Assessment of Muscular Fitness as a Predictor of Flight Duty Limitation

Assessment of Muscular Fitness as a Predictor of Flight Duty Limitation Abstract Introduction The high acceleration (Gz) exposure among military pilots flying fighter aircraft has been associated with an increased risk for cervical and lumbar disorders. It has been suggested that an adequate level of physical performance could reduce the risk of experiencing these disorders. The Finnish Air Force has for several years used aerobic (bicycle ergometer) and muscular fitness tests (battery of five tests) in the selection process of military pilot candidates in order to evaluate their physical fitness level. The aim of the study was to determine if these selection phase tests and anthropometry measures can predispose those individuals who might be at risk of developing severe spinal disorders leading to permanent flight duty limitations later during their military pilots’ career. Materials and Methods The study population consisted of 23 pilots flying with Gz limitation (+2 Gz, +4 Gz or +5 Gz) due to spinal disorders and 50 experienced (+1,000 flight hours) symptomless controls flying actively in operative missions. Data obtained retrospectively for all subjects included anthropometry, physical (aerobic and muscular fitness) test results and self-reported physical activity levels at a pilot selection phase. Aerobic fitness was measured with a maximal ergometer test and muscular endurance was evaluated with a test battery (standing long jump, pull-ups, sit-ups, back extensions, and push-up tests). Results Fighter pilots flying without Gz limitation had significantly better mean (±SE) results in pull-up (14.4 ± 4.2 vs. 11.5 ± 2.0, p < 0.05) and back extension (71.1 ± 14.1 vs. 60.0 ± 12.2, p < 0.05) tests during the pilot selection when compared with the limited pilots. Similarly, the non-limited pilots had a better total muscular fitness test score (13.7 ± 1.7 vs. 12.4 ± 1.6, p < 0.05) during the pilot selection. They had also participated in significantly more competitive sports (54% vs. 22%, p < 0.05) at the time of selection when compared with pilots flying with Gz limitation due to spinal disorders. The aerobic fitness test results and anthropometric measures were not statistically different among the groups. Conclusions Higher levels of muscular fitness, particularly axial strength in military pilot selection may have a protective role for reducing spinal disorders which if developed, can often lead to limiting the availability of pilots for flight duty. The present findings also do not support the assumption that aerobic fitness above the required minimum level would protect pilots from developing spinal disorders and, therefore, from limiting flight duty. INTRODUCTION The high acceleration (+Gz) exposure among fighter pilots has been associated with an increased risk for spinal disorders.1 Fighter pilots report a higher prevalence of neck and low back pain (LBP) compared with both cargo and transport pilots2 and the primary airframe has been found to be an independent predictor for developing clinically significant neck pain.3 In addition to +Gz exposure, awkward head positions when looking-out for enemy aircraft (“check six”) has been suggested to be an underlying factor for cervical pain.4 Similarly, uncomfortable sitting posture in the ejection seat has also been suggested to be a causal factor for increased LBP incidence among fighter pilots.5 According to a review by Hamberg-van Reenen et al,6 the muscular endurance of the lower back and neck can help to predict low back and neck pain among the general working-age population. This supports our previous findings7 among fixed wing military pilots. Specifically, there was an association between increased static isometric back extension test results and decreased LBP reported 5 yr after the initial test.7 Similarly, Ang et al8 found that pilots without a history of neck pain had increased muscle strength than their symptomatic colleagues. However, the cause and effect attribution is not unambiguous. Yet, it has been suggested that strength training might prevent G-induced neck injuries and, therefore, various fitness-training programs have been recommended for fighter pilots.1,9 Taller individuals report LBP symptoms more often than their shorter counterparts among the general population,10 but the results concerning the association between anthropometrics and spinal disorders among military pilots are not clear. The height of a pilot has been reported as a risk indicator for LBP among helicopter pilots11 but among fighter pilots, no association between height and future LBP has been observed.7 The relationship between neck loading and sitting height among fighter pilots is also conflicting. According to Parr et al,12 the sitting height of a pilot has a significant effect on neck loading in high +Gz exposure (1.6 kg helmet at + 8 Gz), but the effect is not significant on the lower +Gz level (+6 Gz). It has been reported that people with LBP have significantly higher body mass index (BMI) among the general population.10 This is also in line with the results among fighter pilots.5 Musculoskeletal disorders are ranked among the top-3 causes for permanent medical flight disqualification among USAF pilots and navigators.13 Instead of permanent flight disqualification, the maximal G level allowed to the pilot during operational flights may be limited for certain period of time by the Finnish Air Force (FINAF). Limitations without airframe restrictions are used during the rehabilitation of spinal disorders. If there is no improvement in the disorder the flight duty limitation becomes permanent. There are no previous studies that have investigated the number of flight duty limitations or the predictive factors that might lead to the flight duty limitation. Typical indicators and measurements of pain are often based on different questionnaires and pain indexes.4 Furthermore, there are no studies as far as the authors are aware of that has investigated the predictive role of physical fitness test results (in the selection phase) on potential future occurrence of neck pain or LBP among fighter pilots. The aim of the present study was to determine if the general physical fitness tests and anthropometry measured in military pilots’ selection tests could predispose those individuals who might be at risk of developing severe spinal disorder leading to permanent flight duty limitation during a military pilots’ career. Data in a group of pilots with permanent flight duty limitations due to spinal disorders were compared with respective data in an age-matched group of experienced fighter pilots without flight duty limitations. It was hypothesized that the lower level of physical fitness in the selection phase could be a risk factor for future flight duty limitation. METHODS Subjects The test results of 73 individuals (Finnish Air Force fighter pilots) who had started the Air Force Academy between the years 1995 and 2004 were included in the study. There were a total of 23 pilots who had received permanent flight duty limitation due to spinal disorders during the 10 yr period. These pilots (N = 23) were assigned to the Flight Duty Limitation (FDL) group. Their age and early career flight experience matched controls (n = 50) were assigned to the non-FDL group (flying without limitation). The range of the starting year at the Air Force Academy was chosen in order to have the same training syllabus and similar testing procedure in the selection phase. At the time of data selection, the pilots had been in service between 10 and 20 yr. Only pilots with more than 150 flight hours with high performance aircraft (HPA) before limitation were selected to the present study in order to increase the probability that the limitation to flying duty was due to flight-related spinal disorders. Flight duty was limited, in average, after 10 yr of flying HPA (minimum 2 yr of flying and maximum of 17 yr flying). Thirteen out of 23 FDL subjects were limited to fly between the sixth and ninth years of flying with HPA. This selection method was chosen to ensure the same syllabus and flight training for subjects (FDL group) and controls (non-FDL group) at the early stage of their career. The non-FDL group consisted of five pilots from each Air Force Academy course during the 10-yr follow-up. The pilots with highest flight hours without limitations from each course were chosen. Therefore, all the non-FDL group pilots were well-experienced F/A-18 fighter pilots who were in an active operative duty. The exact flight hours are not reported since it is classified information, but each of the top-5 pilots had Gz exposed flight experience between 1,000 and 4,000 flight hours. Respectively, the mean (±SD) flight hours with +Gz exposure of limited pilots was 1,354 ± 451 h. There was no statistically significant difference in flight hours flown during the first 5 yr of flying HPA between the FDL and non-FDL groups (t(57) = 1.35, p = 0.18). Therefore, we assume that the +Gz exposure is comparable during the first 5 yr of career. Only male pilots were included in the study because of the very small number of female military pilots in FINAF. Mean (±SD) age, body mass and height of study population is presented in Table I. There was no difference between demographic information between the present groups. In the selection tests, the applicants have to fulfill certain criteria in anthropometry, physical fitness, psychology, and aeromedical examination. The requirements of the different tests are provided in the Methods section. Table I. Basic Characteristics (mean ± SD) of the Non-FDL and FDL Groups Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  Table I. Basic Characteristics (mean ± SD) of the Non-FDL and FDL Groups Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  The FDL group pilots were flying either transport (EADS CASA C-295 M and Fokker F27) or liaison aircraft (Gates Learjet 35 A and Pilatus PC-NG 12) or only lighter missions with limited +Gz level (maximum 5 +Gz) with trainer jets (Hawk Mk 51 jet trainers). The pilots in the non-FDL group were actively flying F-18 Hornet fighters or Hawk Mk 51 jet trainers. They were flying operative missions either with F/A-18 on the operative squadrons or as tactical flight instructors in the training wing, both duties included high +Gz exposure, up to maximum of +8Gz. The data (results of the anthropometric measurements, physical fitness tests and physical activity questionnaire in the pilot selection) were collected from the database of the Aeromedical Centre, which is the only institution responsible for FINAF pilot selection. All subjects volunteered, and they provided written informed consent to participate in the present study. In addition, authorization was obtained from the Finnish Defense Forces’ review board of research permits. Procedures Physical fitness test results from the cycle ergometer test and muscular endurance tests (five test battery including standing long jump, pull-ups, sit-ups, back extensions, and push-up tests) were used for further analysis. These tests have been an integral part of official selection procedure since the year of 1998 in FINAF. Anthropometry and questionnaires of physical activity were also recorded from every subject. However, there were no test results from 6 (of 23) limited pilots and 13 (of 50) non-limited pilots were available because the physical fitness tests have been used as part of the selection process for only the last 7 yr of the present 10-yr follow-up (from the year 1995 instead of 1998). The anthropometry measures taken in FINAF military pilot selection included height, weight, sitting height, and the length of a thigh. Each measure has its maximal and minimal limits which are based on the dimensions of the cockpits of early trainer and jet trainer aircrafts, which the FINAF pilots were flying before entering F/A-18 aircraft. The height and weight requirements in selection are 165–190 cm and 55–92 kg, respectively. Thigh length and sitting height requirements are between 55–67 cm and 81–100 cm, respectively. The subjects’ maximal aerobic capacity was evaluated with a maximal bicycle ergometer test. The initial workload of the test was 20 W, and it was increased with 20 W in every minute until volitional exhaustion. The work capacity in the test was measured as the average load (watts per body mass) of the last minute (W max¹/kg). In the pilot selection, the minimum requirement was 3.5 W/kg. Muscular fitness was determined by a standing long jump as well as repetitive dynamic pull-ups, sit-ups, back extensions, and push-ups tests. All subjects were given careful advice of the test procedure and techniques as well as practice of all tests. For all repetitive dynamic tests, the subjects were instructed to perform as many repetitions as they were able to perform during a 60-s period. Repetitions completed with incorrect technique were not counted. Each test was graded with a scale from 0 to 3 (Table II). All FINAF pilot applicants must reach at least 8 points (maximum 15) in total to pass the selection process. Table II. Fitness Categories and the Scoring of Muscular Fitness Tests   Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤    Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤  Table II. Fitness Categories and the Scoring of Muscular Fitness Tests   Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤    Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤  For the standing long jump test, the subjects were allowed to swing their arms and the upper body to assist the bilateral take-off phase. The distance between the bilateral landing and the starting point was measured and expressed in meters. For the pull-up test, the subjects were required to raise their chin over a bar and then return to the starting point with elbows fully extended. For the sit-ups, the subjects were lying on the floor supine with knees flexed at an angle of 90°, and feet supported. Subjects raised the upper body (hands behind the neck) until their elbows touched the knees and then returned to the starting position where both scapulas touched the floor. For the push-ups, the subjects were required to fully extend their arms while keeping the body straight, and then lower the body down to the position with an elbow angle of 90°. For the back lift, subjects were supported from the ankle joints and during the movement the upper body was lifted until the scapulas were raised 30 cm, and then lowered down back to the starting position. More detailed information of the muscular fitness tests is presented in the study by Taanila et al.14 Physical activity level of an applicant was also analyzed. This was to determine whether the applicants had participated in sports for regular fitness benefits or as a competitive athlete (i.e. part of a sports team or high level junior sports participation) at the time of applying to the FINAF. No information of the exact hours of training (per week or year) was available. Therefore, the result was a dichotomous variable that the subject has either (1) belonged to competitive sports program led by a coach or (ii) did not belong to competitive sports program led by a coach during the last year of high school. The type of sport that each applicant practiced was also categorized. Due to relatively small number of pilots in the present study, the type of sport was presented in four categories: (1) endurance sports (i.e. running, cross-country skiing, etc.), (2) resistance training (i.e. weight lifting or other muscular fitness training), (3) ball games (i.e. football, ice-hockey, etc.), racquet sports (i.e. tennis, etc.) and martial arts, (4) combination of previous sports stated. In FINAF flight duty limitations due to medical reasons are used to guarantee flight safety, and to reduce the burden of flight duties. In case of spinal disorders, the G-forces are regarded as a causative or aggravating factor and the effects of which are reduced by limiting the pilot’s exposure to G-forces. During the first weeks of the G-limitation, the required spinal examinations, including MRI, are performed and physical therapies as well as personal training are initiated. The intention is that the G-limitation would be temporary and the allowed +Gz level could gradually be increased when the spinal function recovers. If rehabilitation is not successful the G-limitation becomes permanent. When flight duty is limited, the exact G-limit is individually determined. The level of limitation varies normally from +2Gz to +5Gz where pilots with +4Gz and +5Gz limits may, in some cases, continue their flying with early trainers (propeller aircraft) or trainer jets (Hawk MK) on lighter missions. However, most of the limited pilots are transferred to fly the liaison or transport aircraft, in particular all pilots with the +2Gz or +3Gz limit. The present study group was not divided by the limitation level due to small study population. All limitations from +2Gz to +5Gz were taken into account. Statistical Analysis Data were analyzed using SPSS Statistics for Windows V.21.0 software. Means with standard deviations are given as descriptive statistics. Student’s t-test was used for comparison between the groups. Bootstrapping with bias corrected (BCa) confidence interval was applied to re-estimate the standard error of the mean difference in t-test. The Levene’s test was used for testing the normality of variances. The χ2 test was used to compare dichotomous variables of sports background between the study groups. Pearson’s correlation coefficient (r) was used to determine statistical dependence between the variables. The level of significance was set at 0.05. RESULTS On average, the pilots in the non-FDL group had a better total score of muscular fitness tests (M = 13.7, SE = ±1.7), than the pilots in FDL group (M = 12.4, SE = ±1.62) in the selection phase. This difference, 1.38, BCa 95% CI [0.386, 2.266], was statistically significant t(49) = 2.80, p = .007. Pilots in the non-FDL group had better results in pull-up (M = 14.4, SE = ±4.2), than the pilots in the FDL group (M = 11.5, SE = ±2.0). This difference, 2.90, BCa 95% CI [1.128, 4.729], was statistically significant t(49) = 3.37, p = .001. Further, the non-FDL group had better results in back extension test (M = 71.1, SE = ±14.1), than the pilots in the FDL group (M = 60.0, SE = ±12.2). This difference, 11.08, BCa 95% CI [2.924, 18.585], was statistically significant t(37), p = 0.007. The maximal aerobic capacity results were not statistically different among the groups in the selection phase. All selection phase results are described in Table III. There was no statistically significant difference between the FDL and non-FDL groups in any of the anthropometric measures analyzed. Table III. Mean (±SD) Physical Fitness Test Results of the Non-FDL and FDL Groups   Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68    Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68  *Indicates significance (Student’s t-test, with Levene’s test for equality of variances). Table III. Mean (±SD) Physical Fitness Test Results of the Non-FDL and FDL Groups   Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68    Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68  *Indicates significance (Student’s t-test, with Levene’s test for equality of variances). The non-FDL pilots had participated significantly (p = 0.012) more in competitive sports when compared with their FDL counterparts. At the time of their selection, 54% of the pilots in the non-FDL group had reported to have a background in competitive sports while only 22% of the FDL group pilots reported to have background in competitive sports. When all results where compared together, the pull-ups correlated with back extensions (r = 0.56, p< 0.01), push-ups (r = 0.59, p< 0.01), and sit-ups (r = 0.31, p = 0.03) test results. Respectively, the results of the back extension test correlated with push-up (r = 0.48, p < 0.01) and sit-up (r = 0.33, p = 0.02) test results. The sit-up results also correlated with the back extensions results (r = 0.33, p = 0.02). The maximal standing long jump did not correlate with any of the other test results. Neither the anthropometric measures (height, weight and BMI) correlated with any of the measured physical test results. DISCUSSION The pilots in the FDL group had a significantly lower score in the muscular fitness tests completed at the pilot selection when compared with the pilots flying without limitation. In particular, the results in the pull-up and back extension tests were significantly lower among the pilots who had flight duty limitation compared with the pilots flying without limitation. In addition, significantly fewer pilots in the flight duty limitation group had a background in competitive sports compared with their symptomless counterparts. No statistically significant difference among the groups in aerobic fitness test results and anthropometrics were recorded. The finding that non-FDL pilots had better muscular fitness in the selection tests is in line with the hypothesis that lower level of physical fitness in the selection phase is a risk factor for developing future flight duty limitation. Furthermore, the association between poor results in the lower back muscle test and LBP supports our previous findings that adequate back muscle strength has been found to be a preventative factor for LBP among military aviators7 and in general population.15 Respectively, there was significant difference among the results in the pull-up test, which measures the strength of the “pulling muscles” of the upper body (i.e. upper back and biceps muscles). While it is logical that low back muscle strength might prevent flight-related spinal issues, the role of the upper back and arm muscles is not clear. One possibility could be that the pull-up test measures the overall athletic ability. When comparing the points (from 0 to a maximum of 3 points in each test), 68% of pilots in the group of the non-limited pilots had scored the best possible one (3 points which is more than 13 pull-ups), while only 31 % of pilots, who had been limited to fly, were able to reach the 3-point category. Interestingly, there were no statistically significant differences in the standing long jump, push-up and sit-up test results between the groups. Yet, the non-FDL group showed a tendency for better results compared with the FDL group (Table II). The aerobic fitness test (bicycle ergometer) results in selection were not statistically different between the groups. In the pilot selection and later in the annual aeromedical examinations, the minimum requirement for aerobic exercise is 3.5 W/kg (the average load of the last minute of exercise). Thus, it seems justified to conclude that above the 3.5 W/kg level, the bicycle ergometer test cannot identify the pilots who are at risk of future flight limitation. Therefore, reaching above the required level may be sufficient to prevent the future limitation. The finding that only muscular fitness has a predictive role on future spinal disorders is in line with the previous studies.6 While studies among other studied groups (for example blue collar workers) suggest that a good level of muscular fitness in the back muscles may help to prevent work-related spinal disorders.15 Anthropometrics (height, weight, sitting height, and the length of a thigh) measured in selection were not statistically different between the groups. This could be due to strict height and weight requirements in the military pilot selection (mentioned in Methods section), which can cause only a small deviation among the results. In addition, because of this small deviation, these findings are not likely comparable to the general population. The present study groups differed significantly in their sport activity background. The FDL group had participated in significantly less competitive and guided sports programs compared with the pilots in the non-FDL group. In further analysis, we categorized the sports on four categories (presented in method section) to find out if certain type of sport (i.e. either strength or endurance training type of sport) would be more beneficial. However, no difference between the sport categories among the groups was observed. The most popular category in both groups was group 4, which included all ball and racquet games as well as all martial arts. It was found out that 65% of the pilots without limitation and 55% of the limited pilots belong to this group. The physical fitness tests used in the present study are found to be reliable. There is a detailed description in literature14 of the muscular fitness tests and these tests are widely used in several armed forces. Respectively, the bicycle ergometer test is used in many sports and there is detailed data of the test in literature.16 Because these tests have been part of the selection process, all the tests have been performed precisely and with care in a controlled environment. In addition, the pilots are highly motivated and performed the tests with their maximal effort in order to pass the selection process. In contrast to the objective measures of physical fitness test, the information regarding the physical activity background was based on the subjective questionnaire where pilot applicants had informed the level of sport and type of the sport they practiced. However, the present information can be considered reliable because the applicants have a risk of being excluded from the selection process by giving false information. The military pilots who are transferred to non-HPAs after full fighter training is a huge economic loss to any Air Force due to the costs of the fighter pilots’ training. Therefore, it is very important to investigate the predictive role of pilots’ application phase data on spinal disorders leading to permanent flight duty limitation. The strength of the present study is that the effects of spinal disorder on military pilots’ work are measured using a hard end point: permanent flight duty limitation that ends or severely limits fighter pilots career (instead of questionnaires). A further strength is that the non-FDL group represents a very experienced group of fighter pilots with 1,000–4,000 flight hours experience. These fighter pilots represent the most experienced pilots of each age group without spinal disorders leading to flight duty limitations. Furthermore, this is the very first study to investigate this relationship and, therefore, the novelty of this study may be considered as a strength too. A limitation to this study is the small number of subjects in the FDL group. Therefore, we recommend further research with a larger sample size to confirm the results found in the present study. Spinal disorders among military pilots are common1 and they may lead to early career limitations and in the worst-case scenario, result in permanent flight disqualification.13 It is, therefore, suggested that the role of the muscular endurance test should be highlighted in military pilot selection. The findings from this study also suggest that muscular endurance tests like the pull-up and lower back muscle tests in addition to a detailed interview of the applicant’s athletic background are a useful part of the selection process for recruiting military pilots. In conclusion, the results of this study support the original hypothesis that pilots with lower muscular fitness in the military pilot selection phase can have an increased risk for flight duty limitation later in their career. Lower recorded results in the dynamic repetitive back extension and pull-up tests were associated with future flight duty limitation due to, spinal disorders. The aerobic fitness test result (above the required minimum performance) was not associated with future flight duty limitation. REFERENCES 1 Kikukawa A, Tachibana S, Yagura S: G-related musculoskeletal spine symptoms in Japan Air Self Defense Force F-15 pilots. Aviat Space Environ Med  1995; 66( 3): 269– 72. Google Scholar PubMed  2 Grossman A, Nakdimon I, Chapnik L, Levy Y: Back symptoms in aviators flying different aircraft. Aviat Space Environ Med  2012; 83( 7): 702– 5. Google Scholar CrossRef Search ADS PubMed  3 Lawson BK, Scott O, Egbulefu FJ, Ramos R, Jenne JW, Anderson ER: Demographic and occupational predictors of neck pain in pilots: analysis and multinational comparison. Aviat Space Environ Med  2014; 85( 12): 1185– 9. Google Scholar CrossRef Search ADS PubMed  4 Shiri R, Frilander H, Sainio M, et al.  : Cervical and lumbar pain and radiological degeneration among fighter pilots: a systematic review and meta-analysis. Occup Environ Med  2015; 72: 145– 50. Google Scholar CrossRef Search ADS PubMed  5 Truszczyńska A, Lewkowicz R, Truszczyński O, Wojtkowiak M: Back pain and its consequences among Polish Air Force pilots flying high performance aircraft. Int J Occup Med Environ Health  2014; 27( 2): 243– 51. Google Scholar CrossRef Search ADS PubMed  6 Hamberg-van Reenen HH, Ariëns GAM, Blatter BM, Twisk JWR, van Mechelen W, Bongers PM: Physical capacity in relation to low back, neck, or shoulder pain in a working population. Occup Environ Med  2006; 63( 6): 371– 7. Google Scholar CrossRef Search ADS PubMed  7 Honkanen T, Kyröläinen H, Avela J, Mäntysaari M: Functional test measures as risk indicators for low back pain among fixed-wing military. J R Army Med Corps  2017; 163( 1): 31– 4. Google Scholar CrossRef Search ADS PubMed  8 Ang B, Linder J, Harms-Ringdahl K: Neck strength and myoelectric fatigue in fighter and helicopter pilots with a history of neck pain. Aviat Space Environ Med  2005; 76( 4): 375– 80. Google Scholar PubMed  9 Coakwell MR, Bloswick DS, Moser R Jr.: High-risk head and neck movements at high G and interventions to reduce associated neck injury. Aviat Space Environ Med  2004; 75( 1): 68– 80. Google Scholar PubMed  10 Hershkovich O, Friedlander A, Gordon B, et al.  : Associations of body mass index and body height with low back pain in 829,791 adolescents. Am J Epidemiol  2013; 178( 4): 603– 9. Google Scholar CrossRef Search ADS PubMed  11 Orsello CA, Phillips AS, Rice GM: Height and in-flight low back pain association among military helicopter pilots. Aviat Space Environ Med  2013; 84( 1): 32– 7. Google Scholar CrossRef Search ADS PubMed  12 Parr JC, Miller ME, Pellettiere JA, Erich RA: Neck injury criteria formulation and injury risk curves for the ejection environment: a pilot study. Aviat Space Environ. Med  2013; 84( 12): 1240– 8. Google Scholar CrossRef Search ADS PubMed  13 McCrary BF, Van Syoc DL: Permanent flying disqualifications of USAF pilots and navigators (1995–1999). Aviat Space Environ Med  2002; 73( 11): 1117– 112. Google Scholar PubMed  14 Taanila H, Suni J, Pihlajamäki H, et al.  : Aetiology and risk factors of musculoskeletal disorders in physically active conscripts: a follow-up study in the Finnish Defence Forces. BMC Musculoskelet Disord  2010; 11( 5): 146. Google Scholar CrossRef Search ADS PubMed  15 Suni J, Oja P, Miilunpalo S, Pasanen M, Vuori I, Bös K: Health-realted fitness test battery for adults: associations with perceived health, mobility, and back function and symptoms. Arch Phys Med Rehabil  1998; 79( 5): 559– 69. Google Scholar CrossRef Search ADS PubMed  16 Santtila M, Häkkinen K, Pihlainen K, Kyröläinen H: Comparison between direct and predicted maximal oxygen uptake measurement during cycling. Mil Med  2010; 175( 4): 273– 9. Google Scholar CrossRef Search ADS PubMed  Author notes The views expressed are solely those of the authors and do not reflect the official policy or position of the Finnish Defense Forces. © Association of Military Surgeons of the United States 2018. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. 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Assessment of Muscular Fitness as a Predictor of Flight Duty Limitation

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Abstract

Abstract Introduction The high acceleration (Gz) exposure among military pilots flying fighter aircraft has been associated with an increased risk for cervical and lumbar disorders. It has been suggested that an adequate level of physical performance could reduce the risk of experiencing these disorders. The Finnish Air Force has for several years used aerobic (bicycle ergometer) and muscular fitness tests (battery of five tests) in the selection process of military pilot candidates in order to evaluate their physical fitness level. The aim of the study was to determine if these selection phase tests and anthropometry measures can predispose those individuals who might be at risk of developing severe spinal disorders leading to permanent flight duty limitations later during their military pilots’ career. Materials and Methods The study population consisted of 23 pilots flying with Gz limitation (+2 Gz, +4 Gz or +5 Gz) due to spinal disorders and 50 experienced (+1,000 flight hours) symptomless controls flying actively in operative missions. Data obtained retrospectively for all subjects included anthropometry, physical (aerobic and muscular fitness) test results and self-reported physical activity levels at a pilot selection phase. Aerobic fitness was measured with a maximal ergometer test and muscular endurance was evaluated with a test battery (standing long jump, pull-ups, sit-ups, back extensions, and push-up tests). Results Fighter pilots flying without Gz limitation had significantly better mean (±SE) results in pull-up (14.4 ± 4.2 vs. 11.5 ± 2.0, p < 0.05) and back extension (71.1 ± 14.1 vs. 60.0 ± 12.2, p < 0.05) tests during the pilot selection when compared with the limited pilots. Similarly, the non-limited pilots had a better total muscular fitness test score (13.7 ± 1.7 vs. 12.4 ± 1.6, p < 0.05) during the pilot selection. They had also participated in significantly more competitive sports (54% vs. 22%, p < 0.05) at the time of selection when compared with pilots flying with Gz limitation due to spinal disorders. The aerobic fitness test results and anthropometric measures were not statistically different among the groups. Conclusions Higher levels of muscular fitness, particularly axial strength in military pilot selection may have a protective role for reducing spinal disorders which if developed, can often lead to limiting the availability of pilots for flight duty. The present findings also do not support the assumption that aerobic fitness above the required minimum level would protect pilots from developing spinal disorders and, therefore, from limiting flight duty. INTRODUCTION The high acceleration (+Gz) exposure among fighter pilots has been associated with an increased risk for spinal disorders.1 Fighter pilots report a higher prevalence of neck and low back pain (LBP) compared with both cargo and transport pilots2 and the primary airframe has been found to be an independent predictor for developing clinically significant neck pain.3 In addition to +Gz exposure, awkward head positions when looking-out for enemy aircraft (“check six”) has been suggested to be an underlying factor for cervical pain.4 Similarly, uncomfortable sitting posture in the ejection seat has also been suggested to be a causal factor for increased LBP incidence among fighter pilots.5 According to a review by Hamberg-van Reenen et al,6 the muscular endurance of the lower back and neck can help to predict low back and neck pain among the general working-age population. This supports our previous findings7 among fixed wing military pilots. Specifically, there was an association between increased static isometric back extension test results and decreased LBP reported 5 yr after the initial test.7 Similarly, Ang et al8 found that pilots without a history of neck pain had increased muscle strength than their symptomatic colleagues. However, the cause and effect attribution is not unambiguous. Yet, it has been suggested that strength training might prevent G-induced neck injuries and, therefore, various fitness-training programs have been recommended for fighter pilots.1,9 Taller individuals report LBP symptoms more often than their shorter counterparts among the general population,10 but the results concerning the association between anthropometrics and spinal disorders among military pilots are not clear. The height of a pilot has been reported as a risk indicator for LBP among helicopter pilots11 but among fighter pilots, no association between height and future LBP has been observed.7 The relationship between neck loading and sitting height among fighter pilots is also conflicting. According to Parr et al,12 the sitting height of a pilot has a significant effect on neck loading in high +Gz exposure (1.6 kg helmet at + 8 Gz), but the effect is not significant on the lower +Gz level (+6 Gz). It has been reported that people with LBP have significantly higher body mass index (BMI) among the general population.10 This is also in line with the results among fighter pilots.5 Musculoskeletal disorders are ranked among the top-3 causes for permanent medical flight disqualification among USAF pilots and navigators.13 Instead of permanent flight disqualification, the maximal G level allowed to the pilot during operational flights may be limited for certain period of time by the Finnish Air Force (FINAF). Limitations without airframe restrictions are used during the rehabilitation of spinal disorders. If there is no improvement in the disorder the flight duty limitation becomes permanent. There are no previous studies that have investigated the number of flight duty limitations or the predictive factors that might lead to the flight duty limitation. Typical indicators and measurements of pain are often based on different questionnaires and pain indexes.4 Furthermore, there are no studies as far as the authors are aware of that has investigated the predictive role of physical fitness test results (in the selection phase) on potential future occurrence of neck pain or LBP among fighter pilots. The aim of the present study was to determine if the general physical fitness tests and anthropometry measured in military pilots’ selection tests could predispose those individuals who might be at risk of developing severe spinal disorder leading to permanent flight duty limitation during a military pilots’ career. Data in a group of pilots with permanent flight duty limitations due to spinal disorders were compared with respective data in an age-matched group of experienced fighter pilots without flight duty limitations. It was hypothesized that the lower level of physical fitness in the selection phase could be a risk factor for future flight duty limitation. METHODS Subjects The test results of 73 individuals (Finnish Air Force fighter pilots) who had started the Air Force Academy between the years 1995 and 2004 were included in the study. There were a total of 23 pilots who had received permanent flight duty limitation due to spinal disorders during the 10 yr period. These pilots (N = 23) were assigned to the Flight Duty Limitation (FDL) group. Their age and early career flight experience matched controls (n = 50) were assigned to the non-FDL group (flying without limitation). The range of the starting year at the Air Force Academy was chosen in order to have the same training syllabus and similar testing procedure in the selection phase. At the time of data selection, the pilots had been in service between 10 and 20 yr. Only pilots with more than 150 flight hours with high performance aircraft (HPA) before limitation were selected to the present study in order to increase the probability that the limitation to flying duty was due to flight-related spinal disorders. Flight duty was limited, in average, after 10 yr of flying HPA (minimum 2 yr of flying and maximum of 17 yr flying). Thirteen out of 23 FDL subjects were limited to fly between the sixth and ninth years of flying with HPA. This selection method was chosen to ensure the same syllabus and flight training for subjects (FDL group) and controls (non-FDL group) at the early stage of their career. The non-FDL group consisted of five pilots from each Air Force Academy course during the 10-yr follow-up. The pilots with highest flight hours without limitations from each course were chosen. Therefore, all the non-FDL group pilots were well-experienced F/A-18 fighter pilots who were in an active operative duty. The exact flight hours are not reported since it is classified information, but each of the top-5 pilots had Gz exposed flight experience between 1,000 and 4,000 flight hours. Respectively, the mean (±SD) flight hours with +Gz exposure of limited pilots was 1,354 ± 451 h. There was no statistically significant difference in flight hours flown during the first 5 yr of flying HPA between the FDL and non-FDL groups (t(57) = 1.35, p = 0.18). Therefore, we assume that the +Gz exposure is comparable during the first 5 yr of career. Only male pilots were included in the study because of the very small number of female military pilots in FINAF. Mean (±SD) age, body mass and height of study population is presented in Table I. There was no difference between demographic information between the present groups. In the selection tests, the applicants have to fulfill certain criteria in anthropometry, physical fitness, psychology, and aeromedical examination. The requirements of the different tests are provided in the Methods section. Table I. Basic Characteristics (mean ± SD) of the Non-FDL and FDL Groups Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  Table I. Basic Characteristics (mean ± SD) of the Non-FDL and FDL Groups Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  Dependent Variables  FDL (n = 23)  Non-FDL (n = 50)  p-Value  Age (yr)  36.8 ± 2.2  36.1 ± 3.0  0.25  Height (cm)  178.4 ± 5.8  178.7 ± 5.5  0.84  Body mass (kg)  69.7 ± 5.4  71.7 ± 7.1  0.18  BMI  22.4 ± 1.6  21.9 ± 1.2  0.16  Seating height (cm)  92.8 ± 2.7  92.8 ± 3.0  0.95  The FDL group pilots were flying either transport (EADS CASA C-295 M and Fokker F27) or liaison aircraft (Gates Learjet 35 A and Pilatus PC-NG 12) or only lighter missions with limited +Gz level (maximum 5 +Gz) with trainer jets (Hawk Mk 51 jet trainers). The pilots in the non-FDL group were actively flying F-18 Hornet fighters or Hawk Mk 51 jet trainers. They were flying operative missions either with F/A-18 on the operative squadrons or as tactical flight instructors in the training wing, both duties included high +Gz exposure, up to maximum of +8Gz. The data (results of the anthropometric measurements, physical fitness tests and physical activity questionnaire in the pilot selection) were collected from the database of the Aeromedical Centre, which is the only institution responsible for FINAF pilot selection. All subjects volunteered, and they provided written informed consent to participate in the present study. In addition, authorization was obtained from the Finnish Defense Forces’ review board of research permits. Procedures Physical fitness test results from the cycle ergometer test and muscular endurance tests (five test battery including standing long jump, pull-ups, sit-ups, back extensions, and push-up tests) were used for further analysis. These tests have been an integral part of official selection procedure since the year of 1998 in FINAF. Anthropometry and questionnaires of physical activity were also recorded from every subject. However, there were no test results from 6 (of 23) limited pilots and 13 (of 50) non-limited pilots were available because the physical fitness tests have been used as part of the selection process for only the last 7 yr of the present 10-yr follow-up (from the year 1995 instead of 1998). The anthropometry measures taken in FINAF military pilot selection included height, weight, sitting height, and the length of a thigh. Each measure has its maximal and minimal limits which are based on the dimensions of the cockpits of early trainer and jet trainer aircrafts, which the FINAF pilots were flying before entering F/A-18 aircraft. The height and weight requirements in selection are 165–190 cm and 55–92 kg, respectively. Thigh length and sitting height requirements are between 55–67 cm and 81–100 cm, respectively. The subjects’ maximal aerobic capacity was evaluated with a maximal bicycle ergometer test. The initial workload of the test was 20 W, and it was increased with 20 W in every minute until volitional exhaustion. The work capacity in the test was measured as the average load (watts per body mass) of the last minute (W max¹/kg). In the pilot selection, the minimum requirement was 3.5 W/kg. Muscular fitness was determined by a standing long jump as well as repetitive dynamic pull-ups, sit-ups, back extensions, and push-ups tests. All subjects were given careful advice of the test procedure and techniques as well as practice of all tests. For all repetitive dynamic tests, the subjects were instructed to perform as many repetitions as they were able to perform during a 60-s period. Repetitions completed with incorrect technique were not counted. Each test was graded with a scale from 0 to 3 (Table II). All FINAF pilot applicants must reach at least 8 points (maximum 15) in total to pass the selection process. Table II. Fitness Categories and the Scoring of Muscular Fitness Tests   Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤    Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤  Table II. Fitness Categories and the Scoring of Muscular Fitness Tests   Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤    Poor  Satisfactory  Good  Excellent  0 points  1 point  2 points  3 points  Pull-up (reps in minute)  <6  6–9  10–13  14≤  Push-up (reps in minute)  <22  22–29  30–37  38≤  Sit-up (reps in minute)  <32  32–39  40–47  48≤  Back extensions (reps in minute)  <40  40–59  50–59  60≤  Standing long jump (meters)  <200  200–219  220–239  240≤  For the standing long jump test, the subjects were allowed to swing their arms and the upper body to assist the bilateral take-off phase. The distance between the bilateral landing and the starting point was measured and expressed in meters. For the pull-up test, the subjects were required to raise their chin over a bar and then return to the starting point with elbows fully extended. For the sit-ups, the subjects were lying on the floor supine with knees flexed at an angle of 90°, and feet supported. Subjects raised the upper body (hands behind the neck) until their elbows touched the knees and then returned to the starting position where both scapulas touched the floor. For the push-ups, the subjects were required to fully extend their arms while keeping the body straight, and then lower the body down to the position with an elbow angle of 90°. For the back lift, subjects were supported from the ankle joints and during the movement the upper body was lifted until the scapulas were raised 30 cm, and then lowered down back to the starting position. More detailed information of the muscular fitness tests is presented in the study by Taanila et al.14 Physical activity level of an applicant was also analyzed. This was to determine whether the applicants had participated in sports for regular fitness benefits or as a competitive athlete (i.e. part of a sports team or high level junior sports participation) at the time of applying to the FINAF. No information of the exact hours of training (per week or year) was available. Therefore, the result was a dichotomous variable that the subject has either (1) belonged to competitive sports program led by a coach or (ii) did not belong to competitive sports program led by a coach during the last year of high school. The type of sport that each applicant practiced was also categorized. Due to relatively small number of pilots in the present study, the type of sport was presented in four categories: (1) endurance sports (i.e. running, cross-country skiing, etc.), (2) resistance training (i.e. weight lifting or other muscular fitness training), (3) ball games (i.e. football, ice-hockey, etc.), racquet sports (i.e. tennis, etc.) and martial arts, (4) combination of previous sports stated. In FINAF flight duty limitations due to medical reasons are used to guarantee flight safety, and to reduce the burden of flight duties. In case of spinal disorders, the G-forces are regarded as a causative or aggravating factor and the effects of which are reduced by limiting the pilot’s exposure to G-forces. During the first weeks of the G-limitation, the required spinal examinations, including MRI, are performed and physical therapies as well as personal training are initiated. The intention is that the G-limitation would be temporary and the allowed +Gz level could gradually be increased when the spinal function recovers. If rehabilitation is not successful the G-limitation becomes permanent. When flight duty is limited, the exact G-limit is individually determined. The level of limitation varies normally from +2Gz to +5Gz where pilots with +4Gz and +5Gz limits may, in some cases, continue their flying with early trainers (propeller aircraft) or trainer jets (Hawk MK) on lighter missions. However, most of the limited pilots are transferred to fly the liaison or transport aircraft, in particular all pilots with the +2Gz or +3Gz limit. The present study group was not divided by the limitation level due to small study population. All limitations from +2Gz to +5Gz were taken into account. Statistical Analysis Data were analyzed using SPSS Statistics for Windows V.21.0 software. Means with standard deviations are given as descriptive statistics. Student’s t-test was used for comparison between the groups. Bootstrapping with bias corrected (BCa) confidence interval was applied to re-estimate the standard error of the mean difference in t-test. The Levene’s test was used for testing the normality of variances. The χ2 test was used to compare dichotomous variables of sports background between the study groups. Pearson’s correlation coefficient (r) was used to determine statistical dependence between the variables. The level of significance was set at 0.05. RESULTS On average, the pilots in the non-FDL group had a better total score of muscular fitness tests (M = 13.7, SE = ±1.7), than the pilots in FDL group (M = 12.4, SE = ±1.62) in the selection phase. This difference, 1.38, BCa 95% CI [0.386, 2.266], was statistically significant t(49) = 2.80, p = .007. Pilots in the non-FDL group had better results in pull-up (M = 14.4, SE = ±4.2), than the pilots in the FDL group (M = 11.5, SE = ±2.0). This difference, 2.90, BCa 95% CI [1.128, 4.729], was statistically significant t(49) = 3.37, p = .001. Further, the non-FDL group had better results in back extension test (M = 71.1, SE = ±14.1), than the pilots in the FDL group (M = 60.0, SE = ±12.2). This difference, 11.08, BCa 95% CI [2.924, 18.585], was statistically significant t(37), p = 0.007. The maximal aerobic capacity results were not statistically different among the groups in the selection phase. All selection phase results are described in Table III. There was no statistically significant difference between the FDL and non-FDL groups in any of the anthropometric measures analyzed. Table III. Mean (±SD) Physical Fitness Test Results of the Non-FDL and FDL Groups   Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68    Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68  *Indicates significance (Student’s t-test, with Levene’s test for equality of variances). Table III. Mean (±SD) Physical Fitness Test Results of the Non-FDL and FDL Groups   Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68    Non-FDL  FDL  p-Value  Aerobic capacity (W/kg)  4.5 ± 0.5  4.3 ± 0.4  0.13  Muscular fitness (pts of 15)  14 ± 1.7  12 ± 1.6  0.01*  Pull-ups (reps/min)  14 ± 4.2  12 ± 1.9  0.01*  Push-ups (reps/min)  44 ± 11.8  41 ± 13.6  0.41  Sit-ups (reps/min)  50 ± 5.0  47 ± 5.2  0.55  Back lifts (reps/min)  71 ± 14.1  60 ± 12.2  0.01*  Standing long jump (cm)  243 ± 9.3  236 ± 15.1  0.68  *Indicates significance (Student’s t-test, with Levene’s test for equality of variances). The non-FDL pilots had participated significantly (p = 0.012) more in competitive sports when compared with their FDL counterparts. At the time of their selection, 54% of the pilots in the non-FDL group had reported to have a background in competitive sports while only 22% of the FDL group pilots reported to have background in competitive sports. When all results where compared together, the pull-ups correlated with back extensions (r = 0.56, p< 0.01), push-ups (r = 0.59, p< 0.01), and sit-ups (r = 0.31, p = 0.03) test results. Respectively, the results of the back extension test correlated with push-up (r = 0.48, p < 0.01) and sit-up (r = 0.33, p = 0.02) test results. The sit-up results also correlated with the back extensions results (r = 0.33, p = 0.02). The maximal standing long jump did not correlate with any of the other test results. Neither the anthropometric measures (height, weight and BMI) correlated with any of the measured physical test results. DISCUSSION The pilots in the FDL group had a significantly lower score in the muscular fitness tests completed at the pilot selection when compared with the pilots flying without limitation. In particular, the results in the pull-up and back extension tests were significantly lower among the pilots who had flight duty limitation compared with the pilots flying without limitation. In addition, significantly fewer pilots in the flight duty limitation group had a background in competitive sports compared with their symptomless counterparts. No statistically significant difference among the groups in aerobic fitness test results and anthropometrics were recorded. The finding that non-FDL pilots had better muscular fitness in the selection tests is in line with the hypothesis that lower level of physical fitness in the selection phase is a risk factor for developing future flight duty limitation. Furthermore, the association between poor results in the lower back muscle test and LBP supports our previous findings that adequate back muscle strength has been found to be a preventative factor for LBP among military aviators7 and in general population.15 Respectively, there was significant difference among the results in the pull-up test, which measures the strength of the “pulling muscles” of the upper body (i.e. upper back and biceps muscles). While it is logical that low back muscle strength might prevent flight-related spinal issues, the role of the upper back and arm muscles is not clear. One possibility could be that the pull-up test measures the overall athletic ability. When comparing the points (from 0 to a maximum of 3 points in each test), 68% of pilots in the group of the non-limited pilots had scored the best possible one (3 points which is more than 13 pull-ups), while only 31 % of pilots, who had been limited to fly, were able to reach the 3-point category. Interestingly, there were no statistically significant differences in the standing long jump, push-up and sit-up test results between the groups. Yet, the non-FDL group showed a tendency for better results compared with the FDL group (Table II). The aerobic fitness test (bicycle ergometer) results in selection were not statistically different between the groups. In the pilot selection and later in the annual aeromedical examinations, the minimum requirement for aerobic exercise is 3.5 W/kg (the average load of the last minute of exercise). Thus, it seems justified to conclude that above the 3.5 W/kg level, the bicycle ergometer test cannot identify the pilots who are at risk of future flight limitation. Therefore, reaching above the required level may be sufficient to prevent the future limitation. The finding that only muscular fitness has a predictive role on future spinal disorders is in line with the previous studies.6 While studies among other studied groups (for example blue collar workers) suggest that a good level of muscular fitness in the back muscles may help to prevent work-related spinal disorders.15 Anthropometrics (height, weight, sitting height, and the length of a thigh) measured in selection were not statistically different between the groups. This could be due to strict height and weight requirements in the military pilot selection (mentioned in Methods section), which can cause only a small deviation among the results. In addition, because of this small deviation, these findings are not likely comparable to the general population. The present study groups differed significantly in their sport activity background. The FDL group had participated in significantly less competitive and guided sports programs compared with the pilots in the non-FDL group. In further analysis, we categorized the sports on four categories (presented in method section) to find out if certain type of sport (i.e. either strength or endurance training type of sport) would be more beneficial. However, no difference between the sport categories among the groups was observed. The most popular category in both groups was group 4, which included all ball and racquet games as well as all martial arts. It was found out that 65% of the pilots without limitation and 55% of the limited pilots belong to this group. The physical fitness tests used in the present study are found to be reliable. There is a detailed description in literature14 of the muscular fitness tests and these tests are widely used in several armed forces. Respectively, the bicycle ergometer test is used in many sports and there is detailed data of the test in literature.16 Because these tests have been part of the selection process, all the tests have been performed precisely and with care in a controlled environment. In addition, the pilots are highly motivated and performed the tests with their maximal effort in order to pass the selection process. In contrast to the objective measures of physical fitness test, the information regarding the physical activity background was based on the subjective questionnaire where pilot applicants had informed the level of sport and type of the sport they practiced. However, the present information can be considered reliable because the applicants have a risk of being excluded from the selection process by giving false information. The military pilots who are transferred to non-HPAs after full fighter training is a huge economic loss to any Air Force due to the costs of the fighter pilots’ training. Therefore, it is very important to investigate the predictive role of pilots’ application phase data on spinal disorders leading to permanent flight duty limitation. The strength of the present study is that the effects of spinal disorder on military pilots’ work are measured using a hard end point: permanent flight duty limitation that ends or severely limits fighter pilots career (instead of questionnaires). A further strength is that the non-FDL group represents a very experienced group of fighter pilots with 1,000–4,000 flight hours experience. These fighter pilots represent the most experienced pilots of each age group without spinal disorders leading to flight duty limitations. Furthermore, this is the very first study to investigate this relationship and, therefore, the novelty of this study may be considered as a strength too. A limitation to this study is the small number of subjects in the FDL group. Therefore, we recommend further research with a larger sample size to confirm the results found in the present study. Spinal disorders among military pilots are common1 and they may lead to early career limitations and in the worst-case scenario, result in permanent flight disqualification.13 It is, therefore, suggested that the role of the muscular endurance test should be highlighted in military pilot selection. The findings from this study also suggest that muscular endurance tests like the pull-up and lower back muscle tests in addition to a detailed interview of the applicant’s athletic background are a useful part of the selection process for recruiting military pilots. In conclusion, the results of this study support the original hypothesis that pilots with lower muscular fitness in the military pilot selection phase can have an increased risk for flight duty limitation later in their career. Lower recorded results in the dynamic repetitive back extension and pull-up tests were associated with future flight duty limitation due to, spinal disorders. The aerobic fitness test result (above the required minimum performance) was not associated with future flight duty limitation. REFERENCES 1 Kikukawa A, Tachibana S, Yagura S: G-related musculoskeletal spine symptoms in Japan Air Self Defense Force F-15 pilots. Aviat Space Environ Med  1995; 66( 3): 269– 72. Google Scholar PubMed  2 Grossman A, Nakdimon I, Chapnik L, Levy Y: Back symptoms in aviators flying different aircraft. Aviat Space Environ Med  2012; 83( 7): 702– 5. Google Scholar CrossRef Search ADS PubMed  3 Lawson BK, Scott O, Egbulefu FJ, Ramos R, Jenne JW, Anderson ER: Demographic and occupational predictors of neck pain in pilots: analysis and multinational comparison. Aviat Space Environ Med  2014; 85( 12): 1185– 9. Google Scholar CrossRef Search ADS PubMed  4 Shiri R, Frilander H, Sainio M, et al.  : Cervical and lumbar pain and radiological degeneration among fighter pilots: a systematic review and meta-analysis. Occup Environ Med  2015; 72: 145– 50. Google Scholar CrossRef Search ADS PubMed  5 Truszczyńska A, Lewkowicz R, Truszczyński O, Wojtkowiak M: Back pain and its consequences among Polish Air Force pilots flying high performance aircraft. Int J Occup Med Environ Health  2014; 27( 2): 243– 51. Google Scholar CrossRef Search ADS PubMed  6 Hamberg-van Reenen HH, Ariëns GAM, Blatter BM, Twisk JWR, van Mechelen W, Bongers PM: Physical capacity in relation to low back, neck, or shoulder pain in a working population. Occup Environ Med  2006; 63( 6): 371– 7. Google Scholar CrossRef Search ADS PubMed  7 Honkanen T, Kyröläinen H, Avela J, Mäntysaari M: Functional test measures as risk indicators for low back pain among fixed-wing military. J R Army Med Corps  2017; 163( 1): 31– 4. Google Scholar CrossRef Search ADS PubMed  8 Ang B, Linder J, Harms-Ringdahl K: Neck strength and myoelectric fatigue in fighter and helicopter pilots with a history of neck pain. Aviat Space Environ Med  2005; 76( 4): 375– 80. Google Scholar PubMed  9 Coakwell MR, Bloswick DS, Moser R Jr.: High-risk head and neck movements at high G and interventions to reduce associated neck injury. Aviat Space Environ Med  2004; 75( 1): 68– 80. Google Scholar PubMed  10 Hershkovich O, Friedlander A, Gordon B, et al.  : Associations of body mass index and body height with low back pain in 829,791 adolescents. Am J Epidemiol  2013; 178( 4): 603– 9. Google Scholar CrossRef Search ADS PubMed  11 Orsello CA, Phillips AS, Rice GM: Height and in-flight low back pain association among military helicopter pilots. Aviat Space Environ Med  2013; 84( 1): 32– 7. Google Scholar CrossRef Search ADS PubMed  12 Parr JC, Miller ME, Pellettiere JA, Erich RA: Neck injury criteria formulation and injury risk curves for the ejection environment: a pilot study. Aviat Space Environ. Med  2013; 84( 12): 1240– 8. Google Scholar CrossRef Search ADS PubMed  13 McCrary BF, Van Syoc DL: Permanent flying disqualifications of USAF pilots and navigators (1995–1999). Aviat Space Environ Med  2002; 73( 11): 1117– 112. Google Scholar PubMed  14 Taanila H, Suni J, Pihlajamäki H, et al.  : Aetiology and risk factors of musculoskeletal disorders in physically active conscripts: a follow-up study in the Finnish Defence Forces. BMC Musculoskelet Disord  2010; 11( 5): 146. Google Scholar CrossRef Search ADS PubMed  15 Suni J, Oja P, Miilunpalo S, Pasanen M, Vuori I, Bös K: Health-realted fitness test battery for adults: associations with perceived health, mobility, and back function and symptoms. Arch Phys Med Rehabil  1998; 79( 5): 559– 69. Google Scholar CrossRef Search ADS PubMed  16 Santtila M, Häkkinen K, Pihlainen K, Kyröläinen H: Comparison between direct and predicted maximal oxygen uptake measurement during cycling. Mil Med  2010; 175( 4): 273– 9. Google Scholar CrossRef Search ADS PubMed  Author notes The views expressed are solely those of the authors and do not reflect the official policy or position of the Finnish Defense Forces. © Association of Military Surgeons of the United States 2018. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Military MedicineOxford University Press

Published: May 8, 2018

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