6 mo  12 mo  24 mo  Overall  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.49 (0.41, 0.57)  0.56 (0.50, 0.63)  0.60 (0.55, 0.65)  Specificity  0.95 (0.94, 0.97)  0.91 (0.89, 0.93)  0.86 (0.84, 0.88)  0.85 (0.82, 0.87)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.45 (0.37, 0.54)  0.53 (0.47, 0.60)  0.61 (0.56, 0.66)  Specificity  0.95 (0.93, 0.96)  0.88 (0.87, 0.90)  0.84 (0.82, 0.86)  0.82 (0.80, 0.84)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.18 (0.13, 0.24)  0.26 (0.21, 0.31)  0.32 (0.27, 0.37)  Specificity  0.96 (0.95, 0.97)  0.94 (0.92, 0.95)  0.91 (0.90, 0.93)  0.91 (0.89, 0.92)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.54 (0.45, 0.63)  0.59 (0.52, 0.66)  0.64 (0.59, 0.70)  Specificity  0.93 (0.91, 0.94)  0.88 (0.86, 0.90)  0.82 (0.80, 0.85)  0.81 (0.78, 0.83)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.48 (0.39, 0.56)  0.56 (0.49, 0.63)  0.65 (0.59, 0.70)  Specificity  0.92 (0.91, 0.94)  0.85 (0.83, 0.87)  0.80 (0.77, 0.82)  0.78 (0.75, 0.80)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. aEvents occurring at prior visits were carried forward into the analyses of subsequent visits to compute cumulative incident events. View Large Table 3 presents the annual transition probabilities to move in and out of a MMD state. For objective MMD, there was approximately a 90% probability of staying MMD free and a 10% probability of developing MMD in 1 year. These probabilities were similar for SR-1/4MILE and SR-BLOCKS. For SR-STAIRS, the probability of staying SR MMD free is as high as 95%. However, for objective MMD, the probability of recovering from a previous MMD was only 22% in a year while this percentage was close to 50% for SR-1/4MILE and SR-BLOCKS and 60% for SR-STAIRS, respectively. The transition probabilities for SR MMD with and without the component of climbing one flight of stairs did not differ substantially. Table 3. Annual Transition Probabilities (95% confidence intervals) for Objective MMD and SR MMD: The LIFE Study   Next Visit  Current Visit  No  Yes    Objective MMD  No  0.89 (0.88,0.91)  0.11 (0.09,0.12)  Yes  0.22 (0.17,0.29)  0.78 (0.71,0.83)    SR-1/4MILE  No  0.90 (0.88,0.91)  0.10 (0.09,0.12)  Yes  0.51 (0.43,0.59)  0.49 (0.41,0.57)    SR-BLOCKS  No  0.89 (0.87,0.90)  0.11 (0.10,0.13)  Yes  0.50 (0.42,0.58)  0.50 (0.42,0.58)    SR-STAIRS  No  0.95 (0.93,0.96)  0.05 (0.04,0.07)  Yes  0.62 (0.51,0.71)  0.38 (0.29,0.49)    SR-1/4MILE-STAIRS  No  0.88 (0.86,0.90)  0.12 (0.10,0.14)  Yes  0.47 (0.40,0.55)  0.53 (0.45,0.60)    SR-BLOCKS-STAIRS  No  0.87 (0.85,0.89)  0.13 (0.11,0.15)  Yes  0.46 (0.39,0.54)  0.54 (0.46,0.61)    Next Visit  Current Visit  No  Yes    Objective MMD  No  0.89 (0.88,0.91)  0.11 (0.09,0.12)  Yes  0.22 (0.17,0.29)  0.78 (0.71,0.83)    SR-1/4MILE  No  0.90 (0.88,0.91)  0.10 (0.09,0.12)  Yes  0.51 (0.43,0.59)  0.49 (0.41,0.57)    SR-BLOCKS  No  0.89 (0.87,0.90)  0.11 (0.10,0.13)  Yes  0.50 (0.42,0.58)  0.50 (0.42,0.58)    SR-STAIRS  No  0.95 (0.93,0.96)  0.05 (0.04,0.07)  Yes  0.62 (0.51,0.71)  0.38 (0.29,0.49)    SR-1/4MILE-STAIRS  No  0.88 (0.86,0.90)  0.12 (0.10,0.14)  Yes  0.47 (0.40,0.55)  0.53 (0.45,0.60)    SR-BLOCKS-STAIRS  No  0.87 (0.85,0.89)  0.13 (0.11,0.15)  Yes  0.46 (0.39,0.54)  0.54 (0.46,0.61)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. View Large Finally, we compared the onset of each SR MMD index with that of objective MMD. Figure 1 illustrates that the development of MMD using either SR-1/4MILE or SR-BLOCKS had a similar trajectory over time to that of objective MMD. The three Kaplan–Meier curves overlap substantially. Therefore, the overall incident rates were also similar for objective MMD, and MMD determined via either SR-1/4MILE or SR-BLOCKS (Table 4). The respective incident events (rates per 100 person years) were 439 (12.6%), 356 (11.8%), and 379 (13.2%). On the other hand, MMD defined by SR-STAIRS alone yielded higher survival probabilities over time compared to objective MMD. As a result, the corresponding incident rate for MMD of SR-STAIRS was only approximately half that defined by 400 MWT, with a total of 227 (6.7%) incident events. Both SR-1/4MILE-STAIRS and SR-BLOCKS-STAIRS produced slightly smaller survival probabilities over time and higher overall incident rates compared to objective MMD, with a total of 379 (13.9%) and 404 (15.3%) incident events for SR-1/4MILE-STAIRS and SR-BLOCKS-STAIRS, respectively. Figure 1. View largeDownload slide Onset of objective and self-reported MMD. MMD = Major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. Figure 1. View largeDownload slide Onset of objective and self-reported MMD. MMD = Major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. Table 4. Incident Event Rate per 100 Person Years (95% confidence intervals) for Objective MMD and SR MMD over the Course of the LIFE study   Event  n  Event Rate  Objective MMD  439  1,594  12.6 (11.5, 13.9)  SR-1/4MILE  356  1,368  11.8 (10.7, 13.1)  SR-BLOCKS  379  1,331  13.2 (11.9, 14.5)  SR-STAIRS  227  1,475  6.7 (5.9, 7.7)  SR-1/4MILE-STAIRS  379  1,281  13.9 (12.6, 15.4)  SR-BLOCKS-STAIRS  404  1,257  15.3 (13.9, 16.9)    Event  n  Event Rate  Objective MMD  439  1,594  12.6 (11.5, 13.9)  SR-1/4MILE  356  1,368  11.8 (10.7, 13.1)  SR-BLOCKS  379  1,331  13.2 (11.9, 14.5)  SR-STAIRS  227  1,475  6.7 (5.9, 7.7)  SR-1/4MILE-STAIRS  379  1,281  13.9 (12.6, 15.4)  SR-BLOCKS-STAIRS  404  1,257  15.3 (13.9, 16.9)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. View Large Discussion Our results demonstrate that the two individual items, SR MMD defined by the ability to walk a quarter mile (SR-1/4MILE) and the ability to walk several blocks (SR-BLOCKS), had differing levels of comparability depending on how the endpoint was defined with respect to agreement with the objective MMD defined by failure on the 400 MWT. When the focus was on prevalence of MMD, both SR-1/4MILE and SR-BLOCKS had relatively low sensitivity but overall high specificity. The specificity remained greater than 0.9 throughout the follow-up period. High specificity represents the high true negative rate and low false-positive rate of these two subjective measures of MMD. This is an important feature since the primary outcome of interest is the identification of objective MMD. A low false-positive rate implies that the participants classified as having MMD via SR-1/4MILE or SR-BLOCKS are likely to fail the 400 MWT. Additionally, since older adults are speculating about their ability to perform a task when asked, the low sensitivity and high specificity generally indicate that people are much more likely to know what they cannot do than what they can do. In a cross-sectional study conducted by Sayers and colleagues in 150 ambulatory and community-dwelling adults aged 75–90 years (5), a three-item self-report measure was developed from an 18-item instrument and one of the items was related to the 400-m walk with a binary response to “Do you think you could walk a quarter of a mile now without sitting down to rest?” They reported sensitivity and specificity of similar magnitude to the current study. When the focus is on the incidence of MMD, we observed a gradual increase in sensitivity and slight decrease in specificity over time for both SR-1/4MILE and SR-BLOCKS. With a median follow-up of 2.5 years, the sensitivity and specificity reached around 0.6 and 0.8, respectively. The increasing sensitivity over time may indicate a learning effect. This also implies that the length of follow-up needs to be planned carefully when designing a study with SR MMD as the outcome. Given the relatively low sensitivity observed before 24 months in the LIFE study, a longer duration might be preferred to ensure that a self-report index provides a good balance between sensitivity and specificity as a proxy for failure with a 400 MWT. Mobility disability is dynamic in nature, with older adults transitioning in and out of MMD status over time (6). Our study shows that there were both similarities as well as some differences for the transition patterns between the objective and self-reported MMD measures. Given a participant that was MMD free, the annual probability of staying MMD free, or equivalently for developing MMD, was similar for objective MMD, SR-1/4MILE, and SR-BLOCKS. This similarity was also reflected in the comparison of incident MMD. The two SR MMD indices not only had similar event rates as the objective MMD, but also captured the trajectory of survival for objective MMD as indicated by the overlapping Kaplan–Meier curves. Therefore, they appear to be excellent candidates to define MMD if the primary outcome of interest is incident MMD. However, if a participant had already developed MMD, the transition patterns were substantially different for SR-1/4MILE or SR-BLOCKS than for objective MMD. When participants had already developed objective MMD, there was a low probability (around 22%) that they would be able to successfully complete the 400 MWT at the next annual visit. In contrast, for participants who self-reported a lot of difficulty or the loss of ability to complete a long-distance walk—SR-1/4MILE or SR-BLOCKS—there was a 50% chance that they would perceive some degree of recovery of such ability at the next annual visit. These findings suggest that neither of these two SR MMD indices should be used as a proxy for 400 MWT if the primary focus is on the pattern of change in MMD over time. Finally, we note that SR MMD defined by ability to climb one flight of stairs alone (SR-STAIRS) has high specificity (>90%), but generally does not agree well with objective MMD. This is not surprising since ability to walk a long distance and ability to climb stairs measure different functional aspect of mobility. Additionally, incorporating the item SR-STAIRS with either SR-1/4MILE or SR-BLOCKS did not significantly improve the agreement between SR MMD and objective MMD. It is important to note that although this study examined the use of SR MMD as a proxy for objectively defined MMD—failure on the 400 MWT—we recognize that there are important differences between the two (20). For example, a 400 MWT is performed in a supervised, controlled environment where there are no competing demands such as interference from other people or the need to negotiate traffic/natural barriers such as curbs and uneven terrain. Thus, a 400 MWT is less ecologically valid than an assessment of one’s capacities in the context of real world environments. Since it is not practical to conduct performance tests in the highly variable real world settings encountered by older adults, measurement of disability by self-report does have utility as an outcome measure when the interest is incident MMD. It is interesting that both measures of SR MMD (SR-1/4MILE and SR-BLOCKS) had a poor relationship with actual recovery across time from MMD as defined by the 400 MWT; as mentioned above, 22% of participants recovered from MMD when using the 400 MWT whereas this value was 50% for both SR-1/4MILE and SR-BLOCKS. It may well be that following MMD, participants are particular sensitive to any improvement in their mobility, an attentional bias that results in overestimating their capacity to actually walk a quarter mile or negotiate several blocks. The precise explanations for this divergence merit further inquiry. Another difference between self-reported and objective MMD is that the 400 MWT has strengths and values beyond just ability to complete within 15 minutes. Time to complete 400 m is very informative in terms of cardiovascular health/endurance and is a validated measure of cardiorespiratory fitness (21). The split times between the laps may also be informative. Finally, observing change in 400 MWT over time provides higher discriminatory power than just ability to complete or self-report MMD. We note that in practice, a study that aims to assess incident SR MMD will include a group of participants who self-report not having a lot of difficulty walking a long distance but may actually fail the objective 400 MWT. However, the LIFE study only recruited participants who successfully completed the 400 MWT at baseline. Therefore, this analysis has limitations given our analytic sample for incident SR MMD was free initially of both objective and self-reported MMD. However, we estimate that the proportion of older adults who have objective MMD but are free of SR MMD is small. For instance, in a cross-sectional sample of participants who represented all possible combinations of objective and self-reported MMD status, the majority (95%) of older adults who reported having no difficulty walking a mile were also able to complete the 400 MWT (5). Thus, we believe that our results still provide some valuable guidance in conducting research on self-reported mobility disability in older populations. The LIFE study is unique in that it used a maximum of 15 minutes to complete the 400-m walk as an operational definition for objective MMD. Researchers could certainly choose other definitions as, for example, is true of how researchers operationalize frailty. Cummings and colleagues recommended using 10 minutes as a cutoff in 400 MWT to define dismobility (22). Therefore, one limitation of this analysis is that participants who completed the 400 MWT between 10 and 15 minutes in LIFE could arguably be considered to have MMD based on slow gait speed. In conclusion, using longitudinal data from a group of older adults at risk of mobility disability at baseline, we found that two of three simple self-reported items on mobility had utility when compared to the objective MMD measure. When individually considered, each had good discriminant properties compared to the objective MMD measure for cumulative incident events and provided a good approximation for the temporal development of incident objective MMD. Therefore, they may constitute a reasonable substitute for the objective MMD measure in large-scale observational studies and implementation trials when the objective assessment of 400-m walk is not practical. Furthermore, the two simple SR MMD indices and the objective MMD measure may provide complementary information on the dynamic transition of functional status and warrant further research. Supplementary Material Supplementary data is available at The Journals of Gerontology, Series A: Biological Sciences and Medical Sciences online. Funding A detailed list of research investigators for the LIFE Study and funding information can be found in Appendix. Acknowledgments The authors thank the anonymous reviewers for their constructive comments that led to an improved presentation of results. Conflict of Interest None reported. References 1. Chang M, Cohen-Mansfield J, Ferrucci Let al.   Incidence of loss of ability to walk 400 meters in a functionally limited older population. J Am Geriatr Soc . 2004; 52: 2094– 2098. doi: 10.1111/j.1532-5415.2004.52570.x Google Scholar CrossRef Search ADS PubMed  2. Pahor M, Blair SN, Espeland Met al.   Effects of a physical activity intervention on measures of physical performance: results of the lifestyle interventions and independence for Elders Pilot (LIFE-P) study. J Gerontol A Biol Sci Med Sci  2006; 61: 1157– 1165. doi:10.1093/gerona/61.11.1157 Google Scholar CrossRef Search ADS PubMed  3. Pahor M, Guralnik JM, Ambrosius WTet al.  ; LIFE study investigators. Effect of structured physical activity on prevention of major mobility disability in older adults: the LIFE study randomized clinical trial. JAMA . 2014; 311: 2387– 2396. doi: 10.1001/jama.2014.5616 Google Scholar CrossRef Search ADS PubMed  4. Simonsick EM, Newman AB, Nevitt MCet al.  ; Health ABC Study Group. Measuring higher level physical function in well-functioning older adults: expanding familiar approaches in the Health ABC study. J Gerontol A Biol Sci Med Sci . 2001; 56: M644– M649. doi:10.1093/gerona/56.10.M644 Google Scholar CrossRef Search ADS PubMed  5. Sayers SP, Brach JS, Newman AB, Heeren TC, Guralnik JM, Fielding RA. Use of self-report to predict ability to walk 400 meters in mobility-limited older adults. J Am Geriatr Soc . 2004; 52: 2099– 2103. doi: 10.1111/j.1532-5415.2004.52571.x Google Scholar CrossRef Search ADS PubMed  6. Gill TM, Allore HG, Hardy SE, Guo Z. The dynamic nature of mobility disability in older persons. J Am Geriatr Soc . 2006; 54: 248– 254. doi: 10.1111/j.1532-5415.2005.00586.x Google Scholar CrossRef Search ADS PubMed  7. Perera S, Mody SH, Woodman RC, Studenski SA. Meaningful change and responsiveness in common physical performance measures in older adults. J Am Geriatr Soc . 2006; 54: 743– 749. doi: 10.1111/j.1532-5415.2006.00701.x Google Scholar CrossRef Search ADS PubMed  8. Kwon S, Perera S, Pahor Met al.   What is a meaningful change in physical performance? Findings from a clinical trial in older adults (the LIFE-P study). J Nutr Health Aging . 2009; 13: 538– 544. doi:10.1007/s12603-009-0104-z Google Scholar CrossRef Search ADS PubMed  9. Hardy SE, McGurl DJ, Studenski SA, Degenholtz HB. Biopsychosocial characteristics of community-dwelling older adults with limited ability to walk one-quarter of a mile. J Am Geriatr Soc . 2010; 58: 539– 544. doi: 10.1111/j.1532-5415.2010.02727.x Google Scholar CrossRef Search ADS PubMed  10. Hardy SE, Kang Y, Studenski SA, Degenholtz HB. Ability to walk ¼ mile predicts subsequent disability, mortality, and health care costs. J Gen Intern Med . 2011; 26: 130– 135. doi: 10.1007/s11606-010-1543-2 Google Scholar CrossRef Search ADS PubMed  11. Guralnik JM, Ferrucci L, Pieper CFet al.   Lower extremity function and subsequent disability: consistency across studies, predictive models, and value of gait speed alone compared with the short physical performance battery. J Gerontol A Biol Sci Med Sci . 2000; 55: M221– M231. Google Scholar CrossRef Search ADS PubMed  12. Reuben DB, Seeman TE, Keeler Eet al.   Refining the categorization of physical functional status: the added value of combining self-reported and performance-based measures. J Gerontol A Biol Sci Med Sci . 2004; 59: 1056– 1061. doi:10.1093/gerona/59.10.M1056 Google Scholar CrossRef Search ADS PubMed  13. Cesari M, Kritchevsky SB, Penninx BWet al.   Prognostic value of usual gait speed in well-functioning older people–results from the Health, Aging and Body Composition Study. J Am Geriatr Soc . 2005; 53: 1675– 1680. doi: 10.1111/j.1532-5415.2005.53501.x Google Scholar CrossRef Search ADS PubMed  14. McDermott MM, Guralnik JM, Tian Let al.   Baseline functional performance predicts the rate of mobility loss in persons with peripheral arterial disease. J Am Coll Cardiol . 2007; 50: 974– 982. doi: 10.1016/j.jacc.2007.05.030 Google Scholar CrossRef Search ADS PubMed  15. Simonsick EM, Newman AB, Visser Met al.  ; Health, Aging and Body Composition Study. Mobility limitation in self-described well-functioning older adults: importance of endurance walk testing. J Gerontol A Biol Sci Med Sci . 2008; 63: 841– 847. Google Scholar CrossRef Search ADS PubMed  16. McDermott MM, Guralnik JM, Criqui MHet al.   Unsupervised exercise and mobility loss in peripheral artery disease: a randomized controlled trial. J Am Heart Assoc  2015; 4: e001659. doi:10.1161/JAHA.114.001659 Google Scholar CrossRef Search ADS PubMed  17. Fielding RA, Rejeski WJ, Blair Set al.  ; LIFE Research Group. The Lifestyle Interventions and Independence for Elders Study: design and methods. J Gerontol A Biol Sci Med Sci . 2011; 66: 1226– 1237. doi: 10.1093/gerona/glr123 Google Scholar CrossRef Search ADS PubMed  18. Marsh AP, Lovato LC, Glynn NWet al.  ; LIFE Study Research Group. Lifestyle interventions and independence for elders study: recruitment and baseline characteristics. J Gerontol A Biol Sci Med Sci . 2013; 68: 1549– 1558. doi: 10.1093/gerona/glt064 Google Scholar CrossRef Search ADS PubMed  19. Newman AB, Simonsick EM, Naydeck BLet al.   Association of long-distance corridor walk performance with mortality, cardiovascular disease, mobility limitation, and disability. JAMA . 2006; 295: 2018– 2026. doi: 10.1001/jama.295.17.2018 Google Scholar CrossRef Search ADS PubMed  20. Rejeski WJ, Ip EH, Marsh AP, Barnard RT. Development and validation of a video-animated tool for assessing mobility. J Gerontol A Biol Sci Med Sci . 2010; 65: 664– 671. doi: 10.1093/gerona/glq055 Google Scholar CrossRef Search ADS PubMed  21. Simonsick EM, Fan E, Fleg JL. Estimating cardiorespiratory fitness in well-functioning older adults: treadmill validation of the long distance corridor walk. J Am Geriatr Soc . 2006; 54: 127– 132. doi: 10.1111/j.1532-5415.2005.00530.x Google Scholar CrossRef Search ADS PubMed  22. Cummings SR, Studenski S, Ferrucci L. A diagnosis of dismobility–giving mobility clinical visibility: a Mobility Working Group recommendation. JAMA . 2014; 311: 2061– 2062. doi: 10.1001/jama.2014.3033 Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2017. 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journals of Gerontology Series A: Biomedical Sciences and Medical Sciences Oxford University Press

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Abstract

Abstract Background The objective assessment of major mobility disability (objective MMD) by a 400-m walk test (400 MWT) is important but not always practical. Previous research on the relationship between self-reported MMD (SR MMD) and objective MMD is sparse and limited to cross-sectional data. Methods We evaluated agreement between SR MMD and objective MMD using longitudinal data from the Lifestyle Interventions for Elders (LIFE) study. The SR MMD indices were defined based on having a lot of difficulty or inability to walk a quarter of a mile (SR-1/4MILE), walk several blocks (SR-BLOCKS), and climb one flight of stairs (SR-STAIRS). Results Using objective MMD as the gold standard, SR-1/4MILE and SR-BLOCKS had relatively low sensitivity (around 0.4) and high specificity (around 0.9) for prevalence. Their overall sensitivity and specificity for cumulative incident objective MMD were approximately 0.6 and 0.8, respectively. While the annual probability of staying MMD free was similar for objective MMD, SR-1/4MILE, and SR-BLOCKS (90% for all), the probability of recovering from SR MMD was higher (50%) than that of objective MMD (22%). The development of objective MMD (439 events), SR-1/4MILE (356 events), and SR-BLOCKS (379 events) had a similar trajectory over time with substantially overlapping survival curves. SR-STAIRS generally did not agree well with objective MMD. Incorporating SR-STAIRS with either SR-1/4MILE or SR-BLOCKS did not significantly improve the agreement between SR MMD and objective MMD. Conclusions Simple SR-1/4MILE and SR-BLOCKS are reasonable candidates to define MMD if the primary outcome of interest is incident MMD. Agreement, Cumulative incidence, Sensitivity, Specificity, Transition probability Preserving the ability to walk 400 m is an excellent proxy for mobility and the loss of this capacity has been used to define major mobility disability (MMD) in geriatric research (1–3). However, the assessment of this outcome requires a walking track or other suitable venue, in-person contact, and supervision. This is neither feasible nor pragmatic for use in large implementation trials or clinical practice. Previous research on the relationship between self-reported and objective assessments of MMD is sparse and limited to cross-sectional data (4,5). The Lifestyle Interventions for Elders (LIFE) study collected concurrent longitudinal data on both self-reported measures of MMD and objective failure to complete the 400-m walk (3). This provided us with a unique opportunity to comprehensively evaluate agreement between subjective and objective assessments of MMD across a period of 36 months. MMD in the LIFE study was defined as failure to complete a 400-m walk test (400 MWT) in ≤15 minutes without assistance. We refer to this as objective MMD and used it as the gold standard to compare with several indices of self-reported MMD (SR MMD). The SR MMD was defined based on self-reported ability to walk a quarter mile, walk several blocks, or climb one flight of stairs. These self-report indices implicitly address the ability to remain mobile in the community, which is what the 400 MWT is meant to address. In the literature, researchers have frequently used walking-based and stair climb-based self-report items separately (6–10) as well as jointly (11–16) to define mobility disability. Therefore, we examined the three items separately and we created composite SR MMD indices as well for ease of comparison with previous work. The current analysis examined several scientific questions related to SR MMD. First, what is the sensitivity and specificity of the SR MMD indices using objective MMD as the gold standard? Second, how would the pattern of change over time for the SR MMD indices compare with objective MMD? Finally, how would the cumulative incidence rates across time for the SR MMD indices compare with objective MMD? Methods Data Source The LIFE study is a multicenter, single-blind randomized trial that compared the effects of a physical activity intervention to a health education control program on (objective) MMD. It was designed to target adults aged 70–89 years who were sedentary, were at risk for mobility disability, and yet were able to walk 400 m in ≤15 minutes without sitting, using a walker, or needing the help of another person. A total of 1,635 participants were randomized to either physical activity or health education. Details regarding the study have been published elsewhere (3,17,18). Briefly, the physical activity intervention involved moderate intensity walking, with a goal of 150 minutes/wk, accompanied by strength, flexibility, and balance training (17). The intervention included both center-based visits and home-based activity for the duration of the study. Participants in the health education group attended workshops on topics in health education other than physical activity that were relevant to older adults. The study protocol was approved by the institutional review boards at all participating sites. Written informed consent was obtained from all study participants. Measures Objective MMD The 400 MWT was assessed every 6 months at clinic visits. This test is a modified version of a walking test originally described by Newman and colleagues (19). Participants were instructed to walk at their usual pace for 400 m (10 laps of a 20-m course defined by two cones). The maximum time allowed for the test was 15 minutes without sitting and without the help of a walker or another person; participants were allowed to stop and stand to rest for up to 1 minute and could use a cane, but they were not allowed to lean against any object to support their weight. When MMD could not be objectively measured because of the inability of the participant to come to the clinic and absence of a suitable walking course at the participant’s home, institution, or hospital, the outcome was adjudicated based on objective inability to walk 4 m in ≤10 seconds, or self-, proxy-, or medical record-reported inability to walk across a room. It would not be feasible for a participant who met any of these latter criteria to complete the 400 MWT within 15 minutes. For the purpose of this analysis, we excluded objective MMD events determined on the basis of self- or proxy-reported responses. Self-reported MMD (SR MMD) The assessment of SR MMD, which was completed at baseline, months 6, 12, 24, 36, and study closeout visits, was achieved using three questions from a disability questionnaire (8): (i) “During the past month, how much difficulty have you had walking a quarter of a mile, which is about 3 or 4 blocks, because of your health?” (SR-1/4MILE), (ii) “During the past month, how much difficulty have you had walking several blocks because of your health?” (SR-BLOCKS), and (iii) “During the past month, how much difficulty have you had climbing one flight of stairs because of your health?” (SR-STAIRS). The possible responses were “no difficulty”, “a little difficulty”, “some difficulty”, “a lot of difficulty”, “unable to do”, “did not do for other reasons”, or “don’t know/refused”. The responses “did not do for other reasons” or “don’t know/refused” were excluded from analyses. For each question, the two response categories “a lot of difficulty” and “unable to do” were combined to define participants as having SR MMD (13,15). Additionally, we created two composite SR MMD indices, SR-1/4MILE-STAIRS and SR-BLOCKS-STAIRS. Specifically, SR-1/4MILE-STAIRS was defined based on development of either SR-1/4MILES or SR-STAIRS disability. The other combination, SR-BLOCKS-STAIRS, was defined in a similar fashion. Statistical Analyses Data on the 400 MWT performed at assessment visits occurring at months 6, 12, 24, 36, and study closeout visits were included in all analyses to align comparisons of self-report and objective measures collected at the same assessment visit. Using objective MMD as the gold standard, sensitivity and specificity for each SR MMD index were assessed in two different scenarios. First, we computed sensitivity and specificity for the prevalence of each SR MMD index against that of objective MMD. That is, sensitivity and specificity were computed for each follow-up visit separately using contingency tables. These served as cross-sectional examinations of the performance of the SR MMD indices. The average cross-sectional sensitivity and specificity throughout the course of the study were calculated using generalized estimating equations (GEE) to account for the correlations among repeated measures. An independence working correlation structure was chosen for all SR MMD indices based on QIC values. The 95% confidence intervals (CI) were reported. Second, we evaluated the sensitivity and specificity for the incident SR MMD compared to the incident objective MMD at each follow-up visit and overall. Specifically, MMD events occurring at prior visits were carried forward into the analyses of subsequent visits so that cumulative incident events were examined. For example, participants who developed MMD at month 6 were treated analytically as incident events thereafter regardless of whether they recovered from MMD subsequently. We note that this approach of evaluating cumulative incidence has the limitation of ignoring the timing of the MMD events that may have occurred at different time points. Next, to describe the pattern of change over time for SR MMD indices and the objective MMD, we used data from annual visits up to month 36 to fit first-order Markov transition models. Annual probabilities that participants transition in and out of a MMD state were estimated and presented in transition matrices. Finally, the onset of SR MMD and objective MMD were graphically compared using Kaplan–Meier curves and the incident rates were estimated using Poisson regression models. All analyses were pooled across treatment groups and were conducted using SAS 9.4 (Cary, NC). Results Although all participants were able to complete the 400 MWT successfully at baseline, 111 (7.4%), 137 (9.1%), and 72 (4.6%) reported having a lot of difficulty or inability to “walk a quarter mile”, “walk several blocks”, and “climb a flight of stairs” at baseline, respectively (Supplementary Table 2). Next, for each SR MMD index, we assessed its agreement with objective MMD among the participants who were free of SR MMD at baseline. Table 1 summarizes the cross-sectional sensitivity and specificity for the prevalence of SR MMD using the prevalence of objective MMD assessed at the same follow-ups as a gold standard. It indicates that the sensitivity for SR-1/4MILE was relatively low, ranging from 0.41 for month 6 to 0.48 for month 24, with an average of 0.40 (95% CI: 0.35–0.45) across the course of the study; the specificity was very high, ranging from 0.95 at month 6 to 0.93 at month 24, with an average of 0.93 (95% CI: 0.92–0.94). Similar patterns were observed for SR-BLOCKS. The overall cross-sectional sensitivity and specificity were 0.42 (95% CI: 0.37–0.47) and 0.92 (95% CI: 0.91–0.93) for SR-1/4MILE and SR-BLOCKS, respectively. Using SR-STAIRS alone to define MMD yielded very low sensitivity (an average of 0.2) and markedly high specificity (an average of 0.96). The inclusion of this item in the composite definitions of SR MMD, that is, SR-1/4MILE-STAIRS and MMD SR-BLOCKS-STAIRS, led to marginal change in sensitivity and specificity. Table 1. Sensitivity and Specificity (95% confidence intervals) for Prevalence of SR MMD Using Objective MMD as the Gold Standard: The LIFE Study   6 mo  12 mo  24 mo  Overalla  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.40 (0.30, 0.49)  0.48 (0.40, 0.56)  0.45 (0.40, 0.51)  Specificity  0.95 (0.94, 0.97)  0.93 (0.92, 0.95)  0.93 (0.91, 0.94)  0.94 (0.93, 0.95)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.44 (0.34, 0.54)  0.43 (0.35, 0.51)  0.46 (0.41, 0.51)  Specificity  0.95 (0.93, 0.96)  0.92 (0.91, 0.94)  0.92 (0.90, 0.93)  0.92 (0.91, 0.93)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.15 (0.08, 0.21)  0.20 (0.14, 0.25)  0.20 (0.17, 0.25)  Specificity  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.96)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.43 (0.33, 0.53)  0.52 (0.44, 0.61)  0.51 (0.46, 0.57)  Specificity  0.93 (0.91, 0.94)  0.91 (0.89, 0.93)  0.91 (0.89, 0.93)  0.92 (0.90, 0.93)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.46 (0.36, 0.56)  0.48 (0.40, 0.57)  0.50 (0.45, 0.56)  Specificity  0.92 (0.91, 0.94)  0.90 (0.89, 0.92)  0.90 (0.88, 0.92)  0.90 (0.89, 0.91)    6 mo  12 mo  24 mo  Overalla  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.40 (0.30, 0.49)  0.48 (0.40, 0.56)  0.45 (0.40, 0.51)  Specificity  0.95 (0.94, 0.97)  0.93 (0.92, 0.95)  0.93 (0.91, 0.94)  0.94 (0.93, 0.95)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.44 (0.34, 0.54)  0.43 (0.35, 0.51)  0.46 (0.41, 0.51)  Specificity  0.95 (0.93, 0.96)  0.92 (0.91, 0.94)  0.92 (0.90, 0.93)  0.92 (0.91, 0.93)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.15 (0.08, 0.21)  0.20 (0.14, 0.25)  0.20 (0.17, 0.25)  Specificity  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.97)  0.96 (0.95, 0.96)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.43 (0.33, 0.53)  0.52 (0.44, 0.61)  0.51 (0.46, 0.57)  Specificity  0.93 (0.91, 0.94)  0.91 (0.89, 0.93)  0.91 (0.89, 0.93)  0.92 (0.90, 0.93)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.46 (0.36, 0.56)  0.48 (0.40, 0.57)  0.50 (0.45, 0.56)  Specificity  0.92 (0.91, 0.94)  0.90 (0.89, 0.92)  0.90 (0.88, 0.92)  0.90 (0.89, 0.91)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. aOverall sensitivity and specificity were calculated using generalized estimating equations. View Large In contrast to the relatively small increase in cross-sectional sensitivity over time, the sensitivity for the cumulative incident events increased steadily over time, reaching 0.60 for SR-1/4MILE and SR-BLOCKS at the end of the study (Table 2). The corresponding specificity for the cumulative incident events decreased somewhat compared to the average cross-sectional specificity, valued at around 0.85 and 0.82 for SR-1/4MILE and SR-BLOCKS, respectively. For SR-STAIRS, the sensitivity remained relatively low (0.32) and the specificity remained relatively high (0.91). Similar to what we observed for prevalence, the addition of SR-STAIRS did not substantially affect the sensitivity and specificity for the cumulative incident events for the two composite definitions of SR MMD. Table 2. Sensitivity and Specificity (95% confidence intervals) for Cumulative Incident Eventsa of SR MMD Using Objective MMD as the Gold Standard: The LIFE Study   6 mo  12 mo  24 mo  Overall  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.49 (0.41, 0.57)  0.56 (0.50, 0.63)  0.60 (0.55, 0.65)  Specificity  0.95 (0.94, 0.97)  0.91 (0.89, 0.93)  0.86 (0.84, 0.88)  0.85 (0.82, 0.87)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.45 (0.37, 0.54)  0.53 (0.47, 0.60)  0.61 (0.56, 0.66)  Specificity  0.95 (0.93, 0.96)  0.88 (0.87, 0.90)  0.84 (0.82, 0.86)  0.82 (0.80, 0.84)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.18 (0.13, 0.24)  0.26 (0.21, 0.31)  0.32 (0.27, 0.37)  Specificity  0.96 (0.95, 0.97)  0.94 (0.92, 0.95)  0.91 (0.90, 0.93)  0.91 (0.89, 0.92)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.54 (0.45, 0.63)  0.59 (0.52, 0.66)  0.64 (0.59, 0.70)  Specificity  0.93 (0.91, 0.94)  0.88 (0.86, 0.90)  0.82 (0.80, 0.85)  0.81 (0.78, 0.83)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.48 (0.39, 0.56)  0.56 (0.49, 0.63)  0.65 (0.59, 0.70)  Specificity  0.92 (0.91, 0.94)  0.85 (0.83, 0.87)  0.80 (0.77, 0.82)  0.78 (0.75, 0.80)    6 mo  12 mo  24 mo  Overall  SR-1/4MILE  Sensitivity  0.41 (0.30, 0.53)  0.49 (0.41, 0.57)  0.56 (0.50, 0.63)  0.60 (0.55, 0.65)  Specificity  0.95 (0.94, 0.97)  0.91 (0.89, 0.93)  0.86 (0.84, 0.88)  0.85 (0.82, 0.87)  SR-BLOCKS  Sensitivity  0.40 (0.29, 0.52)  0.45 (0.37, 0.54)  0.53 (0.47, 0.60)  0.61 (0.56, 0.66)  Specificity  0.95 (0.93, 0.96)  0.88 (0.87, 0.90)  0.84 (0.82, 0.86)  0.82 (0.80, 0.84)  SR-STAIRS  Sensitivity  0.12 (0.05, 0.19)  0.18 (0.13, 0.24)  0.26 (0.21, 0.31)  0.32 (0.27, 0.37)  Specificity  0.96 (0.95, 0.97)  0.94 (0.92, 0.95)  0.91 (0.90, 0.93)  0.91 (0.89, 0.92)  SR-1/4MILE-STAIRS  Sensitivity  0.46 (0.34, 0.58)  0.54 (0.45, 0.63)  0.59 (0.52, 0.66)  0.64 (0.59, 0.70)  Specificity  0.93 (0.91, 0.94)  0.88 (0.86, 0.90)  0.82 (0.80, 0.85)  0.81 (0.78, 0.83)  SR-BLOCKS-STAIRS  Sensitivity  0.44 (0.31, 0.56)  0.48 (0.39, 0.56)  0.56 (0.49, 0.63)  0.65 (0.59, 0.70)  Specificity  0.92 (0.91, 0.94)  0.85 (0.83, 0.87)  0.80 (0.77, 0.82)  0.78 (0.75, 0.80)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. aEvents occurring at prior visits were carried forward into the analyses of subsequent visits to compute cumulative incident events. View Large Table 3 presents the annual transition probabilities to move in and out of a MMD state. For objective MMD, there was approximately a 90% probability of staying MMD free and a 10% probability of developing MMD in 1 year. These probabilities were similar for SR-1/4MILE and SR-BLOCKS. For SR-STAIRS, the probability of staying SR MMD free is as high as 95%. However, for objective MMD, the probability of recovering from a previous MMD was only 22% in a year while this percentage was close to 50% for SR-1/4MILE and SR-BLOCKS and 60% for SR-STAIRS, respectively. The transition probabilities for SR MMD with and without the component of climbing one flight of stairs did not differ substantially. Table 3. Annual Transition Probabilities (95% confidence intervals) for Objective MMD and SR MMD: The LIFE Study   Next Visit  Current Visit  No  Yes    Objective MMD  No  0.89 (0.88,0.91)  0.11 (0.09,0.12)  Yes  0.22 (0.17,0.29)  0.78 (0.71,0.83)    SR-1/4MILE  No  0.90 (0.88,0.91)  0.10 (0.09,0.12)  Yes  0.51 (0.43,0.59)  0.49 (0.41,0.57)    SR-BLOCKS  No  0.89 (0.87,0.90)  0.11 (0.10,0.13)  Yes  0.50 (0.42,0.58)  0.50 (0.42,0.58)    SR-STAIRS  No  0.95 (0.93,0.96)  0.05 (0.04,0.07)  Yes  0.62 (0.51,0.71)  0.38 (0.29,0.49)    SR-1/4MILE-STAIRS  No  0.88 (0.86,0.90)  0.12 (0.10,0.14)  Yes  0.47 (0.40,0.55)  0.53 (0.45,0.60)    SR-BLOCKS-STAIRS  No  0.87 (0.85,0.89)  0.13 (0.11,0.15)  Yes  0.46 (0.39,0.54)  0.54 (0.46,0.61)    Next Visit  Current Visit  No  Yes    Objective MMD  No  0.89 (0.88,0.91)  0.11 (0.09,0.12)  Yes  0.22 (0.17,0.29)  0.78 (0.71,0.83)    SR-1/4MILE  No  0.90 (0.88,0.91)  0.10 (0.09,0.12)  Yes  0.51 (0.43,0.59)  0.49 (0.41,0.57)    SR-BLOCKS  No  0.89 (0.87,0.90)  0.11 (0.10,0.13)  Yes  0.50 (0.42,0.58)  0.50 (0.42,0.58)    SR-STAIRS  No  0.95 (0.93,0.96)  0.05 (0.04,0.07)  Yes  0.62 (0.51,0.71)  0.38 (0.29,0.49)    SR-1/4MILE-STAIRS  No  0.88 (0.86,0.90)  0.12 (0.10,0.14)  Yes  0.47 (0.40,0.55)  0.53 (0.45,0.60)    SR-BLOCKS-STAIRS  No  0.87 (0.85,0.89)  0.13 (0.11,0.15)  Yes  0.46 (0.39,0.54)  0.54 (0.46,0.61)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. View Large Finally, we compared the onset of each SR MMD index with that of objective MMD. Figure 1 illustrates that the development of MMD using either SR-1/4MILE or SR-BLOCKS had a similar trajectory over time to that of objective MMD. The three Kaplan–Meier curves overlap substantially. Therefore, the overall incident rates were also similar for objective MMD, and MMD determined via either SR-1/4MILE or SR-BLOCKS (Table 4). The respective incident events (rates per 100 person years) were 439 (12.6%), 356 (11.8%), and 379 (13.2%). On the other hand, MMD defined by SR-STAIRS alone yielded higher survival probabilities over time compared to objective MMD. As a result, the corresponding incident rate for MMD of SR-STAIRS was only approximately half that defined by 400 MWT, with a total of 227 (6.7%) incident events. Both SR-1/4MILE-STAIRS and SR-BLOCKS-STAIRS produced slightly smaller survival probabilities over time and higher overall incident rates compared to objective MMD, with a total of 379 (13.9%) and 404 (15.3%) incident events for SR-1/4MILE-STAIRS and SR-BLOCKS-STAIRS, respectively. Figure 1. View largeDownload slide Onset of objective and self-reported MMD. MMD = Major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. Figure 1. View largeDownload slide Onset of objective and self-reported MMD. MMD = Major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. Table 4. Incident Event Rate per 100 Person Years (95% confidence intervals) for Objective MMD and SR MMD over the Course of the LIFE study   Event  n  Event Rate  Objective MMD  439  1,594  12.6 (11.5, 13.9)  SR-1/4MILE  356  1,368  11.8 (10.7, 13.1)  SR-BLOCKS  379  1,331  13.2 (11.9, 14.5)  SR-STAIRS  227  1,475  6.7 (5.9, 7.7)  SR-1/4MILE-STAIRS  379  1,281  13.9 (12.6, 15.4)  SR-BLOCKS-STAIRS  404  1,257  15.3 (13.9, 16.9)    Event  n  Event Rate  Objective MMD  439  1,594  12.6 (11.5, 13.9)  SR-1/4MILE  356  1,368  11.8 (10.7, 13.1)  SR-BLOCKS  379  1,331  13.2 (11.9, 14.5)  SR-STAIRS  227  1,475  6.7 (5.9, 7.7)  SR-1/4MILE-STAIRS  379  1,281  13.9 (12.6, 15.4)  SR-BLOCKS-STAIRS  404  1,257  15.3 (13.9, 16.9)  Note: LIFE = Lifestyle Interventions for Elders study; SR MMD = Self-reported major mobility disability. SR-1/4MILE was defined as having a lot of difficulty or inability to walk a quarter mile. SR-BLOCKS was defined as having a lot of difficulty or inability to walk several blocks. SR-STAIRS was defined as having a lot of difficulty or inability to climb one flight of stairs. SR-1/4MILE-STAIRS was defined as having a lot of difficulty or inability to walk a quarter mile or climb one flight of stairs. SR-BLOCKS-STAIRS was defined as having a lot of difficulty or inability to walk several blocks or climb one flight of stairs. View Large Discussion Our results demonstrate that the two individual items, SR MMD defined by the ability to walk a quarter mile (SR-1/4MILE) and the ability to walk several blocks (SR-BLOCKS), had differing levels of comparability depending on how the endpoint was defined with respect to agreement with the objective MMD defined by failure on the 400 MWT. When the focus was on prevalence of MMD, both SR-1/4MILE and SR-BLOCKS had relatively low sensitivity but overall high specificity. The specificity remained greater than 0.9 throughout the follow-up period. High specificity represents the high true negative rate and low false-positive rate of these two subjective measures of MMD. This is an important feature since the primary outcome of interest is the identification of objective MMD. A low false-positive rate implies that the participants classified as having MMD via SR-1/4MILE or SR-BLOCKS are likely to fail the 400 MWT. Additionally, since older adults are speculating about their ability to perform a task when asked, the low sensitivity and high specificity generally indicate that people are much more likely to know what they cannot do than what they can do. In a cross-sectional study conducted by Sayers and colleagues in 150 ambulatory and community-dwelling adults aged 75–90 years (5), a three-item self-report measure was developed from an 18-item instrument and one of the items was related to the 400-m walk with a binary response to “Do you think you could walk a quarter of a mile now without sitting down to rest?” They reported sensitivity and specificity of similar magnitude to the current study. When the focus is on the incidence of MMD, we observed a gradual increase in sensitivity and slight decrease in specificity over time for both SR-1/4MILE and SR-BLOCKS. With a median follow-up of 2.5 years, the sensitivity and specificity reached around 0.6 and 0.8, respectively. The increasing sensitivity over time may indicate a learning effect. This also implies that the length of follow-up needs to be planned carefully when designing a study with SR MMD as the outcome. Given the relatively low sensitivity observed before 24 months in the LIFE study, a longer duration might be preferred to ensure that a self-report index provides a good balance between sensitivity and specificity as a proxy for failure with a 400 MWT. Mobility disability is dynamic in nature, with older adults transitioning in and out of MMD status over time (6). Our study shows that there were both similarities as well as some differences for the transition patterns between the objective and self-reported MMD measures. Given a participant that was MMD free, the annual probability of staying MMD free, or equivalently for developing MMD, was similar for objective MMD, SR-1/4MILE, and SR-BLOCKS. This similarity was also reflected in the comparison of incident MMD. The two SR MMD indices not only had similar event rates as the objective MMD, but also captured the trajectory of survival for objective MMD as indicated by the overlapping Kaplan–Meier curves. Therefore, they appear to be excellent candidates to define MMD if the primary outcome of interest is incident MMD. However, if a participant had already developed MMD, the transition patterns were substantially different for SR-1/4MILE or SR-BLOCKS than for objective MMD. When participants had already developed objective MMD, there was a low probability (around 22%) that they would be able to successfully complete the 400 MWT at the next annual visit. In contrast, for participants who self-reported a lot of difficulty or the loss of ability to complete a long-distance walk—SR-1/4MILE or SR-BLOCKS—there was a 50% chance that they would perceive some degree of recovery of such ability at the next annual visit. These findings suggest that neither of these two SR MMD indices should be used as a proxy for 400 MWT if the primary focus is on the pattern of change in MMD over time. Finally, we note that SR MMD defined by ability to climb one flight of stairs alone (SR-STAIRS) has high specificity (>90%), but generally does not agree well with objective MMD. This is not surprising since ability to walk a long distance and ability to climb stairs measure different functional aspect of mobility. Additionally, incorporating the item SR-STAIRS with either SR-1/4MILE or SR-BLOCKS did not significantly improve the agreement between SR MMD and objective MMD. It is important to note that although this study examined the use of SR MMD as a proxy for objectively defined MMD—failure on the 400 MWT—we recognize that there are important differences between the two (20). For example, a 400 MWT is performed in a supervised, controlled environment where there are no competing demands such as interference from other people or the need to negotiate traffic/natural barriers such as curbs and uneven terrain. Thus, a 400 MWT is less ecologically valid than an assessment of one’s capacities in the context of real world environments. Since it is not practical to conduct performance tests in the highly variable real world settings encountered by older adults, measurement of disability by self-report does have utility as an outcome measure when the interest is incident MMD. It is interesting that both measures of SR MMD (SR-1/4MILE and SR-BLOCKS) had a poor relationship with actual recovery across time from MMD as defined by the 400 MWT; as mentioned above, 22% of participants recovered from MMD when using the 400 MWT whereas this value was 50% for both SR-1/4MILE and SR-BLOCKS. It may well be that following MMD, participants are particular sensitive to any improvement in their mobility, an attentional bias that results in overestimating their capacity to actually walk a quarter mile or negotiate several blocks. The precise explanations for this divergence merit further inquiry. Another difference between self-reported and objective MMD is that the 400 MWT has strengths and values beyond just ability to complete within 15 minutes. Time to complete 400 m is very informative in terms of cardiovascular health/endurance and is a validated measure of cardiorespiratory fitness (21). The split times between the laps may also be informative. Finally, observing change in 400 MWT over time provides higher discriminatory power than just ability to complete or self-report MMD. We note that in practice, a study that aims to assess incident SR MMD will include a group of participants who self-report not having a lot of difficulty walking a long distance but may actually fail the objective 400 MWT. However, the LIFE study only recruited participants who successfully completed the 400 MWT at baseline. Therefore, this analysis has limitations given our analytic sample for incident SR MMD was free initially of both objective and self-reported MMD. However, we estimate that the proportion of older adults who have objective MMD but are free of SR MMD is small. For instance, in a cross-sectional sample of participants who represented all possible combinations of objective and self-reported MMD status, the majority (95%) of older adults who reported having no difficulty walking a mile were also able to complete the 400 MWT (5). Thus, we believe that our results still provide some valuable guidance in conducting research on self-reported mobility disability in older populations. The LIFE study is unique in that it used a maximum of 15 minutes to complete the 400-m walk as an operational definition for objective MMD. Researchers could certainly choose other definitions as, for example, is true of how researchers operationalize frailty. Cummings and colleagues recommended using 10 minutes as a cutoff in 400 MWT to define dismobility (22). Therefore, one limitation of this analysis is that participants who completed the 400 MWT between 10 and 15 minutes in LIFE could arguably be considered to have MMD based on slow gait speed. In conclusion, using longitudinal data from a group of older adults at risk of mobility disability at baseline, we found that two of three simple self-reported items on mobility had utility when compared to the objective MMD measure. When individually considered, each had good discriminant properties compared to the objective MMD measure for cumulative incident events and provided a good approximation for the temporal development of incident objective MMD. Therefore, they may constitute a reasonable substitute for the objective MMD measure in large-scale observational studies and implementation trials when the objective assessment of 400-m walk is not practical. Furthermore, the two simple SR MMD indices and the objective MMD measure may provide complementary information on the dynamic transition of functional status and warrant further research. Supplementary Material Supplementary data is available at The Journals of Gerontology, Series A: Biological Sciences and Medical Sciences online. Funding A detailed list of research investigators for the LIFE Study and funding information can be found in Appendix. Acknowledgments The authors thank the anonymous reviewers for their constructive comments that led to an improved presentation of results. Conflict of Interest None reported. References 1. Chang M, Cohen-Mansfield J, Ferrucci Let al.   Incidence of loss of ability to walk 400 meters in a functionally limited older population. J Am Geriatr Soc . 2004; 52: 2094– 2098. doi: 10.1111/j.1532-5415.2004.52570.x Google Scholar CrossRef Search ADS PubMed  2. Pahor M, Blair SN, Espeland Met al.   Effects of a physical activity intervention on measures of physical performance: results of the lifestyle interventions and independence for Elders Pilot (LIFE-P) study. J Gerontol A Biol Sci Med Sci  2006; 61: 1157– 1165. doi:10.1093/gerona/61.11.1157 Google Scholar CrossRef Search ADS PubMed  3. Pahor M, Guralnik JM, Ambrosius WTet al.  ; LIFE study investigators. Effect of structured physical activity on prevention of major mobility disability in older adults: the LIFE study randomized clinical trial. JAMA . 2014; 311: 2387– 2396. doi: 10.1001/jama.2014.5616 Google Scholar CrossRef Search ADS PubMed  4. Simonsick EM, Newman AB, Nevitt MCet al.  ; Health ABC Study Group. Measuring higher level physical function in well-functioning older adults: expanding familiar approaches in the Health ABC study. J Gerontol A Biol Sci Med Sci . 2001; 56: M644– M649. doi:10.1093/gerona/56.10.M644 Google Scholar CrossRef Search ADS PubMed  5. Sayers SP, Brach JS, Newman AB, Heeren TC, Guralnik JM, Fielding RA. Use of self-report to predict ability to walk 400 meters in mobility-limited older adults. J Am Geriatr Soc . 2004; 52: 2099– 2103. doi: 10.1111/j.1532-5415.2004.52571.x Google Scholar CrossRef Search ADS PubMed  6. Gill TM, Allore HG, Hardy SE, Guo Z. The dynamic nature of mobility disability in older persons. J Am Geriatr Soc . 2006; 54: 248– 254. doi: 10.1111/j.1532-5415.2005.00586.x Google Scholar CrossRef Search ADS PubMed  7. Perera S, Mody SH, Woodman RC, Studenski SA. Meaningful change and responsiveness in common physical performance measures in older adults. J Am Geriatr Soc . 2006; 54: 743– 749. doi: 10.1111/j.1532-5415.2006.00701.x Google Scholar CrossRef Search ADS PubMed  8. Kwon S, Perera S, Pahor Met al.   What is a meaningful change in physical performance? Findings from a clinical trial in older adults (the LIFE-P study). 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Reuben DB, Seeman TE, Keeler Eet al.   Refining the categorization of physical functional status: the added value of combining self-reported and performance-based measures. J Gerontol A Biol Sci Med Sci . 2004; 59: 1056– 1061. doi:10.1093/gerona/59.10.M1056 Google Scholar CrossRef Search ADS PubMed  13. Cesari M, Kritchevsky SB, Penninx BWet al.   Prognostic value of usual gait speed in well-functioning older people–results from the Health, Aging and Body Composition Study. J Am Geriatr Soc . 2005; 53: 1675– 1680. doi: 10.1111/j.1532-5415.2005.53501.x Google Scholar CrossRef Search ADS PubMed  14. McDermott MM, Guralnik JM, Tian Let al.   Baseline functional performance predicts the rate of mobility loss in persons with peripheral arterial disease. J Am Coll Cardiol . 2007; 50: 974– 982. doi: 10.1016/j.jacc.2007.05.030 Google Scholar CrossRef Search ADS PubMed  15. Simonsick EM, Newman AB, Visser Met al.  ; Health, Aging and Body Composition Study. Mobility limitation in self-described well-functioning older adults: importance of endurance walk testing. J Gerontol A Biol Sci Med Sci . 2008; 63: 841– 847. Google Scholar CrossRef Search ADS PubMed  16. McDermott MM, Guralnik JM, Criqui MHet al.   Unsupervised exercise and mobility loss in peripheral artery disease: a randomized controlled trial. J Am Heart Assoc  2015; 4: e001659. doi:10.1161/JAHA.114.001659 Google Scholar CrossRef Search ADS PubMed  17. Fielding RA, Rejeski WJ, Blair Set al.  ; LIFE Research Group. The Lifestyle Interventions and Independence for Elders Study: design and methods. J Gerontol A Biol Sci Med Sci . 2011; 66: 1226– 1237. doi: 10.1093/gerona/glr123 Google Scholar CrossRef Search ADS PubMed  18. Marsh AP, Lovato LC, Glynn NWet al.  ; LIFE Study Research Group. Lifestyle interventions and independence for elders study: recruitment and baseline characteristics. J Gerontol A Biol Sci Med Sci . 2013; 68: 1549– 1558. doi: 10.1093/gerona/glt064 Google Scholar CrossRef Search ADS PubMed  19. Newman AB, Simonsick EM, Naydeck BLet al.   Association of long-distance corridor walk performance with mortality, cardiovascular disease, mobility limitation, and disability. JAMA . 2006; 295: 2018– 2026. doi: 10.1001/jama.295.17.2018 Google Scholar CrossRef Search ADS PubMed  20. Rejeski WJ, Ip EH, Marsh AP, Barnard RT. Development and validation of a video-animated tool for assessing mobility. J Gerontol A Biol Sci Med Sci . 2010; 65: 664– 671. doi: 10.1093/gerona/glq055 Google Scholar CrossRef Search ADS PubMed  21. Simonsick EM, Fan E, Fleg JL. Estimating cardiorespiratory fitness in well-functioning older adults: treadmill validation of the long distance corridor walk. J Am Geriatr Soc . 2006; 54: 127– 132. doi: 10.1111/j.1532-5415.2005.00530.x Google Scholar CrossRef Search ADS PubMed  22. Cummings SR, Studenski S, Ferrucci L. A diagnosis of dismobility–giving mobility clinical visibility: a Mobility Working Group recommendation. JAMA . 2014; 311: 2061– 2062. doi: 10.1001/jama.2014.3033 Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2017. 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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The Journals of Gerontology Series A: Biomedical Sciences and Medical SciencesOxford University Press

Published: Apr 1, 2018

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