Musculoskeletal injuries are a leading cause of morbidity, lost training time, and reduced operational readiness in U.S. military forces (7,11,13,15,19,21,22,28,35,37). These injuries are a significant problem in military recruit populations where individuals are exposed to sudden increases in the volume and intensity of physical training. Although many exercise-related musculoskeletal injuries may be preventable, few studies have determined the precise diagnoses and specific etiologies of these costly problems. The development of effective preventive strategies will require the systematic quantification of diagnosis-specific injuries and the investigation of potential risk factors, such as personal characteristics, equipment, and physical training patterns.
Most of the epidemiological studies of musculoskeletal injuries in military trainees have documented the cumulative incidence of broad categories of injury diagnoses (9,13,15,16,19,21,22,28). Rates of specific injuries have been reported only for a few of the most commonly occurring ones (8,15,19,21,22,28,30,33,36). A more precise characterization of injury patterns would provide a better understanding of specific etiologies and risk factors.
Investigations of predictors of exercise-related injuries in military populations have identified primarily intrinsic risk factors, such as baseline fitness levels and prior exercise behavior (13,15,19,28,36). Only a few studies have examined the association between physical training and injuries (15,16,30,33). These studies, along with research in civilian runners (2,3,12,17,23,25,26,31,40), have provided evidence that patterns of physical training are associated with injury occurrence. Since most physical training variables are modifiable, the identification of specific injury risk factors, such as volume, intensity, type, and rates of progression, would provide a framework for the development of cost effective and feasible preventive strategies.
The distribution and types of injuries reported in civilian runners (3,12,23,26) are similar to those seen in military trainees (9,13,15,16,19,21,22,35), suggesting common etiologies. Investigations in civilian populations of associations between patterns of exercise and musculoskeletal injuries have consisted primarily of observational studies in volunteer recreational runners, using self-report questionnaires. Selection bias, lack of standardized injury definitions, and the heterogeneity of the subjects can limit these types of studies (18). U.S. Marine Corps (USMC) recruits are a homogeneous sample, with a strictly controlled living environment, a standardized exercise schedule, and easy access to sports medicine care. They provide an ideal population for studying the relationship between training-related exercise and injury and for developing preventive interventions that may then be modified for use by other nonmilitary groups.
This investigation was the third in a series of epidemiological studies conducted with USMC recruits by the Naval Health Research Center's Sports Medicine and Research Team (SMART). SMART is a multidisciplinary group established to design and test interventions for the prevention of musculoskeletal injuries in various military populations. The first two SMART studies performed with USMC recruits at the Marine Corps Recruit Depot (MCRD) San Diego identified a high cumulative incidence of podiatric injuries (9.0 per 100 recruit-months) (21) and of overall orthopedic/soft tissue injuries (19.9 per 100 recruit-months) (22) during 12 wk of boot camp training. The purpose of this descriptive study was to determine diagnostically precise rates of training related injuries and to generate hypotheses regarding the potential role of physical training as an etiologic factor.
Subjects. Subjects were 1,347 randomly selected USMC male recruits who arrived at MCRD San Diego for boot camp training between January 12 and September 14, 1993. A roster of recruits reporting for training was obtained weekly, and a sample of study subjects was selected using a table of random numbers. Of the 1,347 recruits selected, 1,296 (96.2%) agreed to participate. After providing written informed consent, volunteers were followed prospectively through 12 wk of recruit training.
Of the 1,296 subjects, 153 (11.8%) separated from USMC service before completing boot camp training. Since medical records of discharged recruits were not available for injury outcome determination, all injury rate calculations, with the exception of those for stress fractures, included only boot camp graduates (N = 1,143). Recruits discharged because of a stress fracture were identified through review of USMC administrative records so that stress fracture incidence could be determined for the entire sample (N = 1,296). Most musculoskeletal injuries likely were captured using this methodology. Prior USMC record reviews indicated that stress fractures were the only musculoskeletal injury of importance with regard to recruit retention. Most recruits who dropped out of boot camp did so during the first several days before any significant amount of physical training and for reasons other than injuries.
Physical training. USMC recruit training consisted of 12 wk of standardized military instruction divided into three distinct phases: First Phase, weeks 1 through 3, included close-order drill/marching, general physical conditioning, and classroom instruction; Second Phase, weeks 4 through 7, consisted of weapons firing and field combat skills training; and Third Phase, weeks 8 through 12, included water survival training and a continuation of the military marching, general physical conditioning, and classroom instruction that began during First Phase.
Hours of scheduled vigorous physical training were quantified by training week and type. Quantification was conducted through review of the USMC recruit training schedule and interviews with USMC training officers. Vigorous physical training was defined as activity requiring an estimated energy expenditure of 6 MET or more (20,29). Energy costs were estimated using a published activity classification system (1).
The type of physical training was categorized into four major groups: close-order drill/marching, general conditioning, military-specific training, and water survival. General physical conditioning included predominantly running, with some calisthenics, obstacle courses, and weight-training circuit courses. Military-specific training consisted of mission-related evolutions, such as rappelling, field movement exercises, and 5- to 10-mile load-bearing conditioning hikes. With the exception of water survival training, vigorous physical training consisted primarily of lower extremity weight-bearing activities.
Injury data. All injured recruits were treated in the military medical clinics located at the training sites; all clinic visits were documented in the recruits' individual medical records. Injury outcomes were determined by reviewing each subject's medical record at the completion of his boot camp training. Data collected included date and training day of clinic visit, anatomical site of injury, final diagnosis, associated training events, and whether the symptoms were of acute or insidious onset.
A musculoskeletal injury was defined as any problem involving bones, muscles, tendons, ligaments, and associated connective tissues for which a recruit presented to the medical clinic. Injuries to the skin and subcutaneous tissues, such as abrasions, blisters, and cellulitis, were not included. Musculoskeletal injuries were classified as either acute or overuse, based on the history of the injury provided in the recruits' medical records. Acute injuries were defined as those precipitated by a sudden, forceful, traumatic event. Overuse injuries were defined as problems of the musculoskeletal system of insidious onset that were associated with repetitive physical activities. In most cases the examining clinician specifically documented that the injury was of acute or insidious onset. Common injuries that typically were considered acute included ankle sprains and contusions. Common overuse injuries were iliotibial band syndrome, stress fractures, patellofemoral syndrome, shin splints, periostitis, plantar fasciitis, capsulitis, and tendinitis. Certain types of injuries, such as muscle strains, were variably diagnosed as acute or overuse.
Diagnostic criteria for stress fractures included (a) a clinical presentation of localized bone pain, without prior acute trauma, aggravated by activity and relieved with rest and (b) a confirmatory radiograph and/or triple-phase bone scan. A positive radiograph was defined as the presence of periosteal new bone formation, sclerotic bands, and/or a fracture line in otherwise normal bone (10). A positive bone scan was defined as the presence of 3+ to 4+ intensity round or fusiform focal uptake (27) at an anatomical site consistent with the clinical presentation.
Data analysis. Injury data were expressed as (a) distribution of injuries by anatomical site, (b) as cumulative incidence (number of injuries/population at risk) of diagnosis-specific injuries per 100 recruits for the entire 12-wk training cycle, (c) as cumulative incidence of acute and overuse injuries per 100 recruits by training week, and (d) as incidence density of injuries per 100 recruit-hours of vigorous physical training (number of injuries/100 recruit-hours of training) by training week. Multiple musculoskeletal diagnoses made in one individual during the same clinic visit were considered discrete injury outcomes. The weekly injury rates reflected the training week that the recruits reported to a medical facility with injury symptoms. The relationship between physical training volume and injury incidence was examined using Pearson correlation coefficients. Correlations between hours of vigorous physical training and cumulative incidence of injuries were calculated by training week and reported separately for acute and overuse injuries.
Subjects. All subjects were USMC male enlisted recruits. Ages ranged from 17 to 28 yr, with a mean of 19.1 yr and an SD of ± 1.7 yr. Subjects were predominantly white (69.7%), with 19.5% Hispanic, 6.0% black, and 4.8% other.
Physical training.Figure 1 presents total hours of vigorous physical training by week of boot camp. Table 1 shows training hours categorized by type. Weeks 1, 2, 3, 6, 7, 8, 10, and 11 all included more than 14 h of vigorous weight-bearing activities.
Injury data. Of the 1,143 subjects who successfully completed boot camp, 453 (39.6% of recruits) incurred at least one musculoskeletal injury during the 12 wk of training. The lower extremity was the most frequent anatomical site of injury, accounting for 82% of the diagnoses. The majority of the injuries occurred in the ankle/foot region (34.3% of injuries), followed by the knee (28.1%), the leg (13.7%), and the back/neck/trunk (9.9%) (Fig. 2).
The 12-wk cumulative incidence rates per 100 recruits (number of injuries/population at risk) of the most common injury diagnoses are shown in Table 2. Overuse injuries comprised 78% of the diagnoses, while acute injuries accounted for only 22%. The most frequent overuse injury, and the second most common overall injury, was iliotibial band syndrome (5.3%, N = 1,143). Stress fractures were the second most common overuse injury, with an incidence of 4.0% (N = 1,296). The only two acute injuries that occurred with any significant frequency were ankle sprains (6.2%, N = 1,143) and contusions (3.7%, N = 1,143).
The cumulative incidence rates for both overuse and acute injuries by training week are displayed in Figure 3. The distribution patterns were approximately the same for overuse and acute injuries. Weekly hours of vigorous physical training correlated significantly with incidence of both acute (r = 0.633, P = 0.027) and overuse (r = 0.667, P = 0.018) injuries. Injuries were reported most often during the first 3 wk of training and during weeks 8, 10, and 11. These weeks were characterized by high total volumes of vigorous physical training and the most hours of running and military marching.
Figure 4 presents the distribution (number of injuries/recruit-hours of training) of total injuries per 100 recruit-hours of vigorous physical training by training week. When expressed per recruit-hours of training, the highest injury rates clustered during the earliest weeks of training, with the exception of one late peak during week 10.
This study was the first to quantify diagnosis-specific musculoskeletal injuries and to examine the role of physical training as a potential causal factor of such injuries in USMC recruits. Injuries occurred at a high rate (39.6% of recruits), with the majority of diagnoses being overuse injuries of the lower extremities. Weekly injury rates were significantly correlated with hours of vigorous physical training, indicating that volume of vigorous physical training may be an etiologic factor for exercise-related injuries. Our findings also suggest that type of training, particularly running, and abrupt increases in training volume may further contribute to injury risk. The results of this controlled epidemiological investigation support observational studies in civilian runners that have shown associations between musculoskeletal injuries and patterns of physical training (4,39).
The overall incidence (39.6% of recruits) (9,13,15,16,19) and anatomical distribution (13,15,22) of injuries found in this study are similar to those described in other investigations of military training populations. A higher injury rate (19.9 per 100 recruit-months, 59.7% of recruits for a 12-wk training cycle) was reported in the first SMART investigation in USMC recruits (22). However, in that study the definition of injury included disorders of the skin and subcutaneous tissues, such as blisters and cellulitis. Dermatological problems were excluded from analyses in this study. Since blisters and cellulitis are common injuries in USMC recruits (21), it is likely that they contributed to the higher injury rate found in the previous study.
The study finding of the ankle/foot region as the most frequently injured area is consistent with results reported in the initial SMART investigation in this population (22). A comparative anatomical distribution of training-related injuries has been found in studies of U.S. Army trainees (15) but not in those of some foreign military recruits (16,19) where the knee has been reported as the primary site of injury. The disparities in injury distribution found between U.S. and other military populations may be a result of several factors, such as differences in footwear, training techniques, terrain, and definitions of injuries.
The lack of diagnostic precision in previous studies in similar populations makes the direct comparison of rates of specific diagnoses difficult. However, some studies in U.S. military trainees have reported the cumulative incidence of a few of the most common diagnoses (15,21,22). Our study results are comparable with these earlier findings. Specific injuries frequently cited as the most commonly occurring include stress fractures, ankle sprains, iliotibial band syndrome, patellar tendinitis, Achilles tendinitis, plantar fasciitis, shin splints, and lower extremity stress syndromes. With the exception of ankle sprains, all of these diagnoses usually are considered overuse injuries. The same injuries also are the most frequently reported in civilian runners (3,12,23,26,39), suggesting that the etiologies may be similar.
This was the first study to examine rates of both acute and overuse injuries by training week (Fig. 3). The finding that the distributions followed approximately the same pattern indicates that these injuries may share some common risk factors. From an injury prevention perspective, this finding is important because it suggests that interventions targeting the more common overuse injuries also may effectively reduce the incidence of acute traumatic events.
This was one of the first studies of military recruits to examine distributions of injuries in relation to patterns of physical training. With the exception of weeks 6 and 7, we found that peaks in injury rates occurred during the weeks with the greatest total volumes of vigorous physical training (Figs. 1 and 3). These findings are consistent with studies in civilian runners that have reported training volume (weekly running distance) as a significant risk factor for exercise-related injuries (2,3,12,17,23,25,26,31,40). In our study, weekly injury rates were based on the day that the recruit presented to the clinic with injury symptoms. Since the presentation week may not have corresponded directly to the week of symptom onset, the rates may reflect the cumulative effects of the current as well as the prior weeks of physical training, particularly in the case of overuse injuries. This delay in injury diagnosis might explain the relatively low injury rates seen during weeks 6 and 7 despite the respective high training loads. During those weeks recruits trained in a fairly remote field environment, and in week 8 they returned to MCRD where they had easy access to the Sports Medicine Clinic.
Although weekly training volume has been most consistently cited as a predictor of exercise-related injuries, some studies of runners have suggested that other training variables, such as type, intensity, frequency, duration, and abrupt changes in training patterns, may be contributing factors (4,14,24,39). Our study results support the concept of a multifactorial effect of physical training. The weeks with the highest injury rates were characterized by the most hours of marching and general physical conditioning (running). Comparable findings were reported in a similar study of recruits in the South African Army (16). Another study in U.S. Army trainees found an association between weekly running volume and injury incidence (15). Furthermore, the distribution and type of injuries reported in military trainees (9,13,15,16,19,21,22,35) are similar to those seen in civilian runners (3,12,23,26). All of these results indicate that exercise type, particularly running, is an important factor in injury causality.
The high injury rates found during the earliest weeks of boot camp may be explained not only by the high volumes of exercise but by the abrupt onset of training. Some studies in recreational runners have cited lack of prior running experience (3,25,26) and "training error" (23) (abrupt increases or changes in training) as risk factors for running-related injuries. A prior study of stress fractures in USMC recruits reported that some recruits had minimal previous running experience (10). Studies of U.S. Army trainees have shown similarly that a number of trainees have little or no prior history of exercise (13,15). It is reasonable to assume that some recruits in this investigation had limited prior exercise experience. The high volume of vigorous physical activity, more than 15 h·wk−1, conducted during the first 3 wk of USMC training may have constituted training error for these recruits.
When the injury incidence was expressed as a function of hours of exposure to physical training, the highest weekly rates were seen most consistently during the earliest weeks of boot camp (Fig. 4). A similar pattern was reported in the South African Army study (16). Another investigation in civilian runners showed that as running experience increased, injuries per unit of running exposure declined (3). All these findings suggest that as individuals become fit through regular physical activity they become more resistant to exercise-related injuries. This theory is supported by laboratory evidence that progressive physical conditioning and overloading of musculoskeletal tissues stimulates a protective adaptive response (5,6,32,34).
The late peak in injuries per hour of training exposure seen in week 10 (Fig. 4) may also be attributable to training error. On the last day of week 9, following a week of no physical training, the recruits returned to a schedule of nearly daily, high-volume weight-bearing activities. The sudden rise in the injury rate in week 10 provides further evidence that abrupt increases in training volume, particularly weight-bearing activities, may contribute to injury risk.
Our study findings support the hypothesis that patterns of physical training, particularly volume of vigorous weight-bearing activities, are associated with both overuse and acute musculoskeletal injuries. In this study, hours of scheduled vigorous physical training were used as an estimate of total training volume. Only vigorous training was quantified, since nearly all scheduled physical events required an estimated energy expenditure of 6 MET or more. Since training intensity (such as pace of running events) increased slightly through the training cycle, a greater volume of exercise per hour may have been performed later in the schedule. Furthermore, 6 MET may have constituted only moderate-intensity activity for relatively fit individuals. Future studies should include a more precise quantification of physical training in terms of volume, type, frequency, intensity, and duration. Nonscheduled physical activities, such as incidental movement miles from training site to training site, should also be measured to provide a more accurate assessment of total exercise exposure. Specific types of training, especially running and marching, should be quantified and correlated with injury patterns. Training patterns should be further examined for risk factors common to both overuse and acute injuries. Since our results indicate that recruits were particularly susceptible to injury during the early phases of training, intrinsic factors, such as baseline fitness levels and prior exercise histories, also should be investigated as contributors to injuries in this population.
With the current public health emphasis on the promotion of physical activity (38), there is an increased need for the scientific investigation of strategies that will minimize the risks of exercise-related patient morbidity and health care costs. The systematic identification of exercise injury risk factors through controlled, epidemiological studies such as this will form the basis for the development of injury prevention strategies that will benefit both the civilian and military communities.
The authors thank John G. Aronen, M.D. and CDR John Caldwell, and their staff at the MCRD San Diego Sports Medicine Clinic for their invaluable clinical support of this work. We also thank the research staff at NHRC, particularly Denise Leone and Sara Ronaghy, for their technical assistance with data collection and analysis. We also express our gratitude to the U.S. Marines at MCRD San Diego, whose interest and support made this research project possible.
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