1 Journal of Physical Activity and Health, 2011, 8, Human Kinetics, Inc. Association Between Ambulatory Physical Activity and Injuries During United States Army Basic Combat Training Joseph J. Knapik, Keith G. Hauret, Sara Canada, Roberto Marin, and Bruce Jones Background: Associations between physical and injuries have been previously examined using selfreports. The present investigation examined this association using objective measures of and injury. Methods: To quantify ambulatory, pedometers were worn daily by recruits in 10 Army Basic Combat Training companies during the 9-week training cycle. Injuries were obtained from a medical surveillance system, defined as traumatic or overuse events resulting in a medical care provider visit. A daily questionnaire documented whether or not recruits wore the pedometers and trained with their companies for the entire day. Results: Training companies were categorized by level into 3 groups with higher (HA, 17,948 ± 550 steps/day), 4 with moderate (MA, 16,346 ± 768 steps/day) and 3 with lower (LA, 14,772 ± 400 steps/day). Among men, the MA and HA groups were at 1.52 (95% confidence interval [95% CI] = ) and 1.94 (95% CI = ) times higher injury risk, respectively, compared with the LA group. Among women, the MA and HA groups were at 1.36 (95% CI = ) and 1.53 (95% CI = ) times higher injury risk, respectively, compared with low LA group. The relationships remained significant after considering physical characteristics and physical fitness. Conclusions: In consonance with previous self-report studies, higher physical was associated with higher injury risk. Keywords: pedometer, fitness, body mass index The health benefits of physical have been well described and documented. Regular physical reduced morbidity associated with cardiovascular disease, coronary heart disease, hypertension, obesity, Type 2 diabetes, colon cancer, osteoporosis, depression, and anxiety. 1 3 All cause mortality risk is also lower among those who are physically active. 3,4 Despite these positive associations, there are risks associated with physical, the most common being injuries. Among long distance runners the incidence of lower extremity running injuries has ranged from 19% to 79% of those participating. 5 National United States (US) surveys of sports and recreation injuries requiring medical attention found yearly incidences of 2.6% to 2.9% with an estimated 7 million individuals experiencing these traumatic events. 6,7 A national German survey found that the yearly incidence of sports injuries among adults was 3.1% involving an estimated 2 million people per year and making it the second leading cause of injury after domestic accidents (3.7%). 8 Knapik, Hauret, and Jones are with the Dept of Injury Prevention, US Army Public Health Command (Provisional), Aberdeen Proving Ground, MD. Canada is with the Physician Assistant Program, Towson University, Towson, MD. Marin is with the Dept of Occupational Medicine, Womack Army Medical Center, Fort Bragg, NC. Previous studies have shown that as the amount of physical increases, so does injury incidence. Studies have been conducted largely among runners 9 17 but also among athletic club members, 18 military recruits, 19,20 combinations of runners and walkers and participants in sports and other leisure-time. 7,8 Most of these investigations have used self-reports of injuries and physical, although a few studies have documented injuries from medical records 19,20 and obtained physical from training logbooks. 15,19,20 The purpose of the present project was to examine the association between objectively measured physical and medically diagnosed injuries in Basic Combat Training (BCT). Ambulatory was measured with pedometers and injuries requiring visits to medical care providers were obtained from a medical surveillance system. Descriptive data on ambulatory in BCT has been previously reported. 25 BCT is a physically active, 9-week training program designed to physically condition new Army recruits and introduce them to basic soldiering skills. Vigorous physical training is conducted 3 to 6 times per week and other physical activities are performed such as negotiating obstacle courses, high tower operations, rifle/bayonet training, road marches with rucksacks and load-bearing equipment, drill and ceremony, assault courses, land navigation, infiltration exercises, and other activities. 496
2 Physical Activity and Injury 497 Procedures Methods On the first or second day of BCT, recruits in each of 10 BCT companies were briefed on the purposes and risks of the project. Eight recruits in each company were provided detailed instructions on how to wear the pedometers (Yamax Digi-Walker Model SW-200, New-Lifestyles, Inc., Lee s Summit, MO). Selected recruits were instructed to wear the pedometer on their hip, either on the belt of their Battle Dress Uniforms (BDUs), the waistband of their physical training outfit or the pistol belt of the load bearing equipment. A cord was used to further secure the pedometer to the belt loop of the uniforms, and recruits were instructed to put the pedometer back on their belt or waistband if they saw that it had slipped off during training. Although 8 recruits were provided instructions on how to use the pedometer, on any given day of the 9-week BCT cycle, only 4 recruits in each company were selected to wear the pedometer for the entire day. The mean number of steps taken by the 4 recruits in each company was treated as data reflective of the amount of a company s ambulatory. In BCT, all members of single company participate in training activities together. In addition, they consume meals in the same dining facility and live together in the same barracks areas. Thus, the ambulatory of any single recruit could be considered representative of the of the entire company. There were occasions when 1 or more recruits selected to wear a pedometer did not train with their companies. These situations included visits to medical care providers, kitchen police (KP), or special activities (details). These instances were identified by questionnaire (as described below) and the data for that recruit was not included in the company data for that day as described previously. 25 Every morning shortly before the first formation, members of the project team exchanged the pedometer worn on the previous day with a new one. The project team members read and recorded the data from the pedometer worn the previous day. Besides exchanging the pedometers, the recruits filled out a very short questionnaire. The questionnaire asked 1) Did you do something other than train with your company yesterday? (yes/no response), 2) If you did do something other than train with your company yesterday, what did you do? (open ended response), 3) Did you wear the pedometer all day yesterday? (yes/no response), and 4) Did you travel to a training area by motor vehicle yesterday? (yes/no response). Evaluators had daily contact with the recruits to handle pedometer problems and to answer questions. All pedometers were tested before the project by walking 100 steps twice on a level surface. All pedometers were found to record the steps to within ±2%, with the exception of 1 pedometer that was not used. To estimate distances traveled from recorded pedometer steps, each participant s step length (toe of one boot or shoe to toe of the other boot or shoe) was measured during the briefing. Participants were asked to walk 10 steps at a normal pace. The total distance of the 10 steps was obtained with a tape measure and divided by 10 to obtain an average single step length. Physical Characteristics, Physical Fitness, and Attrition All information other than the pedometer data came from existing administrative data sources. Age and gender were obtained from records in the training companies. Height and weight were obtained from a database called the Reception Battalion Automated Support System (RECBASS). Body mass index (BMI) was calculated as body weight/height height. 26 An initial physical fitness test (IPFT) was administered within 3 days of arrival at the BCT company. The IPFT consisted of the maximum numbers of push-ups completed in 1 minute, the maximum number of situps completed in 1 minute, and a 1-mile run. A final Army Physical Fitness Test (APFT) was administered on the seventh week of the 9-week BCT cycle. The APFT consisted of the maximum numbers of pushups completed in 2 minutes, the maximum number of sit-ups completed in 2 minutes, and a 2-mile run. For the push-ups, a recruit was required to lower his or her body in a generally straight line to a point where his or her upper arm was parallel to the ground, and then return to the starting point with elbows fully extended. For sit-ups, the recruit s knees were bent at a 90 angle, fingers were interlocked behind the head, and a second person held the participant s ankles, keeping his or her heels firmly on the ground. The recruit raised his upper body to a vertical position so that the base of the neck was anterior to the base of the spine and then returned to the starting position. The number of push-ups and sit-ups that were successfully completed within the time period were recorded. For the run, time to complete the distance was the performance measure. Time between the 3 events was no less than 10 min. Attrition was defined as leaving one s BCT company before completion of the 9-week training cycle. Recruits attriting were obtained from a company-level database called Warrior Training Room. Recruits could fail to complete training because of discharge or recycling. A discharged recruit was one who was not suitable for service in the Army and was formally released from his or her service commitment before completing BCT. A recycled recruit was one who was reassigned to another training unit when additional training was needed for that recruit to meet graduation requirements.
3 498 Knapik et al Injury Data The Defense Medical Surveillance System (DMSS) regularly incorporates data on ambulatory (outpatient) encounters that occur within military treatment facilities, as well as those that occur outside the medical treatment facilities but are paid for by the Department of Defense. 27 The DMSS provided visit dates and diagnostic codes from the International Classification of Diseases, Revision 9, Clinical Modification (ICD-9-CM) for all outpatient medical visits within the BCT timeframe for all recruits in the 10 companies. The first 4 listed diagnoses for each visit were considered, although a single visit usually resulted in only 1 diagnosis. Specific ICD-9-CM diagnostic codes comprising the Overuse Injury Index (OII), and the Comprehensive Injury Index (CII) were selected to indicate injury events. These indices have been described previously in detail. 28 The OII specifically captures the subset of ICD-9-CM codes involving physical damage to the body presumably due to repetitive microtrauma. These include such diagnoses as stress fractures, stress reactions, tendonitis, bursitis, fasciitis, arthralgias, neuropathies, radiculopathies, shin splints, and synovitis. The CII captures all ICD-9-CM codes related to physical damage to the body as a result of energy exchanges, 29 both traumatic and overuse. Data Analysis Since the pedometers were given to recruits on the first or second day of training and these days contained only partial data, only pedometer readings on the third and subsequent days were considered in the analysis for a total of 61 surveys. Pedometer readings were not included in the data if 1) the recruit did not train with their company during the entire day (as indicated by the questionnaire responses), 2) the recruit forgot to wear the pedometer at any point in the day (as indicated by questionnaire responses), 3) the pedometer was lost or broken, 4) a single reading differed by a factor of 2 from other pedometer readings in that company on a particular day as previously described. 25 To obtain estimated distance, the number of steps was multiplied by average step length obtained for each recruit. Cumulative injury incidence (%) was calculated as follows: recruits with 1 or more injuries/all recruits 100%. Injury incidence rates (injuries/1000 persondays) were calculated as follows: recruits with 1 or more injuries/total time in company for all recruits The 10 companies were reduced to 3 groups for analysis by combining the 3 companies with the greatest number of steps (higher or HA), the 3 companies with the lowest number of steps (lower or LA) and the 4 companies between these (moderate or MA). Attrition incidence was compared among the 3 groups using the chi-square statistic. A 1-way analysis of variance was used to compare overall group differences in steps, distances, physical characteristics and IPFT data; the Tukey test was used to isolate differences between groups. If there were no significant differences on the IPFT (P <.05), a 1-way analysis of variance was used to compare overall group differences on the APFT (final physical fitness test) and the Tukey test was used to isolate differences between groups. If there were significant differences between groups, analysis of covariance was used to compare group differences on the APFT. Cox regression (survival analysis) was used to examine group differences in time to first injury with days of training as the time scale. Univariate Cox regression involved groups (HA, MA, LA) as the only independent variable. The LA group was defined as the reference group (hazard ratio = 1.00) and simple contrasts were made between the LA versus the MA and HA groups. Multivariate Cox regression included any prospectively measured variable that differed among the 3 groups in the chi-square or t test analyses. For each Cox regression analysis, once a recruit had an injury, his or her contribution to time in BCT was terminated. Those attriting from BCT had their times terminated at the day they left the unit. Results There were 1174 men and 898 women in the 10 BCT companies. Cumulative CII incidence was 27.1% and 50.2% for men and women, respectively. Cumulative OII incidence for the OII was 18.7% and 40.0% for men and women, respectively. CII incidence rates were 4.5/1000 person-days and 8.7/1000 person-days for men and women, respectively. OII incidence rates were 3.1/1000 person-days and 6.9/1000 person-days for men and women, respectively. Male attrition was 14.5%, 13.8%, and 13.6% for the HA, MA and LA groups, respectively (P =.96). Female attrition was 23.5%, 24.2%, and 28.3% for the HA, MA, and LA groups, respectively (P =.37). Table 1 shows the descriptive statistics on the number of steps and distances for the 3 groups. The Tukey test indicated that all 3 groups differed from each other on the number of steps (P <.01). For the distance estimate, the Tukey Test indicated that LA group differed from both the HA and MA groups (P <.01) but there was little difference between the HA and MA groups (P =.47). Table 2 shows the comparison of the groups on the physical characteristics and physical fitness. Among the 3 groups of men, there was little difference in any of the measures. Among the women, there were no differences in the physical characteristics; however, the 3 groups of women differed on all 3 of the physical fitness measures. Table 3 shows the results of the APFT (final fitness test) by group. For the men, the 3 groups differed by an average of no more than 1 repetition on the pushups and sit-ups and 0.1 min on the 2-mile run. Since the women had significant group differences on all 3 IPFT
4 Physical Activity and Injury 499 Table 1 Descriptive Data (Mean ± SD) on Steps and Distances Among the 3 Activity Groups Variable Higher Moderate Lower P Steps (n/day) 17,948 ± ,346 ± ,772 ± 400 <0.01 Distance (km/day) 12.6 ± ± ± Table 2 Comparison of Activity Groups on Physical Characteristics and Physical Fitness Men Women Higher Moderate Lower Higher Moderate Lower Variable (n = 307) (n = 411) (n = 456) P (n = 313) (n = 384) (n = 201) P Age (yrs) 22 ± 4 22 ± 4 22 ± ± 4 22 ± 4 22 ± Height (cm) 177 ± ± ± ± ± ± Weight (kg) 79 ± ± ± ± ± ± Body Mass Index (kg/m 2 ) 25.1 ± ± ± ± ± ± Push-Ups (n) 29 ± ± ± ± 7 a 10 ± 10 b 10 ± 9 b <0.01 Sit-Ups (n) 31 ± 7 31 ± 7 31 ± ± 8 b 23 ± 10 a 25 ± 9 b Mile Run (min) 8.4 ± ± ± ± 2.0 b 10.7 ± 1.6 b 10.2 ± 1.4 a <0.01 a,b Different subscripts indicate differences between groups by Tukey test (P <.05). Table 3 Army Physical Fitness Test Scores (Final Physical Fitness Test) Gender Group a Push-ups (n) Sit-ups (n) 2-Mile run (min) Men HA 47 ± ± ± 1.3 MA 47 ± ± ± 1.3 LA 47 ± ± ± 1.4 P Women (adjusted means) HA 25 ± 11 a 58 ± 11 b 17.9 ± 1.9 MA 23 ± 11 b 61 ± 12 a 18.1 ± 1.9 LA 25 ± 11 a 59 ± 11 b 18.2 ± 1.7 P 0.02 < Abbreviations: HA, higher ; MA, moderate ; LA, lower. a,b Different letters indicate differences between groups (P <.05). events, the APFT data were analyzed with analysis of covariance using the IPFT scores as covariates. Table 3 presents the adjusted means. On push-ups, the LA and HA women had significantly higher performance than the MA women but the average difference was only 2 repetitions. On sit-ups, the MA women had significantly higher performance than the HA and LA women but the average difference was no more than 3 repetitions. On the 2-mile run, performance among the 3 groups of women was similar, differing by no more than 0.3 minutes. Table 4 shows the results of the Cox regressions. Univariate analyses showed that for both men and women, injury risk increased progressively with higher level. Since the 3 groups of women differed on the fitness measures, a multivariate Cox regression was performed including the 3 fitness measures as covariates with the groups. Although the hazard ratios were reduced when the fitness measures were included, injury risk was still progressively higher with higher level among the women.
5 500 Knapik et al Table 4 Association Between Ambulatory Physical Activity and Injury Risk (Cox Regression) Injury index CII Group HA MA LA Men (univariate analysis) Injury incidence rate (cases/1000 Hazard ratio person-days) (95% CI) P ( ) < ( ) < Women (univariate analysis) Injury incidence rate (cases/1000 Hazard ratio person-days) (95% CI) P ( ) < ( ) Women (multivariate analysis) a Hazard ratio (95% CI) P 1.43 ( ) < ( ) OII HA MA LA ( ) 1.34 ( ) 1.00 < ( ) 1.33 ( ) ( ) 1.24 ( ) 1.00 Abbreviations: CII, comprehensive injury index; OII, overuse injury index; HA, higher ; MA, moderate ; LA, lower. a Includes initial push-up, sit-up, and 2-mile run performances Discussion The present investigation was the first to examine objectively measured physical in relation to injury risk using documented injuries requiring medical attention. Physical and injuries were considered for 61 of the 64 days of BCT. Results indicated that as the total ambulatory physical increased, injury risk also increased in BCT. These results are consistent with past investigations that have used self-reports of physical and injuries. 7 16,18,21 24 One study 30 reported not to have found this relationship but risk was elevated in the highest group (odds ratio = 1.4, 95% CI = ) and the reference group selected for comparison was not the least active. One other study provided objective measures of and injury-like events. Pollock et al 17 prescribed varying running frequencies and durations for 70 prison inmates, and then examined training-related events that prevented running for 1 week. Groups running at 85% to 90% maximum heart rate and exercising 0, 1, 3, and 5 days per week had respective injury incidences of 0%, 0%, 12%, and 39%. Groups running 0, 15, 30, and 45 minutes per day had respective injury incidences of 0%, 22%, 24%, and 54%. Despite differences in populations, injury definitions, and type, intensity, and measures of physical, the results of the present investigation were similar to those of Pollock et al: 17 as increased, injury risk increased. Previous basic training studies have examined associations between physical using training logs and injuries documented from medical records. Almedia et al 19 found that the weekly hours of vigorous physical training was related to the incidence of both acute and overuse injuries in Marine basic training. Jones et al 20,31 found that a BCT unit with higher running mileage had a higher incidence of lower extremity overuse injuries compared with a unit that had lower running mileage. Trank et al 32 found that in naval recruit training, higher running mileage was related to higher incidence of lower extremity musculoskeletal injury but total movement mileage was not related to these injuries. The current study demonstrated that overall group ambulatory physical was related to the overall incidence of injury in BCT and this generally supports the results of Almeida 19 but not that of Trank et al. 32 While we obtained our levels from daily pedometers recordings, Trank et al and Almeida et al depended on training logs. Specifically, Almedia et al obtained data from training schedules and Trank et al obtained data from logs of formation marches. Examination of the reported movement distances in Trank et al (their Table 3) indicated that their naval recruits moved an average of 3.6 to 6.7 km/day, a much shorter distance than in the current study. Either the total ambulatory distance was not measured (ie, they only measured selected group movements) or the naval recruits walked a much shorter distance. If the distance was shorter, it may be that these distances produce smaller differences in injury incidence. While there were no group differences between the men on the physical characteristics and physical fitness, there were group differences in fitness among the 3 groups of women. In BCT, low entry level physical fitness has been shown to be a very robust and independent injury risk factor 20,33,34 and thus these group differences were a serious consideration in the analysis. Nonetheless, once fitness was controlled for in the multivariate analysis of the female data the basic relationship remained despite some reduction in the magnitude of the association between physical and injury. Of interest was the fact that that there were only very small group differences in the final fitness test scores, especially among the 2-mile run times. We previously showed that a group of infantry recruits who ran a total of 56 miles during the 12 weeks of infantry basic training had similar final 2-mile run times compared with a group who ran 130 miles during the same period. 31 However, in that observational study we were unable to obtain initial
6 Physical Activity and Injury 501 score, so we could not be sure if the initial scores of the 2 groups were similar. In naval recruits, Trank et al 32 showed no association between 1.5-mile run time and either total movement mileage or total running mileage. As noted above, Trank et al 32 may not have obtained the total ambulatory distance or the naval recruits may have traversed considerably less distance than recruits in the present investigation. Nonetheless, the current study found that regardless of the total ambulatory distance, there were only very small differences in the final 2-mile run times, in agreement with Trank et al. 32 The overall injury rates reported in the current study are consistent with those reported in previous BCT studies. Ten previous BCT investigations reported cumulative injury incidences between 21% and 42% for men and 41% and 65% for women. Risk ratios (women/men) have ranged from 1.6 to No BCT study has previously examined 1-minute push-ups and sit-ups in BCT but a few studies have included 1-mile run times. 36,37 Run times were slower in the present investigation compared with these past studies conducted in 1984 and There are temporal trends indicating that the running performance of recruits has progressively declined over time, presumably associated with an increase in body weight. 38 Acknowledgments We would like to thank Ms. Carol Pace, Ms Salima Darakjy, and First Sergeant (ret) Mark Kenyon for their outstanding assistance in collecting many of the daily pedometer readings. Ms. Carol Pace compiled much of the data included in this report. The leadership of Colonel Heaney, Lieutenant Colonel Philip Kaiser and Lieutenant Colonel Brian Reinwald was greatly appreciated. Ms Kerns graciously provided the RECBASS data. Ms Anita Spiess edited the manuscript. 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