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Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees
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Contents lists available at ScienceDirect
Journal of Science and Medicine in Sport journal homepage: www.elsevier.com/locate/jsams
Original research
Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees Bruce H. Jones a,∗ , Keith G. Hauret a , Shamola K. Dye a , Veronique D. Hauschild a , Stephen P. Rossi a , Melissa D. Richardson a , Karl E. Friedl b a b
Army Public Health Center, Clinical Public Health and Epidemiology Directorate, Injury Prevention Division, United States U.S. Army Research Institute of Environmental Medicine, United States
a r t i c l e
i n f o
Article history: Received 31 March 2017 Received in revised form 19 August 2017 Accepted 20 September 2017 Available online xxx Keywords: Physical fitness Running Body composition Musculoskeletal Overuse Gender
a b s t r a c t Objectives: To determine the combined effects of physical fitness and body composition on risk of trainingrelated musculoskeletal injuries among Army trainees. Design: Retrospective cohort study. Methods: Rosters of soldiers entering Army basic combat training (BCT) from 2010 to 2012 were linked with data from multiple sources for age, sex, physical fitness (heights, weights (mass), body mass index (BMI), 2 mile run times, push-ups), and medical injury diagnoses. Analyses included descriptive means and standard deviations, comparative t-tests, risks of injury, and relative risks (RR) and 95% confidence intervals (CI). Fitness and BMI were divided into quintiles (groups of 20%) and stratified for chi-square (2 ) comparisons and to determine trends. Results: Data were obtained for 143,398 men and 41,727 women. As run times became slower, injury risks increased steadily (men = 9.8–24.3%, women = 26.5–56.0%; 2 trends (p < 0.00001)). For both genders, the relationship of BMI to injury risk was bimodal, with the lowest risk in the average BMI group (middle quintile). Injury risks were highest in the slowest groups with lowest BMIs (male trainees = 26.5%; female trainees = 63.1%). Compared to lowest risk group (average BMI with fastest run-times), RRs were significant (male trainees = 8.5%; RR 3.1, CI: 2.8–3.4; female trainees = 24.6%; RR 2.6, CI: 2.3–2.8). Trainees with the lowest BMIs exhibited highest injury risks for both genders and across all fitness levels. Conclusions: While the most aerobically fit Army trainees experience lower risk of training-related injury, at any given aerobic fitness level those with the lowest BMIs are at highest risk. This has implications for recruitment and retention fitness standards. © 2017 Published by Elsevier Ltd on behalf of Sports Medicine Australia.
1. Introduction Musculoskeletal (MSK) injuries among active duty soldiers result in over 1.3 million medical visits and over 10 million limited duty days each year.1,2 These injuries have been estimated to account for over 70% of those who are medically non-deployable.2 As with civilian athletes and exercise participants, MSK injuries commonly result from vigorous physical training and overuse.3 Among soldiers in the U.S. Army, over 50% of the injuries are training-related overuse injuries.4,5 These injuries occur primarily in the lower extremities and the low back.4,5 Such MSK overuse injuries have been called “the single most significant medical impediment to military readiness.”6,7
∗ Corresponding author. E-mail address: [email protected] (B.H. Jones).
Recruits entering the Army are at particular risk of injury.3,6 A number of studies have shown that having poor physical fitness on entry to the Army is a leading risk factor for injuries among trainees.1,8 Although the ten week Army basic combat training (BCT) programs accommodate some gradual acclimation to the extremely physically-demanding routines, the short time period to develop fitness may contribute to injury risk.5,9 To advance, Army recruits must demonstrate basic measures of physical fitness upon entry, achieve Army fitness standards by completion of BCT, and then maintain standards when in operational units.10,11 The primary health-related components of physical fitness include cardiorespiratory endurance (i.e., aerobic fitness), muscle endurance, muscle strength, flexibility, and body composition.7,12 Of these fitness components, aerobic fitness has been shown to most strongly correlate with the performance of physicaldemanding tasks required of Army soldiers.13 Additionally, among these components low levels of aerobic fitness (typically measured
http://dx.doi.org/10.1016/j.jsams.2017.09.015 1440-2440/© 2017 Published by Elsevier Ltd on behalf of Sports Medicine Australia.
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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as timed-runs) are most consistently and most strongly associated with higher risk of injuries among both male and female military trainees.1,8,14–18 Other components of physical fitness (i.e., muscle endurance, muscle strength, flexibility and body composition) have not been as strongly or consistently associated with injury risk.8,14,16,17,19 The U.S. Department of Defense and Army also place great emphasis on maintaining acceptable body composition (measured as body fat percentage or body mass index (BMI) determined from height and weight standards).10,11,20 In the past, military height and weight and body fat standards have focused on appearance as well as physical fitness.21 Only in the last few decades have studies began to look at the association of body composition with injuries. Several military studies have shown bimodal or J-shaped patterns of risk for injury, where the highest and lowest extremes of BMI are at greatest risk.18,22–24 Other studies suggest highest risk occurs among those running average run times or slower and who exhibit the lowest BMIs.1,7 These data suggest a need to further evaluate the extremes of current U.S. Army BMI standards. Specifically, injury risk associated with low BMI, and how it relates to lean body mass, may need to be factored into the well-established relationship between higher body fat and lower aerobic fitness.21 Because training-related MSK injuries represent the leading threat to military readiness, a means to reduce injury risk will enhance force strength.1,4 Current military policies may place too much emphasis on and encourage lower BMIs, and not adequately recognize that soldiers with higher BMIs may be less likely to experience MSK injuries, or in other terms, they may be more “musculoskeletally-resilient.” To provide a scientific basis for future military physical fitness and body composition policy, the interrelationships between the fitness components and associations with injury risk must be better understood. Therefore, the purpose of this study was to examine the association between physical fitness as measured by the Army Physical Fitness Test (APFT), which includes a 2 mile run, push-ups, and sit-ups, and BMI with MSK injury risk.
2. Methods This study employed a retrospective cohort design. The study was approved by the Army Public Health Center’s Public Health Review Board as a public health surveillance project. Rosters of all male and female trainees entering the 10 week U.S. Army BCT program between October 2009 and September 2012 were provided by the Training and Doctrine Command (TRADOC). Data were analyzed for all trainees (over 180,000) with complete data for age, gender, heights, weights, and APFT results (2 mile run times, and the number of push-ups and sit ups in two minutes). These variables were linked to any injury the trainee experienced and received a diagnosis recorded in their medical record during the 10 week BCT period. Age and gender data were received from the Defense Manpower Data Center and Defense Medical Surveillance System (DMSS), maintained by the Armed Forces Health Surveillance Branch of the Defense Health Agency. Heights and weights were acquired from the TRADOC Military Entrance Processing Stations (MEPS) records. Body mass index (BMI) was calculated from weights or mass (kg) and heights (m) [BMI = weight (kg)/height (m)2 ]. BMI was used as an estimate of % body fat because in was not possible to directly measure % body fat in a study of this size. Trainees’ APFT results were obtained from the TRADOC Resident Individual Training Management System (RITMS). The medically-documented injury data were extracted from the DMSS. Injuries in this study were operationally defined by the specific International Classification of Diseases, 9th revision,
Clinical Modification (ICD-9-CM) diagnosis codes for the most frequently identified training-related MSK injuries to the low back and lower extremities. Examples include acute mechanical trauma (e.g., fractures, ankle sprains, and muscle strains) or cumulative mechanical micro-trauma (e.g., overuse injuries such as stress fractures, Achilles tendonitis, and plantar fasciitis). The diagnosis codes used for this study are referred to as the Training Related Injury Index, originally described in a 2004 report by Knapik et al.25 Descriptive statistics included means and standard deviations for height, weight (mass), BMI, and APFT results. Risk of injury (% injured) was calculated as the number of trainees with one or more training-related MSK injury during the 10 week BCT period divided by the number of trainees in a particular group of men or women. Relative risk (RR) of injury was calculated for women compared to men and by categories of risk factor, equal size quintile groups (∼20%) for BMI and 2 mile run times [RR = % injured in risk group/% risk of referent group, with 95% confidence interval (95% CI)]. Chi-square (2 ) tests were used to determine the significance of compared risks. When appropriate, Mantel–Haenszel (MH) 2 tests for trend were performed.
3. Results Descriptive statistics for the 184,670 trainees (143,398 men and 41,272 women) are shown in Table 1. Compared to men, women were shorter, weighed less, had lower BMIs, and had lower levels of fitness by APFT scores. The injury risk (%) of women was 2.6 times higher than for men (40.3%/15.7%, 95% CI: 2.5–2.6). The second table (Table 2) displays the injury risks (%) of the men grouped into quintiles (20% groups) of 2-mile run times (fastest to slowest group) stratified by quintiles of BMI (lowest to highest BMIs). As run times become slower, risk of injury increases steadily from 9.8% to 24.3% (RRslowest/fastest = 2.5 (95% CI: 2.45–2.51; MH 2 trend p < 0.0001)). The relationship of BMI to injury risk is a slightly bi-modal curve, with the lowest risk (14.4%) in the middle quintile (BMIQ3) and highest risk at the extremes (BMIQ1 and BMIQ5). The lowest injury risk (8.5%) occurred in the referent group (Fastest RUNQ1–Middle BMIQ3). The highest risk (26.6%) is the group of male trainees with the slowest run times and lowest BMI (Slowest RUNQ5–Lowest BMIQ1), a risk 3.1 times higher than the referent (95% CI: 2.8–3.4). Across all run-time quintiles, the highest risk was among the male trainees with the lowest BMIs, while those with the highest BMIs exhibit some of the lowest injury risk. Table 3 displays similar results for the female trainees. As run times become slower, risks of injury increase steadily from 26.5% to 56.0% (MH 2 trend, p < 0.00001). The relationship of BMI to injury risk for women is also a bimodal curve (lowest risk in the middle quintiles, BMIQ2 and BMIQ3). As with men, the extremes of BMI (BMIQ1 and BMIQ5) exhibit the highest risk. The lowest injury risk (24.6%) occurs in the referent group (Fastest RUNQ1–Middle BMIQ3). The group with the slowest run times and the lowest BMI (RUNQ5–BMIQ1) had the highest risk (63.1%), a 2.6 times higher risk than the referent (95% CI: 2.3–2.8, p < 0.00001). As had been shown among male trainees, the highest risk among female trainees across all run time groups were to those with the lowest BMIs. The women with the highest BMIs exhibited some of the lowest injury risk. Associations of injury risk with muscle endurance fitness (measured by push-ups and sit ups) (data not included) yielded some similarities as shown between injury risk and aerobic fitness (run times). For example, as the number of push-ups performed increased (indicating increased fitness), injury risk decreased sequentially for each quintile [male trainees = 20.9–12.4% (RR = 1.7lowest number of push-ups/highest , 95% CI: 1.6–1.7; MH 2 trend, p-value < 0.0001); female trainees = 48.8%
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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Table 1 Descriptive characteristics of male and female trainees analyzed, Army basic combat training, FY 2010 to FY 2012. Variable
Men
Women
Difference men − women (p-value)
N (%) Age Years, mean (±SD) Height Meters (m), mean (±SD) Weight Kilograms (kg), mean (±SD) Body mass index kg/m2 , mean (±SD) Push-ups Number/2 min, mean (±SD) Sit-ups Number/2 min, mean (±SD) 2 mile run time Minutes, mean (±SD) Risk (%) injured Relative risk (RR, men/women) 95% confidence Interval p-value
143,398 (77) 22.6 (4.9)
41,727 (23) 22.3 (4.8)
0.3 years (p < 0.0001)
1.76 (0.069)
1.62 (0.064)
0.14 m (p < 0.00001)
77.8 (13.2)
61.8 (8.6)
16 kg (p < 0.0001)
25.1 (3.7)
23.3 (2.7)
1.8 kg/m2 (p < 0.00001)
52.8 (13.6)
30.6 (12.2)
22.0 push-ups (p < 0.0001)
63.0 (11.7)
60.8 (12.1)
2.2 sit-ups (p < 0.0001)
14.7 (1.4)
17.7 (1.8)
−3 min (p < 0.0001)
15.7%
40.3% RR = 2.6 95% CI (2.5–2.6) p < 0.00001
Table 2 Injury risk by quintile of 2 mile run times stratified by BMI level, male Army trainees (N = 143,398). Quintiles of BMI (kg/m2 )
Lowest BMIQ1 [<21.7] BMIQ2 [21.6–23.8] BMIQ3 [23.9–25.9] BMIQ4 [26.0–28.3] Highest BMIQ5 [>28.3] Total injury risk by run time level
Quintiles (Q) of 2 mile run time (min)
Total injury risk by BMI level
Fastest RUNQ1 [<13.5] % Risk RR (CI) n
RUNQ2 [13.5–14.2] % Risk RR (CI) n
RUNQ3 [14.3–15.0] % Risk RR (CI) n
RUNQ4 [15.0–15.8] % Risk RR (CI) n
Slowest RUNQ5 [>15.8] % Risk RR (CI) n
11.1 1.3 (1.2–1.4) 7712 9.9 1.2 (1.1–1.3) 8158 8.5 1.0 referent 6779 9.1 1.1 (1.0–1.2) 4276 10.5 1.2 (1.1–1.4) 1788 9.8 1.0 28,713
14.3 1.7 (1.5–1.8) 6849 13.1 1.5 (1.4–1.7) 6830 11.6 1.4 (1.2–1.5) 6231 11.0 1.3 (1.2–1.5) 5261 12.7 1.5 (1.3–1.7) 3366 12.6 1.3 (1.2,1.3) 28,537
16.2 1.9 (1.7–2.1) 6161 15.0 1.8 (1.6–1.9) 5738 14.5 1.7 (1.5–1.9) 5873 14.1 1.7 (1.5–1.8) 5920 14.0 1.6 (1.5–1.8) 5176 14.8 1.5 (1.4,1.5) 28,868
18.7 2.2 (2.0–2.4) 4836 18.3 2.2 (1.9–2.4) 4756 16.4 1.9 (1.7–2.1) 5366 16.9 1.9 (1.8–2.2) 6495 15.9 1.9 (1.7–2.1) 7195 17.1 1.8 (1.7,1.8) 28,648
26.6 3.1 (2.8–3.4) 3157 23.6 2.8 (2.5–3.1) 3237 25.3 3.0 (2.7–3.3) 4272 22.7 2.7 (2.4–2.9) 6642 24.5 2.9 (2.6–3.1) 11,085 24.3 2.5 (2.4,2.5) 28,393
16.0 1.1 (1.1,1.3) 28,715 14.6 1.01 (1.0,1.0) 28,719 14.4 1.0 28,521 15.4 1.1 (1.1,1.1) 28,594 18.2 1.3 (1.2,1.3) 28,610 15.7 143,159
Quintile (Q) = group containing one fifth of subjects. RR = relative risk; CI = 95% confidence interval; n = sub-population group size.
to 31.6% (RR = 1.5lowest/highest , 95% CI: 1.49–1.61; MH 2 trend, p < 0.00001)]. Also as with aerobic fitness, the lowest risk (male trainees = 11.2%; female trainees = 29.2%) occurred among those with average (middle) BMI who demonstrated greatest muscular endurance fitness (i.e., did the most push-ups). However, unlike aerobic fitness, the highest risk among male trainees (22.7%) occurred among those with the highest BMI who did the least pushups (RR = 2.0, 95% CI: 1.9–2.2). Across levels of muscle endurance, the male trainees in the highest BMI quintile had the highest risk of injury. Among female trainees, results were somewhat different, as the highest risk (50%) was to women in the lowest two BMI quintiles who did the least number of push-ups (RR = 1.7 (95% CI: 1.6–1.9)). 4. Discussion This investigation took place in the U.S. Army BCT setting which provides an ideal environment to identify intrinsic risk factors for
physical training-related injuries such as level of physical fitness, body composition, and health risk behaviors. This is because all trainees of both genders perform the same types and amounts of physical and operational training, so the primary extrinsic risk factors for such injuries are controlled. In addition to controlling for physical activity patterns, sleep and nutrition are more standardized, and individuals’ demographic, fitness test scores, and medical data (ICD diagnostic codes) are available. As shown by this study, body composition alone (i.e., BMI) does not adequately reflect Army trainees’ overall risk of experiencing MSK injury. The main finding was that the highest risk of trainingrelated MSK injury occurred in male and female Army trainees who had both the lowest levels of aerobic fitness (slowest run times) and the lowest levels of BMI (men < 21.7; women ≤ 20.7). The study findings are consistent with the general body of evidence that shows the most aerobically fit U.S. Army male and female trainees are protected from training injuries.14,16–19,23,26,27 This finding has also been reported by researchers from Great Britain28
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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Table 3 Injury risk by quintile of 2 mile run times stratified by BMI level, female Army trainees (N = 41,727). Quintiles of BMI (kg/m2 )
Lowest BMI Q1 (<20.7) BMIQ2 (20.8–22.6) BMIQ3 22.6–24.2 BMIQ4 (24.3–25.6) Highest BMIQ5 (>25.6) Total injury risk by run time level
Quintiles (Q) of 2 mile run time (min)
Total injury risk by BMI level
Fastest RUNQ1 [<16.2] % Risk RR (CI) n
RUNQ2 [16.2–17.3] % Risk RR (CI) n
RUNQ3 [17.3–18.1] % Risk RR (CI) n
RUNQ4 [18.2–19.0] % Risk RR (CI) n
Slowest RUNQ5 [>19.0] % Risk RR (CI) n
29.2 1.2 (1.1–1.3) 2334 26.0 1.1 (1.0–1.2) 2141 24.6 1.0 referent 1680 26.1 1.1 (1.0–1.2) 1301 25.0 1.1 (1.0–1.2) 853 26.5 1.0 8309
38.9 1.6 (1.4–1.7) 1981 35.8 1.5 (1.3–1.6) 1828 33.9 1.4 (1.2–1.5) 1684 33.4 1.4 (1.2–1.5) 1564 30.7 1.3 (1.1–1.4) 1224 35.0 1.3 (1.3,1.4) 8281
45.3 1.8 (1.7–2.0) 1660 40.2 1.6 (1.5–1.8) 1680 38.3 1.6 (1.4–1.7) 1734 36.5 1.5 (1.4–1.7) 1793 36.5 1.5 (1.3–1.6) 1546 39.3 1.5 (1.4,1.5) 8413
49.3 2.0 (1.8–2.2) 1358 47.7 1.9 (1.7–2.1) 1427 46.6 1.9 (1.7–2.1) 1645 40.8 1.6 (1.5–1.8) 1807 40.8 1.6 (1.5–1.8) 1928 44.6 1.7 (1.6,1.8) 8165
63.1 2.6 (2.3–2.8) 934 56.0 2.3 (2.1–2.5) 1175 55.4 2.3 (2.2–2.5) 1548 56.0 2.3 (2.1–2.4) 1886 54.0 2.2 (2.0–2.4) 2728 56.0 2.1 (2.0,2.2) 8271
41.9 1.1 (1.02,1.1) 8267 39.1 1.0 (0.9,1.0) 8251 39.5 1.0 8291 39.6 1.0 (0.9,1.1) 8351 41.2 1.04 (1.01,1.08) 8279 40.3 41,439
Quintile (Q) = group containing one fifth of subjects. RR = relative risk; CI = 95% confidence interval; n = sub-population group size.
and Norway29 as well as for soldiers in U.S. operational units.30 Run times, a validated measure of aerobic (or cardiorespiratory) fitness, are consistently correlated with maximum oxygen uptake (VO2 max), and lower VO2 max.13 In addition, slower run times have been consistently associated with higher risk of military injury.8,16 The association of higher injury risk with lower aerobic fitness is probably a derivative of the high amount of exposure soldiers have to stressful weight-bearing aerobic activities inherent to Army BCT (e.g., running, road marching with loads, and walking to and from training sites).8,31 The potential for a higher BMI to have protective effect against injury hypothetically may result in part from greater absolute amounts of muscle among soldiers with higher BMIs.20 Furthermore, a high volume of aerobic training involving running and marching has been found to suppress muscular strength development that would be expected from a modest resistance exercise program.32 This suggests that military recruit training programs may not yet be achieving an optimal balance between aerobic and resistance training to produce physically resilient soldiers.33 Measures of muscle endurance such as push-ups and sit-ups have not been shown to be as strongly or as consistently associated with risk of injury as aerobic fitness measures. A number of studies have, however, shown that trainees who complete the fewest numbers of push-ups or sit-ups experience higher risk of injury than those who perform the greatest number.14,16–18,26,27 The reason these measures of muscle endurance are not as strongly associated with risk of injury during BCT as running may be that the activities requiring upper body or abdominal muscle endurance are less common or limited. Though strong correlations between push-ups and performance of common military tasks are demonstrated in a recent systematic review, the correlations are less strong than with aerobic fitness.13 In the systematic review, sit-ups were not shown to strongly correlate with performance of any common military tasks. However, sit-ups have essentially been the only metric used to measure military core muscle endurance.13 The limited relationship between sit-up and performance or injury may be because the sit-up test itself is not the most appropriate metric. The relationship between muscle strength and injury risk in military populations has not been as well studied as aerobic fitness and
muscle endurance. This is probably because it is not as easy to assess strength in large populations and it requires equipment. However, the few studies that have been performed among men in BCT suggest that strength is not associated with injury risk.16,26,27,34 Blacker et al.28 found that strength was associated with risk of injury among British recruits on a univariate analysis, but was not significant in multivariate models. It may be that the number of exposures on a daily basis to tasks requiring soldiers to exert their maximal muscle strength is limited, whereas weight-bearing aerobic activities are inescapable for soldiers and especially trainees. Future studies should examine the association of muscle strength with injury risk. The relationship between body composition, as assessed by BMI, and injury risk is more complex. Most studies of military trainees have demonstrated either a J-shaped or a bi-modal relationship between BMI and injury risk. Specifically, MSK injury risk is highest at the high and low extremes of BMI, and lowest among those with “average” BMI.7,22,24,29,30 One study of aerobic fitness and injury risk that stratified on level of BMI showed results similar to this study.1,7 Although differences were small in magnitude, this study supports the prior studies’ finding of a bi-modal curve of association of BMI with injury risk for both genders. This study also demonstrates that though aerobic fitness is perhaps more critical, lower BMI may be a greater risk than higher BMI in Army trainees of both genders. This finding may seem counterintuitive, but it could mean that men and women with higher BMIs – who maintain adequate aerobic fitness – are more musculoskeletally-resilient (i.e., are more protected from MSK injury). This may be because Army trainees with the lowest BMIs do not have enough lean body mass, and particularly muscle mass, to endure the physiologic stresses to their MSK system during Army training, so are more susceptible to injury. Those in the upper BMI extremes may also experience some additional MSK stress from carrying extra body weight (thus explaining the bimodal injury risk curve). The benefit, or protective effect, of a moderately high BMI may be related to the additional lean body mass (probably muscle).20,21 A higher muscle mass among trainees with high BMI who are also aerobically fit could protect
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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them from injuries secondary to strenuous activities making them more “musculoskeletally resilient.” It is important to provide a context in which to understand the significance of the low BMI groups evaluated in this study. The lower quintiles of BMI in this study, where there was a marked increase in MSK injury risk, correspond to BMI values observed in a study of previously healthy fit men at the end of a vigorous eightweek ranger training program.35 The ranger trainees in that study had lost all of their accessible body fat stores and begun to consume muscle mass (a mean BMI of 20.3 kg/m2 at the end of training).35 It is therefore reasonable to assume that the lowest BMI values in this study (BMI < 21.7) could represent individuals with extremely low fat and also reduced muscle mass levels. Underweight or low weight U.S. Army recruits and soldiers have not received as much attention as those with high BMIs or body fat. The high obesity rates in the civilian population which is also represented in the military’s recruitment pool, has previously placed the focus on physical training as well as exercise and diet programs for those with excessive BMI and body fat. However, recruits with extremely low BMI are another high risk group that should be monitored. Strength training programs may be a means to reduce injuries in trainees and soldiers with extremely low BMIs. However, caution should be exercised with this recommendation since improperly designed strength training programs may lead to higher rates of injuries. Just as high exposure to lower body aerobic training activities (running, marching) likely contributes to the high rates of lower extremity MSK injury, an increase in exposure to strength training may increase the rates of other MSK injuries. Currently the minimum BMI calculated from the height and weight tables for retention of men and women in the Army is 18.9 kg/m2 . The maximum acceptable BMI for 17–27 year olds ranges from 25.9 to 26.5 for men and 25.0 to 25.3 for women.11 Trainees do not need to meet these standards immediately on entrance to the Army, but are expected to be able to meet the standards once they have gone to their first duty assignments. If they fail to pass the height-weight standards they have their percentage of body fat assessed by tape measurements.11 Men age 17–27 may be up to 22% body fat and women of the same age range may be up to 32% body fat.11 Given the recent data on physical fitness and body composition as assessed by BMI presented in this paper (as well as in the article J. Pierce et al. in this journal supplement), it may make sense for the Army to reconsider its standards to better balance the roles of body composition and physical fitness. 5. Conclusion Substantial evidence has demonstrated that military trainees with poor aerobic fitness have an increased risk of training-related MSK overuse injuries. This study provides additional evidence that there is a higher risk of injury to male and female trainees with the lowest BMIs and the lowest levels of aerobic fitness (measured by run times). The lowest risk of injury was found among the most aerobically fit trainees who exhibited “average” weight levels (the middle quintile of BMI) or over-average levels (BMI greater than the middle quintile). This suggests the military’s desire for lean service members should be balanced with the need to ensure adequate fitness levels and to lower injury risk. Practical implications - Higher levels of aerobic fitness and a moderate BMI can be protective against musculoskeletal (MSK) injury risk. - Employers and coaches who monitor height-weight (BMI) and physical fitness standards should balance requisite physical fitness and body fat with the need for MSK resilience.
5
- Sports medicine and military personnel who recommend exercise programs should consider injury risk as well as fitness development when formulating individual physical activity and diet plans. - Given the high injury risk for Army personnel and the need to maintain high fitness levels it may be wise to for the military to re-consider its height-weight (i.e., BMI) standards. Disclaimer The views expressed in this presentation are those of the authors and do not necessarily reflect the official policy of the Department of Defense, Department of Army, or the U.S. Army Medical Department. Acknowledgments No outside funding was received for this project. The authors would like to acknowledge the assistance of Ms. Mimi Eng in making publication of this manuscript possible. References 1. Nindl BC, Jones BH, Van Arsdale SJ et al. Operational physical performance and fitness in military women: physiological, musculoskeletal injury, and optimized physical training considerations for successfully integrating women into combat-centric military occupations. Mil Med 2016; 181(Suppl. 1):50–62. 2. Department of the Army. Health of the Force Report. November 2015. Retrieved from https://www.army.mil/e2/c/downloads/419337.pdf. 3. Powell KE. Adverse events, in Physical Activity Guidelines Advisory Committee Report G10-1–G10-58, US Department of Health and Human Services (DHHS), editor, Washington, DC, Physical Activity Guidelines Advisory Committee, 2008. 4. Hauret KG, Bedno S, Loringer K et al. Epidemiology of exercise- and sportsrelated injuries in a population of young, physically active adults. Am J Sports Med 2015; 43(11):2645–2653. 5. Jones BH, Canham-Chervak M, Canada S et al. Medical surveillance of injuries in the U.S. military: descriptive epidemiology and recommendations for improvement. Am J Prev Med 2010; 38(Suppl. 3):S42–S60. 6. Molloy JM, Feltwell DN, Scott SJ et al. Physical training injuries and interventions for military recruits. Mil Med 2012; 177(5):553–558. 7. National Research Council. Assessing fitness for military enlistment: physical, medical, and mental health standards, Washington, DC, The National Academies Press, 2006. 8. Jones BH, Hauschild VD. Physical training, fitness, and injuries: lessons learned from military studies. J Strength Cond Res 2015; 29(Suppl. 11):S57–S64. 9. Department of the Army. Army physical readiness training, field manual 7-22 (FM 7-22). Washington, DC; 2012. Retrieved from armypubs.army.mil/doctrine/ DR pubs/dr a/pdf/fm7 22.pdf. 10. Department of Defense Instruction 1308.3. DoD physical fitness and body fat programs procedures. November 5, 2012. 11. Department of the Army. Army Regulation 600-9. The Army body composition program. Washington, DC; 2013. 12. Caspersen CJ, Powell KE, Christenson GM. Physical activity, exercise, and physical fitness: definitions and distinctions for health-related research. Public Health Rep 1985; 100(2):126–131. 13. Hauschild VD, DeGroot DW, Hall SM et al. Fitness tests and occupational tasks of military interest: a systematic review of correlations. Occup Environ Med 2017; 74(2):144–153. 14. Knapik JJ, Swedler DI, Grier TL et al. Injury reduction effectiveness of selecting running shoes based on plantar shape. J Strength Cond Res 2009; 23(3):685–697. 15. Rauh MJ, Macera CA, Trone DW et al. Epidemiology of stress fracture and lower-extremity overuse injury in female recruits. Med Sci Sports Exerc 2006; 38(9):1571–1577. 16. Knapik JJ, Sharp MA, Canham-Chervak M et al. Risk factors for training-related injuries among men and women in basic combat training. Med Sci Sports Exerc 2001; 33(6):946–954. 17. Gilchrist J, Jones BH, Sleet DA et al. Exercise-related injuries among women: strategies for prevention from civilian and military studies. MMWR Recomm Rep 2000; 49(RR-2):15–33. 18. Jones BH, Bovee MW, Harris 3rd JM et al. Intrinsic risk factors for exerciserelated injuries among male and female army trainees. Am J Sports Med 1993; 21(5):705–710. 19. Bell NS, Mangione TW, Hemenway D et al. High injury rates among female army trainees: a function of gender? Am J Prev Med 2000; 18(3):141–146. 20. Grier T, Canham-Chervak M, Sharp M et al. Does body mass index misclassify physically active young men? Prev Med Rep 2015; 2:483–487. 21. Friedl KE. Body composition and military performance—many things to many people. J Strength Cond Res 2012; 26(Suppl. 2):S87–S100.
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22. Hruby A, Bulathsinhala L, McKinnon CJ et al. BMI and lower extremity injury in US Army soldiers, 2001–2011. Am J Prev Med 2016; 50(6):e163–e171. 23. Knapik JJ, Brosch LC, Venuto MA et al. Effect on injuries of assigning shoes based on foot shape in Air Force basic training. Am J Prev Med 2010; 38(Suppl. 1):S197–S211. 24. Jones BH, Bovee MW, Knapik JJ. The association between body composition, physical fitness, and injuries among young men and women in the Army, in Body composition and physical performance, Mariotte BM, Grumstrup-Scott J, editors, Washington DC, National Academy Press, 1992, [Chapter 9]. 25. Knapik JJ, Darakjy S, Scott S et al. Evaluation of two Army fitness programs: the TRADOC standardized physical training program for basic combat training and the fitness assessment program, epidemiologic report no. 12-HF-5772B-04, Aberdeen Proving Ground, MD, U.S. Army Center for Health Promotion and Preventive Medicine, 2004. 26. Knapik JJ, Sharp ML, Canham ML et al. Injury incidence and injury risk factors among U.S. Army basic trainees, Ft. Jackson, South Carolina 1998, epidemiologic report no. 29-HE-8370-98, Aberdeen Proving Ground, MD, U.S. Army Center for Health Promotion and Preventive Medicine, 1999. 27. Jones BH, Amoroso PJ, Canham ML et al. Atlas of injuries in the US armed forces. Mil Med 1999; 164(Suppl. 3), 1–1 to 9–25 (642 pages). URL: http://oai.dtic.mil/ oai/oai?verb=getrecord&metadataPrefix=html&identifier=Ada367256.
28. Blacker SD, Wilkinson DM, Bilzon JL et al. Risk factors for training injuries among British Army recruits. Mil Med 2008; 173(3):278–286. 29. Heir T, Eide G. Injury proneness in infantry conscripts undergoing a physical training programme: smokeless tobacco use, higher age, and low levels of physical fitness are risk factors. Scand J Med Sci Sports 1997; 7(5):304–311. 30. Reynolds KL. Cigarette smoking, physical fitness, and injuries in infantry soldiers. Am J Prev Med 1994; 10:145–150. 31. Knapik JJ, Graham BS, Rieger J et al. Activities associated with injuries in initial entry training. Mil Med 2013; 178(5):500–506. 32. Santtila M, Kyröläinen H, Häkkinen K. Changes in maximal and explosive strength, electromyography, and muscle thickness of lower and upper extremities induced by combined strength and endurance training in soldiers. J Strength Cond Res 2009; 23(4):1300–1308. 33. Friedl KE, Knapik JJ, Häkkinen K et al. Perspectives on aerobic and strength influences on military physical readiness: report of an international military physiology roundtable. J Strength Cond Res 2015; 29:S10–S23. 34. Jones BH, Cowan DN, Tomlinson JP et al. Epidemiology of injuries associated with physical training among young men in the army. Med Sci Sports Exerc 1993; 25(2):197–203. 35. Friedl KE, Moore RJ, Martinez-Lopez LE et al. Lower limit of body fat in healthy active men. J Appl Physiol 1994; 77(2):933–940.
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Contents lists available at ScienceDirect
Journal of Science and Medicine in Sport journal homepage: www.elsevier.com/locate/jsams
Original research
Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees Bruce H. Jones a,∗ , Keith G. Hauret a , Shamola K. Dye a , Veronique D. Hauschild a , Stephen P. Rossi a , Melissa D. Richardson a , Karl E. Friedl b a b
Army Public Health Center, Clinical Public Health and Epidemiology Directorate, Injury Prevention Division, United States U.S. Army Research Institute of Environmental Medicine, United States
a r t i c l e
i n f o
Article history: Received 31 March 2017 Received in revised form 19 August 2017 Accepted 20 September 2017 Available online xxx Keywords: Physical fitness Running Body composition Musculoskeletal Overuse Gender
a b s t r a c t Objectives: To determine the combined effects of physical fitness and body composition on risk of trainingrelated musculoskeletal injuries among Army trainees. Design: Retrospective cohort study. Methods: Rosters of soldiers entering Army basic combat training (BCT) from 2010 to 2012 were linked with data from multiple sources for age, sex, physical fitness (heights, weights (mass), body mass index (BMI), 2 mile run times, push-ups), and medical injury diagnoses. Analyses included descriptive means and standard deviations, comparative t-tests, risks of injury, and relative risks (RR) and 95% confidence intervals (CI). Fitness and BMI were divided into quintiles (groups of 20%) and stratified for chi-square (2 ) comparisons and to determine trends. Results: Data were obtained for 143,398 men and 41,727 women. As run times became slower, injury risks increased steadily (men = 9.8–24.3%, women = 26.5–56.0%; 2 trends (p < 0.00001)). For both genders, the relationship of BMI to injury risk was bimodal, with the lowest risk in the average BMI group (middle quintile). Injury risks were highest in the slowest groups with lowest BMIs (male trainees = 26.5%; female trainees = 63.1%). Compared to lowest risk group (average BMI with fastest run-times), RRs were significant (male trainees = 8.5%; RR 3.1, CI: 2.8–3.4; female trainees = 24.6%; RR 2.6, CI: 2.3–2.8). Trainees with the lowest BMIs exhibited highest injury risks for both genders and across all fitness levels. Conclusions: While the most aerobically fit Army trainees experience lower risk of training-related injury, at any given aerobic fitness level those with the lowest BMIs are at highest risk. This has implications for recruitment and retention fitness standards. © 2017 Published by Elsevier Ltd on behalf of Sports Medicine Australia.
1. Introduction Musculoskeletal (MSK) injuries among active duty soldiers result in over 1.3 million medical visits and over 10 million limited duty days each year.1,2 These injuries have been estimated to account for over 70% of those who are medically non-deployable.2 As with civilian athletes and exercise participants, MSK injuries commonly result from vigorous physical training and overuse.3 Among soldiers in the U.S. Army, over 50% of the injuries are training-related overuse injuries.4,5 These injuries occur primarily in the lower extremities and the low back.4,5 Such MSK overuse injuries have been called “the single most significant medical impediment to military readiness.”6,7
∗ Corresponding author. E-mail address: [email protected] (B.H. Jones).
Recruits entering the Army are at particular risk of injury.3,6 A number of studies have shown that having poor physical fitness on entry to the Army is a leading risk factor for injuries among trainees.1,8 Although the ten week Army basic combat training (BCT) programs accommodate some gradual acclimation to the extremely physically-demanding routines, the short time period to develop fitness may contribute to injury risk.5,9 To advance, Army recruits must demonstrate basic measures of physical fitness upon entry, achieve Army fitness standards by completion of BCT, and then maintain standards when in operational units.10,11 The primary health-related components of physical fitness include cardiorespiratory endurance (i.e., aerobic fitness), muscle endurance, muscle strength, flexibility, and body composition.7,12 Of these fitness components, aerobic fitness has been shown to most strongly correlate with the performance of physicaldemanding tasks required of Army soldiers.13 Additionally, among these components low levels of aerobic fitness (typically measured
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as timed-runs) are most consistently and most strongly associated with higher risk of injuries among both male and female military trainees.1,8,14–18 Other components of physical fitness (i.e., muscle endurance, muscle strength, flexibility and body composition) have not been as strongly or consistently associated with injury risk.8,14,16,17,19 The U.S. Department of Defense and Army also place great emphasis on maintaining acceptable body composition (measured as body fat percentage or body mass index (BMI) determined from height and weight standards).10,11,20 In the past, military height and weight and body fat standards have focused on appearance as well as physical fitness.21 Only in the last few decades have studies began to look at the association of body composition with injuries. Several military studies have shown bimodal or J-shaped patterns of risk for injury, where the highest and lowest extremes of BMI are at greatest risk.18,22–24 Other studies suggest highest risk occurs among those running average run times or slower and who exhibit the lowest BMIs.1,7 These data suggest a need to further evaluate the extremes of current U.S. Army BMI standards. Specifically, injury risk associated with low BMI, and how it relates to lean body mass, may need to be factored into the well-established relationship between higher body fat and lower aerobic fitness.21 Because training-related MSK injuries represent the leading threat to military readiness, a means to reduce injury risk will enhance force strength.1,4 Current military policies may place too much emphasis on and encourage lower BMIs, and not adequately recognize that soldiers with higher BMIs may be less likely to experience MSK injuries, or in other terms, they may be more “musculoskeletally-resilient.” To provide a scientific basis for future military physical fitness and body composition policy, the interrelationships between the fitness components and associations with injury risk must be better understood. Therefore, the purpose of this study was to examine the association between physical fitness as measured by the Army Physical Fitness Test (APFT), which includes a 2 mile run, push-ups, and sit-ups, and BMI with MSK injury risk.
2. Methods This study employed a retrospective cohort design. The study was approved by the Army Public Health Center’s Public Health Review Board as a public health surveillance project. Rosters of all male and female trainees entering the 10 week U.S. Army BCT program between October 2009 and September 2012 were provided by the Training and Doctrine Command (TRADOC). Data were analyzed for all trainees (over 180,000) with complete data for age, gender, heights, weights, and APFT results (2 mile run times, and the number of push-ups and sit ups in two minutes). These variables were linked to any injury the trainee experienced and received a diagnosis recorded in their medical record during the 10 week BCT period. Age and gender data were received from the Defense Manpower Data Center and Defense Medical Surveillance System (DMSS), maintained by the Armed Forces Health Surveillance Branch of the Defense Health Agency. Heights and weights were acquired from the TRADOC Military Entrance Processing Stations (MEPS) records. Body mass index (BMI) was calculated from weights or mass (kg) and heights (m) [BMI = weight (kg)/height (m)2 ]. BMI was used as an estimate of % body fat because in was not possible to directly measure % body fat in a study of this size. Trainees’ APFT results were obtained from the TRADOC Resident Individual Training Management System (RITMS). The medically-documented injury data were extracted from the DMSS. Injuries in this study were operationally defined by the specific International Classification of Diseases, 9th revision,
Clinical Modification (ICD-9-CM) diagnosis codes for the most frequently identified training-related MSK injuries to the low back and lower extremities. Examples include acute mechanical trauma (e.g., fractures, ankle sprains, and muscle strains) or cumulative mechanical micro-trauma (e.g., overuse injuries such as stress fractures, Achilles tendonitis, and plantar fasciitis). The diagnosis codes used for this study are referred to as the Training Related Injury Index, originally described in a 2004 report by Knapik et al.25 Descriptive statistics included means and standard deviations for height, weight (mass), BMI, and APFT results. Risk of injury (% injured) was calculated as the number of trainees with one or more training-related MSK injury during the 10 week BCT period divided by the number of trainees in a particular group of men or women. Relative risk (RR) of injury was calculated for women compared to men and by categories of risk factor, equal size quintile groups (∼20%) for BMI and 2 mile run times [RR = % injured in risk group/% risk of referent group, with 95% confidence interval (95% CI)]. Chi-square (2 ) tests were used to determine the significance of compared risks. When appropriate, Mantel–Haenszel (MH) 2 tests for trend were performed.
3. Results Descriptive statistics for the 184,670 trainees (143,398 men and 41,272 women) are shown in Table 1. Compared to men, women were shorter, weighed less, had lower BMIs, and had lower levels of fitness by APFT scores. The injury risk (%) of women was 2.6 times higher than for men (40.3%/15.7%, 95% CI: 2.5–2.6). The second table (Table 2) displays the injury risks (%) of the men grouped into quintiles (20% groups) of 2-mile run times (fastest to slowest group) stratified by quintiles of BMI (lowest to highest BMIs). As run times become slower, risk of injury increases steadily from 9.8% to 24.3% (RRslowest/fastest = 2.5 (95% CI: 2.45–2.51; MH 2 trend p < 0.0001)). The relationship of BMI to injury risk is a slightly bi-modal curve, with the lowest risk (14.4%) in the middle quintile (BMIQ3) and highest risk at the extremes (BMIQ1 and BMIQ5). The lowest injury risk (8.5%) occurred in the referent group (Fastest RUNQ1–Middle BMIQ3). The highest risk (26.6%) is the group of male trainees with the slowest run times and lowest BMI (Slowest RUNQ5–Lowest BMIQ1), a risk 3.1 times higher than the referent (95% CI: 2.8–3.4). Across all run-time quintiles, the highest risk was among the male trainees with the lowest BMIs, while those with the highest BMIs exhibit some of the lowest injury risk. Table 3 displays similar results for the female trainees. As run times become slower, risks of injury increase steadily from 26.5% to 56.0% (MH 2 trend, p < 0.00001). The relationship of BMI to injury risk for women is also a bimodal curve (lowest risk in the middle quintiles, BMIQ2 and BMIQ3). As with men, the extremes of BMI (BMIQ1 and BMIQ5) exhibit the highest risk. The lowest injury risk (24.6%) occurs in the referent group (Fastest RUNQ1–Middle BMIQ3). The group with the slowest run times and the lowest BMI (RUNQ5–BMIQ1) had the highest risk (63.1%), a 2.6 times higher risk than the referent (95% CI: 2.3–2.8, p < 0.00001). As had been shown among male trainees, the highest risk among female trainees across all run time groups were to those with the lowest BMIs. The women with the highest BMIs exhibited some of the lowest injury risk. Associations of injury risk with muscle endurance fitness (measured by push-ups and sit ups) (data not included) yielded some similarities as shown between injury risk and aerobic fitness (run times). For example, as the number of push-ups performed increased (indicating increased fitness), injury risk decreased sequentially for each quintile [male trainees = 20.9–12.4% (RR = 1.7lowest number of push-ups/highest , 95% CI: 1.6–1.7; MH 2 trend, p-value < 0.0001); female trainees = 48.8%
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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Table 1 Descriptive characteristics of male and female trainees analyzed, Army basic combat training, FY 2010 to FY 2012. Variable
Men
Women
Difference men − women (p-value)
N (%) Age Years, mean (±SD) Height Meters (m), mean (±SD) Weight Kilograms (kg), mean (±SD) Body mass index kg/m2 , mean (±SD) Push-ups Number/2 min, mean (±SD) Sit-ups Number/2 min, mean (±SD) 2 mile run time Minutes, mean (±SD) Risk (%) injured Relative risk (RR, men/women) 95% confidence Interval p-value
143,398 (77) 22.6 (4.9)
41,727 (23) 22.3 (4.8)
0.3 years (p < 0.0001)
1.76 (0.069)
1.62 (0.064)
0.14 m (p < 0.00001)
77.8 (13.2)
61.8 (8.6)
16 kg (p < 0.0001)
25.1 (3.7)
23.3 (2.7)
1.8 kg/m2 (p < 0.00001)
52.8 (13.6)
30.6 (12.2)
22.0 push-ups (p < 0.0001)
63.0 (11.7)
60.8 (12.1)
2.2 sit-ups (p < 0.0001)
14.7 (1.4)
17.7 (1.8)
−3 min (p < 0.0001)
15.7%
40.3% RR = 2.6 95% CI (2.5–2.6) p < 0.00001
Table 2 Injury risk by quintile of 2 mile run times stratified by BMI level, male Army trainees (N = 143,398). Quintiles of BMI (kg/m2 )
Lowest BMIQ1 [<21.7] BMIQ2 [21.6–23.8] BMIQ3 [23.9–25.9] BMIQ4 [26.0–28.3] Highest BMIQ5 [>28.3] Total injury risk by run time level
Quintiles (Q) of 2 mile run time (min)
Total injury risk by BMI level
Fastest RUNQ1 [<13.5] % Risk RR (CI) n
RUNQ2 [13.5–14.2] % Risk RR (CI) n
RUNQ3 [14.3–15.0] % Risk RR (CI) n
RUNQ4 [15.0–15.8] % Risk RR (CI) n
Slowest RUNQ5 [>15.8] % Risk RR (CI) n
11.1 1.3 (1.2–1.4) 7712 9.9 1.2 (1.1–1.3) 8158 8.5 1.0 referent 6779 9.1 1.1 (1.0–1.2) 4276 10.5 1.2 (1.1–1.4) 1788 9.8 1.0 28,713
14.3 1.7 (1.5–1.8) 6849 13.1 1.5 (1.4–1.7) 6830 11.6 1.4 (1.2–1.5) 6231 11.0 1.3 (1.2–1.5) 5261 12.7 1.5 (1.3–1.7) 3366 12.6 1.3 (1.2,1.3) 28,537
16.2 1.9 (1.7–2.1) 6161 15.0 1.8 (1.6–1.9) 5738 14.5 1.7 (1.5–1.9) 5873 14.1 1.7 (1.5–1.8) 5920 14.0 1.6 (1.5–1.8) 5176 14.8 1.5 (1.4,1.5) 28,868
18.7 2.2 (2.0–2.4) 4836 18.3 2.2 (1.9–2.4) 4756 16.4 1.9 (1.7–2.1) 5366 16.9 1.9 (1.8–2.2) 6495 15.9 1.9 (1.7–2.1) 7195 17.1 1.8 (1.7,1.8) 28,648
26.6 3.1 (2.8–3.4) 3157 23.6 2.8 (2.5–3.1) 3237 25.3 3.0 (2.7–3.3) 4272 22.7 2.7 (2.4–2.9) 6642 24.5 2.9 (2.6–3.1) 11,085 24.3 2.5 (2.4,2.5) 28,393
16.0 1.1 (1.1,1.3) 28,715 14.6 1.01 (1.0,1.0) 28,719 14.4 1.0 28,521 15.4 1.1 (1.1,1.1) 28,594 18.2 1.3 (1.2,1.3) 28,610 15.7 143,159
Quintile (Q) = group containing one fifth of subjects. RR = relative risk; CI = 95% confidence interval; n = sub-population group size.
to 31.6% (RR = 1.5lowest/highest , 95% CI: 1.49–1.61; MH 2 trend, p < 0.00001)]. Also as with aerobic fitness, the lowest risk (male trainees = 11.2%; female trainees = 29.2%) occurred among those with average (middle) BMI who demonstrated greatest muscular endurance fitness (i.e., did the most push-ups). However, unlike aerobic fitness, the highest risk among male trainees (22.7%) occurred among those with the highest BMI who did the least pushups (RR = 2.0, 95% CI: 1.9–2.2). Across levels of muscle endurance, the male trainees in the highest BMI quintile had the highest risk of injury. Among female trainees, results were somewhat different, as the highest risk (50%) was to women in the lowest two BMI quintiles who did the least number of push-ups (RR = 1.7 (95% CI: 1.6–1.9)). 4. Discussion This investigation took place in the U.S. Army BCT setting which provides an ideal environment to identify intrinsic risk factors for
physical training-related injuries such as level of physical fitness, body composition, and health risk behaviors. This is because all trainees of both genders perform the same types and amounts of physical and operational training, so the primary extrinsic risk factors for such injuries are controlled. In addition to controlling for physical activity patterns, sleep and nutrition are more standardized, and individuals’ demographic, fitness test scores, and medical data (ICD diagnostic codes) are available. As shown by this study, body composition alone (i.e., BMI) does not adequately reflect Army trainees’ overall risk of experiencing MSK injury. The main finding was that the highest risk of trainingrelated MSK injury occurred in male and female Army trainees who had both the lowest levels of aerobic fitness (slowest run times) and the lowest levels of BMI (men < 21.7; women ≤ 20.7). The study findings are consistent with the general body of evidence that shows the most aerobically fit U.S. Army male and female trainees are protected from training injuries.14,16–19,23,26,27 This finding has also been reported by researchers from Great Britain28
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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Table 3 Injury risk by quintile of 2 mile run times stratified by BMI level, female Army trainees (N = 41,727). Quintiles of BMI (kg/m2 )
Lowest BMI Q1 (<20.7) BMIQ2 (20.8–22.6) BMIQ3 22.6–24.2 BMIQ4 (24.3–25.6) Highest BMIQ5 (>25.6) Total injury risk by run time level
Quintiles (Q) of 2 mile run time (min)
Total injury risk by BMI level
Fastest RUNQ1 [<16.2] % Risk RR (CI) n
RUNQ2 [16.2–17.3] % Risk RR (CI) n
RUNQ3 [17.3–18.1] % Risk RR (CI) n
RUNQ4 [18.2–19.0] % Risk RR (CI) n
Slowest RUNQ5 [>19.0] % Risk RR (CI) n
29.2 1.2 (1.1–1.3) 2334 26.0 1.1 (1.0–1.2) 2141 24.6 1.0 referent 1680 26.1 1.1 (1.0–1.2) 1301 25.0 1.1 (1.0–1.2) 853 26.5 1.0 8309
38.9 1.6 (1.4–1.7) 1981 35.8 1.5 (1.3–1.6) 1828 33.9 1.4 (1.2–1.5) 1684 33.4 1.4 (1.2–1.5) 1564 30.7 1.3 (1.1–1.4) 1224 35.0 1.3 (1.3,1.4) 8281
45.3 1.8 (1.7–2.0) 1660 40.2 1.6 (1.5–1.8) 1680 38.3 1.6 (1.4–1.7) 1734 36.5 1.5 (1.4–1.7) 1793 36.5 1.5 (1.3–1.6) 1546 39.3 1.5 (1.4,1.5) 8413
49.3 2.0 (1.8–2.2) 1358 47.7 1.9 (1.7–2.1) 1427 46.6 1.9 (1.7–2.1) 1645 40.8 1.6 (1.5–1.8) 1807 40.8 1.6 (1.5–1.8) 1928 44.6 1.7 (1.6,1.8) 8165
63.1 2.6 (2.3–2.8) 934 56.0 2.3 (2.1–2.5) 1175 55.4 2.3 (2.2–2.5) 1548 56.0 2.3 (2.1–2.4) 1886 54.0 2.2 (2.0–2.4) 2728 56.0 2.1 (2.0,2.2) 8271
41.9 1.1 (1.02,1.1) 8267 39.1 1.0 (0.9,1.0) 8251 39.5 1.0 8291 39.6 1.0 (0.9,1.1) 8351 41.2 1.04 (1.01,1.08) 8279 40.3 41,439
Quintile (Q) = group containing one fifth of subjects. RR = relative risk; CI = 95% confidence interval; n = sub-population group size.
and Norway29 as well as for soldiers in U.S. operational units.30 Run times, a validated measure of aerobic (or cardiorespiratory) fitness, are consistently correlated with maximum oxygen uptake (VO2 max), and lower VO2 max.13 In addition, slower run times have been consistently associated with higher risk of military injury.8,16 The association of higher injury risk with lower aerobic fitness is probably a derivative of the high amount of exposure soldiers have to stressful weight-bearing aerobic activities inherent to Army BCT (e.g., running, road marching with loads, and walking to and from training sites).8,31 The potential for a higher BMI to have protective effect against injury hypothetically may result in part from greater absolute amounts of muscle among soldiers with higher BMIs.20 Furthermore, a high volume of aerobic training involving running and marching has been found to suppress muscular strength development that would be expected from a modest resistance exercise program.32 This suggests that military recruit training programs may not yet be achieving an optimal balance between aerobic and resistance training to produce physically resilient soldiers.33 Measures of muscle endurance such as push-ups and sit-ups have not been shown to be as strongly or as consistently associated with risk of injury as aerobic fitness measures. A number of studies have, however, shown that trainees who complete the fewest numbers of push-ups or sit-ups experience higher risk of injury than those who perform the greatest number.14,16–18,26,27 The reason these measures of muscle endurance are not as strongly associated with risk of injury during BCT as running may be that the activities requiring upper body or abdominal muscle endurance are less common or limited. Though strong correlations between push-ups and performance of common military tasks are demonstrated in a recent systematic review, the correlations are less strong than with aerobic fitness.13 In the systematic review, sit-ups were not shown to strongly correlate with performance of any common military tasks. However, sit-ups have essentially been the only metric used to measure military core muscle endurance.13 The limited relationship between sit-up and performance or injury may be because the sit-up test itself is not the most appropriate metric. The relationship between muscle strength and injury risk in military populations has not been as well studied as aerobic fitness and
muscle endurance. This is probably because it is not as easy to assess strength in large populations and it requires equipment. However, the few studies that have been performed among men in BCT suggest that strength is not associated with injury risk.16,26,27,34 Blacker et al.28 found that strength was associated with risk of injury among British recruits on a univariate analysis, but was not significant in multivariate models. It may be that the number of exposures on a daily basis to tasks requiring soldiers to exert their maximal muscle strength is limited, whereas weight-bearing aerobic activities are inescapable for soldiers and especially trainees. Future studies should examine the association of muscle strength with injury risk. The relationship between body composition, as assessed by BMI, and injury risk is more complex. Most studies of military trainees have demonstrated either a J-shaped or a bi-modal relationship between BMI and injury risk. Specifically, MSK injury risk is highest at the high and low extremes of BMI, and lowest among those with “average” BMI.7,22,24,29,30 One study of aerobic fitness and injury risk that stratified on level of BMI showed results similar to this study.1,7 Although differences were small in magnitude, this study supports the prior studies’ finding of a bi-modal curve of association of BMI with injury risk for both genders. This study also demonstrates that though aerobic fitness is perhaps more critical, lower BMI may be a greater risk than higher BMI in Army trainees of both genders. This finding may seem counterintuitive, but it could mean that men and women with higher BMIs – who maintain adequate aerobic fitness – are more musculoskeletally-resilient (i.e., are more protected from MSK injury). This may be because Army trainees with the lowest BMIs do not have enough lean body mass, and particularly muscle mass, to endure the physiologic stresses to their MSK system during Army training, so are more susceptible to injury. Those in the upper BMI extremes may also experience some additional MSK stress from carrying extra body weight (thus explaining the bimodal injury risk curve). The benefit, or protective effect, of a moderately high BMI may be related to the additional lean body mass (probably muscle).20,21 A higher muscle mass among trainees with high BMI who are also aerobically fit could protect
Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015
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them from injuries secondary to strenuous activities making them more “musculoskeletally resilient.” It is important to provide a context in which to understand the significance of the low BMI groups evaluated in this study. The lower quintiles of BMI in this study, where there was a marked increase in MSK injury risk, correspond to BMI values observed in a study of previously healthy fit men at the end of a vigorous eightweek ranger training program.35 The ranger trainees in that study had lost all of their accessible body fat stores and begun to consume muscle mass (a mean BMI of 20.3 kg/m2 at the end of training).35 It is therefore reasonable to assume that the lowest BMI values in this study (BMI < 21.7) could represent individuals with extremely low fat and also reduced muscle mass levels. Underweight or low weight U.S. Army recruits and soldiers have not received as much attention as those with high BMIs or body fat. The high obesity rates in the civilian population which is also represented in the military’s recruitment pool, has previously placed the focus on physical training as well as exercise and diet programs for those with excessive BMI and body fat. However, recruits with extremely low BMI are another high risk group that should be monitored. Strength training programs may be a means to reduce injuries in trainees and soldiers with extremely low BMIs. However, caution should be exercised with this recommendation since improperly designed strength training programs may lead to higher rates of injuries. Just as high exposure to lower body aerobic training activities (running, marching) likely contributes to the high rates of lower extremity MSK injury, an increase in exposure to strength training may increase the rates of other MSK injuries. Currently the minimum BMI calculated from the height and weight tables for retention of men and women in the Army is 18.9 kg/m2 . The maximum acceptable BMI for 17–27 year olds ranges from 25.9 to 26.5 for men and 25.0 to 25.3 for women.11 Trainees do not need to meet these standards immediately on entrance to the Army, but are expected to be able to meet the standards once they have gone to their first duty assignments. If they fail to pass the height-weight standards they have their percentage of body fat assessed by tape measurements.11 Men age 17–27 may be up to 22% body fat and women of the same age range may be up to 32% body fat.11 Given the recent data on physical fitness and body composition as assessed by BMI presented in this paper (as well as in the article J. Pierce et al. in this journal supplement), it may make sense for the Army to reconsider its standards to better balance the roles of body composition and physical fitness. 5. Conclusion Substantial evidence has demonstrated that military trainees with poor aerobic fitness have an increased risk of training-related MSK overuse injuries. This study provides additional evidence that there is a higher risk of injury to male and female trainees with the lowest BMIs and the lowest levels of aerobic fitness (measured by run times). The lowest risk of injury was found among the most aerobically fit trainees who exhibited “average” weight levels (the middle quintile of BMI) or over-average levels (BMI greater than the middle quintile). This suggests the military’s desire for lean service members should be balanced with the need to ensure adequate fitness levels and to lower injury risk. Practical implications - Higher levels of aerobic fitness and a moderate BMI can be protective against musculoskeletal (MSK) injury risk. - Employers and coaches who monitor height-weight (BMI) and physical fitness standards should balance requisite physical fitness and body fat with the need for MSK resilience.
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- Sports medicine and military personnel who recommend exercise programs should consider injury risk as well as fitness development when formulating individual physical activity and diet plans. - Given the high injury risk for Army personnel and the need to maintain high fitness levels it may be wise to for the military to re-consider its height-weight (i.e., BMI) standards. Disclaimer The views expressed in this presentation are those of the authors and do not necessarily reflect the official policy of the Department of Defense, Department of Army, or the U.S. Army Medical Department. Acknowledgments No outside funding was received for this project. The authors would like to acknowledge the assistance of Ms. Mimi Eng in making publication of this manuscript possible. References 1. Nindl BC, Jones BH, Van Arsdale SJ et al. Operational physical performance and fitness in military women: physiological, musculoskeletal injury, and optimized physical training considerations for successfully integrating women into combat-centric military occupations. Mil Med 2016; 181(Suppl. 1):50–62. 2. Department of the Army. Health of the Force Report. November 2015. Retrieved from https://www.army.mil/e2/c/downloads/419337.pdf. 3. Powell KE. Adverse events, in Physical Activity Guidelines Advisory Committee Report G10-1–G10-58, US Department of Health and Human Services (DHHS), editor, Washington, DC, Physical Activity Guidelines Advisory Committee, 2008. 4. Hauret KG, Bedno S, Loringer K et al. Epidemiology of exercise- and sportsrelated injuries in a population of young, physically active adults. Am J Sports Med 2015; 43(11):2645–2653. 5. Jones BH, Canham-Chervak M, Canada S et al. Medical surveillance of injuries in the U.S. military: descriptive epidemiology and recommendations for improvement. Am J Prev Med 2010; 38(Suppl. 3):S42–S60. 6. Molloy JM, Feltwell DN, Scott SJ et al. Physical training injuries and interventions for military recruits. Mil Med 2012; 177(5):553–558. 7. National Research Council. Assessing fitness for military enlistment: physical, medical, and mental health standards, Washington, DC, The National Academies Press, 2006. 8. Jones BH, Hauschild VD. Physical training, fitness, and injuries: lessons learned from military studies. J Strength Cond Res 2015; 29(Suppl. 11):S57–S64. 9. Department of the Army. Army physical readiness training, field manual 7-22 (FM 7-22). Washington, DC; 2012. Retrieved from armypubs.army.mil/doctrine/ DR pubs/dr a/pdf/fm7 22.pdf. 10. Department of Defense Instruction 1308.3. DoD physical fitness and body fat programs procedures. November 5, 2012. 11. Department of the Army. Army Regulation 600-9. The Army body composition program. Washington, DC; 2013. 12. Caspersen CJ, Powell KE, Christenson GM. Physical activity, exercise, and physical fitness: definitions and distinctions for health-related research. Public Health Rep 1985; 100(2):126–131. 13. Hauschild VD, DeGroot DW, Hall SM et al. Fitness tests and occupational tasks of military interest: a systematic review of correlations. Occup Environ Med 2017; 74(2):144–153. 14. Knapik JJ, Swedler DI, Grier TL et al. Injury reduction effectiveness of selecting running shoes based on plantar shape. J Strength Cond Res 2009; 23(3):685–697. 15. Rauh MJ, Macera CA, Trone DW et al. Epidemiology of stress fracture and lower-extremity overuse injury in female recruits. Med Sci Sports Exerc 2006; 38(9):1571–1577. 16. Knapik JJ, Sharp MA, Canham-Chervak M et al. Risk factors for training-related injuries among men and women in basic combat training. Med Sci Sports Exerc 2001; 33(6):946–954. 17. Gilchrist J, Jones BH, Sleet DA et al. Exercise-related injuries among women: strategies for prevention from civilian and military studies. MMWR Recomm Rep 2000; 49(RR-2):15–33. 18. Jones BH, Bovee MW, Harris 3rd JM et al. Intrinsic risk factors for exerciserelated injuries among male and female army trainees. Am J Sports Med 1993; 21(5):705–710. 19. Bell NS, Mangione TW, Hemenway D et al. High injury rates among female army trainees: a function of gender? Am J Prev Med 2000; 18(3):141–146. 20. Grier T, Canham-Chervak M, Sharp M et al. Does body mass index misclassify physically active young men? Prev Med Rep 2015; 2:483–487. 21. Friedl KE. Body composition and military performance—many things to many people. J Strength Cond Res 2012; 26(Suppl. 2):S87–S100.
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Please cite this article in press as: Jones BH, et al. Impact of physical fitness and body composition on injury risk among active young adults: A study of Army trainees. J Sci Med Sport (2017), http://dx.doi.org/10.1016/j.jsams.2017.09.015