Occuptionl Medicine Advnce Access published July 14, 2014 Occuptionl Medicine doi:10.1093/occmed/kqu062 Fitness, obesity nd risk of het illness mong rmy trinees S. A. Bedno 1, N. Urbn 2,3, M. R. Boivin 2 nd D. N. Cown 2,3 1 Deprtment of Clinicl Investigtion, Willim Beumont Army Medicl Center, El Pso, TX 79920, USA, 2 Preventive Medicine Progrm, Wlter Reed Army Institute of Reserch, Silver Spring, MD 20901, USA, 3 Mission Solutions & Services, MnTech Interntionl Corportion, Herndon, VA 22033, USA. Correspondence to: S. A. Bedno, Willim Beumont Army Medicl Center, 5005 Piedrs Street, El Pso, TX 79920, USA. Tel: +1 915-742-6208; fx: +1 915-742-1500; e-mil: sheryl..bedno.mil@mil.mil Bckground Exertionl het illness (EHI) ffects militry personnel, thletes nd occuptionl groups such s griculturl workers, despite knowledge of preventive mesures. Aims Methods Results To evlute EHI dignoses during US Army bsic trining nd its ssocitions with fitness nd body ft on entering militry service. From Februry 2005 to September 2006, US Army recruits t six different militry entrnce sttions took pre-ccession fitness test, including 5-min step test scored s pss or fil. Subsequent EHI incidence nd incidence rte rtios were nlysed with reference to subjects fitness (step test performnce) nd whether they met (weight qulified [WQ]) or exceeded body ft (EBF) stndrds. Among the 8621 WQ nd 834 EBF mle subjects, there were 67 incidents of EHI within 180 dys of entering militry service. Among WQ subjects, step test filure ws significntly ssocited with EHI (odds rtio [OR] 2.00, 95% confidence intervl [CI] 1.13, 3.53). For those pssing the step test, the risk of EHI ws significntly higher in EBF thn in WQ subjects (OR 3.98, 95% CI 2.17, 7.29). Expected ORs for the joint effects of step test filure nd EBF clssifiction under dditive nd multiplictive models were 4.98 nd 7.96, respectively. There were too few women to evlute their dt in detil. Conclusions This study demonstrted tht fitness nd body ft re independently ssocited with incident EHI, nd the effect of both ws substntilly higher. Those with low fitness levels nd/or obesity should be evluted further before engging in intense physicl ctivity, especilly in wrmer months. Key words Introduction Body ft; fitness test; het illness; militry; obesity. Despite the widespred knowledge of preventive mesures (t lest mong thletic nd militry triners), exertionl het illness (EHI) continues to ffect thletes, militry personnel nd certin groups of workers, such s griculturl workers. EHI in militry popultions cn impir opertionl rediness, especilly in trining nd combt in hot wether. Among ctive duty US militry personnel, the incidence rte of het stroke ws higher in 2012 thn in 2011, lthough the incidence of other het illness ws lower [1]. In study of US high school thletes from 2005 to 2009, there were 118 cses of het illness tht resulted in more thn 1 dy of time lost from thletic ctivity, rte of 1.6 per 100 000 thlete exposures [2]. According to Nelson et l., EHI is risk to ll physiclly ctive individuls, with the mjority of instnces occurring in those performing sports or exercising [3]. Severl risk fctors for EHI hve been identified. Obese individuls nd those who re less physiclly fit hve been found to be t greter risk [4,5]. Given the rising prevlence of obesity in the US popultion, severl studies hve demonstrted n increse in EHI mong militry personnel [6 8]. There hve been few published studies where obesity hs been described s risk fctor in het illness mong thletes, lthough in one study of high school thlete obesity ws found to be fctor in the onset of EHI [9]. In study of Mrine recruits, the gretest risk of EHI ws found mong men with the highest body mss index (BMI) [6]. In 2010, we published prospective study of EHI mong mle US Army trinees in the first 90 dys of Published by Oxford University Press on behlf of the Society of Occuptionl Medicine 2014. This work is written by () US Government employee(s) nd is in the public domin in the US.
Pge 2 of 7 OCCUPATIONAL MEDICINE militry service [10]. Since then, we hve followed the subjects for further 90 dys, cquiring new dt not previously considered, nd hve included some informtion on women. The dditionl dt include newly identified cses nd mesured fitness (bsed on 5-min step test) prior to militry entry. We hve conducted number of dditionl nlyses, including comprisons between fit nd unfit weight-qulified individuls nd comprisons between fit weight-qulified recruits nd fit recruits with excess body ft [11 16]. These nlyses provide substntil new insight regrding rtes of nd risk fctors for het illness. Methods These nlyses were bsed on dt from the Assessment of Recruit Motivtion nd Strength (ARMS) study. Study subjects included ll men nd women enlisting in the US Army for the first time between Februry 2005 nd September 2006 t six Militry Entrnce Processing Sttions (MEPS). Additionl detils on the study methods hve been published elsewhere [10 17]. Subjects were followed for 180 dys fter entry. This study ws pproved by the Wlter Reed Army Institute of Reserch Institutionl Review Bord. Subjects were ged 18 or more nd provided written informed consent for completing questionnire nd for medicl nd dministrtive follow-up. Individuls with no vlid weight, height or dte of birth recorded were excluded from nlysis (n = 7). Individuls with missing mbultory helth cre record or no mtching ccession dte within 30 dys of the ARMS study entry dte (n = 62) nd one with seprtion dte preceding ccession were lso excluded. Becuse few cses of EHI occurred mong women only summry dt re provided. Everyone entering the US Army through ny of the study sites ws required to tke pre-ccession physicl fitness test (ARMS test). The fitness test of interest in this study involved 5-min step test set by metronome t pce of 120 steps per minute, with step height of 12 inches. The ARMS test is fully described elsewhere [10 16]. Those who exceeded body ft (EBF) percent stndrds were required to pss the physicl fitness test in order to enter the Army under n ARMS wiver, wheres weightqulified (WQ) study subjects were permitted to enter irrespective of their step test performnce. Therefore, the study included three groups of subjects: fit WQ, unfit WQ nd fit EBF. The referent group for ll comprisons ws those who were both fit nd WQ. Due to ethicl concerns bout risks to those who were both unfit nd EBF, those who filed the step test nd did not meet body ft requirement were not provided with n ARMS wiver. Subjects were mtched to ccession dt provided by the Center for Accession Reserch, US Army Accession Commnd. The US Militry Entrnce Processing Commnd (US MEPCOM) nd the Defense Mnpower Dt Center (DMDC) provided Militry Occuptionl Specilty (MOS) codes. The Ptient Administrtion Systems nd Biosttistics Activity provided mbultory helth cre encounter dt from the Stndrd Ambultory Dt Record (SADR) nd inptient dt from the Stndrd Inptient Dt Record (SIDR). EHI incident cses were defined s t lest one mbultory encounter resulting in dignosis of het stroke (992.0), het exhustion (992.3 5), het syncope (992.1), het crmps (992.2), het ftigue, trnsient (992.6), het oedem (992.7), other specified het effects (992.8) nd unspecified effects of het nd light (992.9) entered in ny dignosis position. Inptient medicl records were lso serched with the sme dignostic criteri. Rhbdomyolysis ws excluded from the cse definition becuse, while it cn be het-relted, other cuses not ssocited with het exposure lso occur [10]. The independent vribles of interest were performnce on the step test portion of the screening fitness test (pss/fil, described s fit/unfit herefter), ARMS wiver sttus (WQ/EBF), ge (18 19, 20 24 nd 25 yers), smoking history (ever/never), BMI (underweight [ 18.5 kg/ m 2 ], norml weight [18.5 25.0 kg/m 2 ], overweight [25.0 29.9 kg/m 2 ], obese [ 30.0 kg/m 2 ]), rce (blck, white or other), nd MOS ctegory (combt support/combt support services, combt rms, nd other/missing). Becuse BMI is highly correlted with receiving n ARMS wiver, BMI ws not considered in models compring fit EBF to fit WQ. The primry predictors of interest were performnce on the step test, scored s pss/fil nd ARMS wiver sttus (yes/no). For nlyses evluting step test performnce s predictor of het illness dignosis, only those meeting weight for height or body ft stndrds were included. For nlyses compring the incidence of het illness dignosis between EBF nd WQ subjects, only those individuls pssing the step test were considered in the nlyses. To explore the potentil joint effects of EBF nd being unfit, we exmined dditive (independent effect) nd multiplictive (effect modifiction) models to estimte the expected effects of filing the step test nd being EBF, using the formule (RR step test + RR EBF 1) for n dditive model, nd (RR step test RR EBF ) for multiplictive model[18]. Chi-squre nd Fishers exct tests were used to nlyze ctegoricl dt. Logistic regression ws used to clculte the crude nd djusted odds rtios (cor nd OR) of het illness in the first 6 months of service. We lso conducted Poisson regression to estimte the incidence (using person-time t risk) nd the incidence rte rtios. Becuse the results were the sme to the second deciml point s the OR, for convenience, we only report the OR. ORs re reported with their 95% confidence intervl (CI). Vribles evluted included step test sttus, ARMS sttus, ge, smoking history, BMI, rce nd MOS ctegory. For multivrite models, prsimonious models were developed using bckwrds stepwise elimintion in which ll vribles were entered into the model nd then sequentilly removed until only the primry predictor of
S. A. BEDNO ET AL.: FITNESS, OBESITY AND RISK OF HEAT ILLNESS AMONG ARMY TRAINEES Pge 3 of 7 interest nd covrites with P vlue of 0.05 remined in the model. Becuse the physicl requirements for combt occuptions re generlly higher thn for noncombt occuptions, MOS ws retined in ll models regrdless of sttisticl significnce. Additionlly, ech covrite ws entered into seprte regression to ssess its impct on the primry predictor. However, including ech covrite in the model filed to produce t lest 5% chnge in the OR of the min exposure outcome ssocition. All sttisticl nlyses were performed using SAS version 9.3 (SAS Institute, Cry, NC). Results There were 8621 WQ nd 834 EBF mle study prticipnts (Tble 1). Among WQ prticipnts, ll reported nd mesured chrcteristics differed significntly between fit nd unfit subjects. Among the fit, prticipnts smoking history, BMI, rce nd MOS ctegory differed significntly between groups. EHI episodes were identified in 67 men nd 13 women (from totl of 1913 women). All incident het illness episodes occurred between April nd October including nerly two-thirds in July nd August. Tble 2 summrizes numbers nd percentges for specific het illness dignostic ctegories mong men. There were 32 cses of EHI mong the fit WQ group (0.5%), Tble 1. Chrcteristics of mle study prticipnts (N = 9455) WQ: pssed step test N = 6645 19 mong the unfit WQ group (1.0%) nd 16 mong the fit EBF group (1.9%). Among women, there were too few to ctegorize into meningful groups (results not shown). As Tble 3 shows, mong mle WQ subjects, being unfit ws significntly ssocited with EHI dignosis in the first 6 months of service (OR 2.00 [1.13, 3.53]). No other vribles were predictive when entered in the multivrible model (P > 0.05). The cor for unfit women ws 1.69 (0.52, 5.58) (dt not shown). Tble 4 shows tht mong fit subjects, the risk of EHI in EBF ws significntly higher thn in WQ individuls (cor 4.04, [2.21 7.40]). None of the mesured demogrphic fctors cptured were significntly ssocited with het illness. Compred with subjects ssigned combt support service MOS, those with combt rms MOS hd higher risk of EHI in the first 6 months of service (cor 1.90 [1.06, 3.39]). Adjusting for MOS ctegory hd no pprecible effect on the OR for EHI compring EBF with WQ (OR 3.98 [2.17, 7.29]). There were no significnt ssocitions between EHI nd ge, smoking history or rce. The cor for EBF women ws 1.12 (0.22, 5.56) (dt not shown). The expected ORs for the joint effects of being unfit nd EBF under the dditive nd multiplictive models were 4.98 nd 7.96, respectively (Tble 5). Although the dt were sprse, we were ble to compre EHI mong women to men. Overll, the cor for WQ: filed step test N = 1976 n (%) n (%) n (%) P EBF: pssed step test N = 834 Age (yers) 18 19 3080 (46) 804 (41) <0.001 365 (44) NS 20 24 2754 (41) 875 (44) 379 (45) 25 811 (12) 297 (15) 90 (11) Smoker c No 4804 (73) 1468 (74) NS 635 (77) <0.05 Yes 1745 (27) 506 (26) 193 (23) BMI Underweight (x 18.5) 234 (4) 55 (3) <0.001 0 (0) <0.001 Norml weight (18.5< x 25) 3766 (57) 808 (41) 6 (1) Overweight (25< x <30) 1861 (28) 733 (37) 111 (13) Obese (x 30) 784 (12) 380 (19) 717 (86) Rce White 4863 (73) 1352 (68) <0.001 618 (74) <0.001 Blck 774 (12) 274 (14) 62 (7) Other 1008 (15) 350 (18) 154 (18) MOS CS/CSS 3559 (54) 1168 (59) <0.001 467 (56) <0.001 Combt rms 2888 (43) 768 (39) 362 (43) Other/missing 198 (3) 40 (2) 5 (1) P b CS, Combt Support; CSS; Combt Service Support; NS, non-significnt. Comprisons mde between WQ step test pssers nd WQ step test filures. b Comprisons mde between WQ step test pssers nd EBF step test pssers. c n = 104 missing smoking vlues not included in clcultion.
Pge 4 of 7 OCCUPATIONAL MEDICINE Tble 2. Frequency of het illness by step test sttus nd ARMS wiver sttus mong mle ARMS subjects WQ: pssed step test N = 6645 WQ: filed step test N = 1976 n (%) n (%) n (%) P EBF: pssed step test N = 834 P b Het illness ctegory c Het stroke 2 (0) 0 (0) 1.00 4 (0) <0.01 Het exhustion 17 (0) 10 (1) <0.01 7 (1) <0.01 Other het illness 20 (0) 12 (1) <0.05 12 (1) <0.001 All het illness 32 (0) 19 (1) <0.05 16 (2) <0.001 Comprisons mde between WQ step test pssers nd WQ step test filures. b Comprisons mde between WQ step test pssers nd EBF step test pssers. c Ctegories not mutully exclusive. Tble 3. Crude nd djusted ORs for het illness dignosis mong mle weight-qulified subjects (N = 8621) No het illness N = 8570 n (%) n (%) One or more het illness episode N = 51 Crude OR P vlue 95% CI Adjusted OR P vlue 95% CI Step test sttus Pss 6613 (99.5) 32 (0.5) Ref. Ref. Fil 1957 (99.0) 19 (1.0) 2.01 <0.05 1.14, 3.55 2.00 <0.05 1.13, 3.53 Age (yers) 18 19 3863 (99.5) 21 (0.5) Ref. 20 24 3604 (99.3) 25 (0.7) 1.28 0.71, 2.28 25 1103 (99.5) 5 (0.5) 0.83 0.31 2.22 Smoker No 6232 (99.4) 40 (0.6) Ref. Yes 2240 (99.5) 11 (0.5) 0.77 0.39, 1.49 BMI Underweight (x < 18.5) 286 (99.0) 3 (1.0) 1.91 0.57, 6.36 Norml weight (18.5 < x 25) 4549 (99.5) 25 (0.5) Ref. Overweight (25 < x <30) 2583 (99.6) 11 (0.4) 0.78 0.38, 1.58 Obese (x 30) 1152 (99.0) 12 (1.0) 1.90 0.95, 3.78 Rce White 6182 (99.5) 33 (0.5) Ref. Blck 1042 (99.4) 6 (0.6) 1.08 0.45, 2.58 Other 1346 (99.1) 12 (0.9) 1.67 0.86, 3.24 MOS CS/CSS 4699 (99.4) 28 (0.6) Ref. Ref. Combt rms 3633 (99.4) 23 (0.6) 1.06 0.61, 1.85 1.10 0.63, 1.91 Other/missing 238 (100.0) 0 (0.0) Adjusted for MOS women ws 1.04 (0.57, 1.89); mong WQ fit women, it ws 0.86 (0.36, 2.05); mong WQ unfit, it ws 1.01 (0.38, 2.73), nd mong EBF fit, it ws 3.10 (0.71, 13.56). Few cses of hospitliztion resulting in n EHI dignosis were identified (N = 10). All subjects with inptient dignoses for EHI were lso identified s outptient EHI cses included in this study. Discussion This study found tht being unfit, s mesured by the step test, ws n importnt risk fctor for het illness in mle US Army recruits with n OR of 2.00. Other fctors, including ge t ccession, rce, smoking history, BMI nd militry occuption, were not significntly ssocited with het illness. The OR for the obese BMI ctegory ws not sttisticlly significntly rised. It should be noted tht none of the WQ individuls EBF percent limits, nd BMI cn be n inccurte indictor of body ft, especilly mong fit young men [19]. Although we hve found tht older ge t ccession is ssocited with risk of musculoskeletl injury [12 15], we found no significnt ssocitions between ge nd het illness.
S. A. BEDNO ET AL.: FITNESS, OBESITY AND RISK OF HEAT ILLNESS AMONG ARMY TRAINEES Pge 5 of 7 Tble 4. Crude nd djusted ORs for het illness dignosis mong mle subjects who pssed the ARMS test (N = 7479) No het illness N = 7431 One or more het illness episode N = 48 Crude OR P vlue 95% CI Adjusted OR P vlue 95% CI n (%) n (%) ARMS wiver sttus WQ 6613 (99.5) 32 (0.5) Ref. Ref. EBF 818 (98.1) 16 (1.9) 4.04 <0.001 2.21, 7.40 3.98 <0.001 2.17, 7.29 Age (yers) 18 19 3421 (99.3) 24 (0.7) Ref. 20 24 3112 (99.3) 21 (0.7) 0.96 0.53, 1.73 25 898 (99.7) 3 (0.3) 0.48 0.14, 1.59 Smoker No 5402 (99.3) 37 (0.7) Ref. Yes 1927 (99.4) 11 (0.6) 0.83 0.42, 1.64 Rce White 5448 (99.4) 33 (0.6) Ref. Blck 833 (99.6) 3 (0.4) 0.60 0.18, 1.94 Other 1150 (99.0) 12 (1.0) 1.72 0.89, 3.35 MOS CS/CSS 4007 (99.5) 19 (0.5) Ref. Ref. Combt rms 3221 (99.1) 29 (0.9) 1.90 <0.05 1.06, 3.39 1.92 <0.05 1.08, 3.44 Other/missing 203 (100.0) 0 (0.0) Adjusted for MOS. Tble 5. Expected OR of joint effects for being unfit nd hving excess body ft for multiplictive nd dditive models Fitness sttus Arms wiver sttus Multiplictive model OR Additive model OR Fit WQ 1 (ref.) 1 (ref.) Fit EBF 3.98 3.98 Unfit WQ 2 2 Unfit EBF 7.96 4.98 Among fit men, obesity ws strongly ssocited with het illness, with n OR = 3.98. This finding is supported by the literture, indicting tht overweight people re t incresed risk of EHI [6,7]. Apprently, this increse in risk exists even mong fit young men. Although dt were lcking regrding the risk mong those who were both EBF nd unfit, the expected reltive risks ssuming both n dditive (independent) nd multiplictive (single or joint effect modifiction) reltionship between EBF nd lck of fitness were substntil. Given the 5- to 8-fold incresed expected OR for the joint effects of low fitness nd EBF, it ws pproprite to not include those pplicnts in the study. Occuption ws lso significntly ssocited with het illness, s those with combt MOS (which includes prticulrly physiclly demnding jobs such s infntry, rmour, combt engineers nd field rtillery) hd n OR = 1.92. This is biologiclly plusible s the trining requirements for these jobs re prticulrly strenuous nd their period of intense trining is often longer thn for those entering support militry occuptions. Although no ssocition with sex ws observed, the sprse dt nd subsequent low sttisticl power prevents us reching ny conclusions bout EHI risks in women. The strengths of this study include its prospective design nd the lrge popultion studied. In ddition, informtion not usully gthered on US Army recruits ws cptured, including n objective mesure of fitness nd history of smoking. Becuse this study ws of n opertionl test progrmme, individuls who normlly would hve been disqulified from Army service due to excess body ft were included if they pssed the fitness test. This EBF study group is of specil vlue s it llowed the identifiction of n importnt risk fctor. The weknesses of this study include the reltively few numbers of EHI cses identified, especilly mong demogrphic subgroups nd prticulrly mong women. This results in low power to detect potentilly importnt risk fctors nd llows for devitions from expected ssocitions to be explined by rndom fluctutions in events. This low power is reflected in mny of the wide CIs reported. In ddition, no morbidly obese individuls were included, nor ny who EBF limits nd who could
Pge 6 of 7 OCCUPATIONAL MEDICINE not pss the fitness test. These exclusions preclude studying the effects mong two groups probbly t higher risk of het illness. It is likely tht not ll EHI episodes, prticulrly mild events, resulted in medicl encounters or lost duty time. The mgnitude of this probble underreporting, nd ny resulting bises in our findings, cnnot be quntified. However, since it is likely tht ll serious cses of EHI re cptured, these findings re likely to be vlid for those events of most concern. The ARMS Progrm ws terminted in 2009 nd therefore individuls who exceed body ft stndrds nd presumbly would be t incresed risk of EHI re no longer llowed to join the Army. At present, there re no forml physicl fitness stndrds or pre-ccession tests of fitness for US Army pplicnts. After further evlution of the fitness test nd its reltionship to vrious dverse endpoints [10 16,20], including cost-effectiveness nlyses, considertion my be given to requiring demonstrtion of fitness prior to enlistment to reduce the risk of EHI s well s musculoskeletl injuries, stress frctures [13,14] nd psychitric disorders [16]. Our findings re relevnt to the generl US popultion, especilly mles. Those who re unfit or hve sedentry lifestyle should exercise cution when beginning ny strenuous ctivity progrmme, prticulrly during wrmer wether. The sme dvice lso holds for those who re obese, regrdless of degree of fitness. Those who re both unfit nd obese my be t even greter risk thn either of the two risk groups we evluted. Key points Among fit mle US Army trinees, the risk of exertionl het illness ws significntly higher in those who exceeded body ft stndrds compred with those who were weight qulified. Among those who were weight qulified, filing step test of fitness ws ssocited with n incresed risk of exertionl het illness. There ws 5- to 8-fold increse in expected odds of exertionl het illness for the joint effects of low fitness nd exceeding body ft stndrds. Funding United Sttes Army Accession Commnd. Acknowledgements The views expressed re those of the uthors nd should not be construed to represent the positions of the Deprtment of the Army or Deprtment of Defense. Conflicts of interest None declred. References 1. Armed Forces Helth Surveillnce Center. Updte: het injuries, ctive component, U.S. Armed Forces, 2012. Medicl Surveillnce Monthly Report 2013;20:17 20. 2. Yrd EE, Gilchrist J, Hileyesus T et l. Het illness mong high school thletes United Sttes, 2005 2009. J Sfety Res 2010;41:471 474. 3. Nelson NG, Collins CL, Comstock RD, McKenzie LB. Exertionl het-relted injuries treted in emergency deprtments in the U.S., 1997 2006. Am J Prev Med 2011;40:54 60. 4. Howe AS, Boden BP. Het-relted illness in thletes. Am J Sports Med 2007;35:1384 1395. 5. Coris EE, Rmirez AM, Vn Durme DJ. Het illness in thletes. Sports Med 2004;34:9 16. 6. Grdner JW, Krk JA, Krnei K et l. Risk fctors predicting exertionl het illness in mle Mrine Corps recruits. Med Sci Sports Exerc 1996;28:939 944. 7. Wllce RF, Kriebel D, Punnett L et l. Risk fctors for recruit exertionl het illness by gender nd trining period. Avit Spce Environ Med 2006;77:415 421. 8. Epstein Y, Morn DS, Shpiro Y, Sohr E, Shemer J. Exertionl het stroke: cse series. Med Sci Sports Exerc 1999;31:224 228. 9. Kerr ZY, Cs DJ, Mrshll SW, Comstock RD. Epidemiology of exertionl het illness mong U.S. high school thletes. Am J Prev Med 2013;44:8 14. 10. Bedno SA, Li Y, Hn W et l. Exertionl het illness mong overweight U.S. Army recruits in bsic trining. Avit Spce Environ Med 2010;81:107 111. 11. Niebuhr DW, Scott CT, Li Y, Bedno SA, Hn W, Powers TE. Preccession fitness nd body composition s predictors of ttrition in U.S. Army recruits. Mil Med 2009;174:695 701. 12. Niebuhr DW, Scott CT, Powers TE et l. Assessment of Recruit Motivtion nd Strength study: preccession physicl fitness ssessment predicts erly ttrition. Mil Med 2008;173:555 562. 13. Bedno SA, Cown DN, Urbn N, Niebuhr DW. Effect of pre-ccession physicl fitness on trining injuries mong US Army recruits. Work 2013;44:509 515. 14. Cown DN, Bedno SA, Urbn N, Lee DS, Niebuhr DW. Step test performnce nd risk of stress frctures mong femle rmy trinees. Am J Prev Med 2012;42:620 624. 15. Cown DN, Bedno SA, Urbn N, Yi B, Niebuhr DW. Musculoskeletl injuries mong overweight rmy trinees: incidence nd helth cre utiliztion. Occup Med (Lond) 2011;61:247 252. 16. Gubt ME, Urbn N, Cown DN, Niebuhr DW. A prospective study of physicl fitness, obesity, nd the subsequent risk of mentl disorders mong helthy young dults in rmy trining. J Psychosom Res 2013;75:43 48. 17. Gubt ME, Cown DN, Bedno SA, Urbn N, Niebuhr DW. Self-reported physicl ctivity nd preccession fitness testing in U.S. Army pplicnts. Mil Med 2011;176:922 925. 18. De Gonzlez AB. Additive nd multiplictive models for the joint effect of two risk fctors. Biosttistics 2005;6:1 9.
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