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Epidemiology

Gender differences in musculoskeletal injury rates: a function of symptom reporting?

ALMEIDA, SANDRA A.; TRONE, DANIEL W.; LEONE, DENISE M.; SHAFFER, RICHARD A.; PATHEAL, SHERRY L.; LONG, KEN

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Medicine & Science in Sports & Exercise: December 1999 - Volume 31 - Issue 12 - p 1807
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Abstract

Musculoskeletal injuries are among the most common adverse effects of regular exercise and physical activity for men and women of all ages (1,15,16). Female gender has been examined as a possible risk factor for exercise-related injuries in several investigations (3,9–13,15,17,18,21,26,27,32,33). While most studies of collegiate athletes (8,18,33) and recreational runners (3,9,15,21,32) have shown no difference in injury rates between the sexes, a number of investigations in military training populations (5,6,10–13,17,25–27) have shown that women are at increased risk of musculoskeletal injuries relative to men. For certain specific diagnoses, such as stress fractures, rates among female military trainees have been cited to be up to 12 times higher than rates among male trainees (26). Numerous factors have been considered, although not well studied, as possible etiologies for the increased injury rates reported in military women. Authors have discussed women’s poorer baseline fitness levels (5,6,10–12,17), gender differences in musculoskeletal anatomy (6,10,17,27), characteristics of military training, and footwear (17,27).

In both military (24) and civilian populations (14,28,31) it has been well documented that women use health care services at higher rates than men. However, gender differences in symptom reporting have not been studied as a possible explanation for the increased musculoskeletal injuries found among women military trainees. The purpose of this investigation was to determine whether gender differences in the likelihood of reporting musculoskeletal injury were apparent among USMC recruits, and, if so, whether these reporting differences contributed to the higher rates of injury found among female trainees.

Exercise-related musculoskeletal injuries can be associated with significant morbidity, work absenteeism, increased use of medical resources, and attrition of injured individuals from exercise programs (4,15,22,30). For military trainees these injuries are also associated with lost training time, medical attrition, and increased training costs (10,11,17,23,29). It is important to determine whether sex differences in exercise-related musculoskeletal injury rates and risk factors really do exist. If women are at increased risk, then gender-specific preventive strategies must be developed. If women are not at increased risk but are simply more likely to seek medical attention when injured, then injury management programs must include mechanisms to identify individuals, particularly men, with unreported injuries.

METHODS

Subjects.

This investigation was a substudy within two larger studies, conducted at Marine Corps Recruit Depot (MCRD) San Diego and MCRD Parris Island, of musculoskeletal injuries in USMC recruits. Male subjects (N = 176) were a subset of 513 randomly selected male USMC recruits who reported to MCRD San Diego for boot camp training between October 5, 1993, and April 20, 1994. The 513 recruits were enrolled in a prospective cohort study to determine rates and risk factors for training-related musculoskeletal injuries in USMC male recruits during 11 wk of boot camp. Of the cohort of 513 recruits, 404 (78.8%) successfully completed training by the end of the study. From this group of 404 graduates, all recruits (N = 192) who were scheduled to graduate during the 9 wk in which the active surveillance substudy was conducted were selected for substudy participation. Of the 192 recruits selected, 176 recruits (91.7%) participated. Sixteen recruits did not participate because of training schedule conflicts. All study subjects signed an informed consent document approved by the Committee for the Protection of Human Subjects of the Naval Health Research Center, San Diego.

Female subjects (N = 241) were a subset of all 400 female USMC recruits who reported to MCRD Parris Island for boot camp training between January 29 and March 25, 1996 (platoons 4012 to 4019). The 400 women were enrolled in a prospective cohort study to determine rates and risk factors for musculoskeletal injuries in female USMC recruits during their 12 wk of boot camp training. Of the cohort of 400 female recruits, 242 (60.5%) successfully completed training by the graduation date for platoon 4019, which was the completion date for the study. All 242 graduates were selected for the substudy evaluation of unreported musculoskeletal injuries, and 241 recruits (99.6%) participated. All study participants provided written, informed consent.

Active surveillance for unreported injuries.

The active surveillance procedures consisted of a screening questionnaire followed by a physician evaluation, if indicated by the questionnaire responses. The four-item questionnaire, developed in consultation with military sports medicine physicians, targeted symptoms of lower extremity musculoskeletal injuries typically seen in USMC recruits (Table 1). Only lower extremity injuries were measured since almost all of the injuries reported in military basic training populations occur in the lower extremities (11,17,19,27,29). The questionnaire was designed to identify true musculoskeletal injuries and to screen out the minor muscular discomforts experienced by many recruits during the first few weeks of training (Phase 1), when they are first exposed to the USMC physical training regimen. Recruits who answered “yes” to the first two questionnaire items, and “yes” to either item 3 or item 4, were referred to a sports medicine physician for evaluation of a possible musculoskeletal injury. The physician evaluation consisted of a medical history and physical examination with diagnostic studies, such as radiographs and scintigrams, as indicated.

Table 1
Table 1:
Screening questionnaire for unreported lower extremity musculoskeletal injuries in USMC male recruits, MCRD San Diego, and USMC female recruits, MCRD Parris Island.

All subjects participating in the active surveillance substudy were first asked to complete the four-item questionnaire. To ensure the recruits’ full disclosure of injury symptoms, arrangements were made with USMC authorities to allow all recruits participating in the substudy, regardless of any injuries found, to graduate with their platoons. The study subjects were informed of the graduation guarantee before the administration of the questionnaire. They were also informed that if they were diagnosed with a previously unreported injury, they would receive medical treatment and then be allowed to continue to their next phase of USMC training.

Active surveillance for unreported injuries for male recruits at MCRD San Diego occurred during 9 wk between January 7 and May 13, 1994. The active surveillance time period was determined by the availability of the examining physician and clinic facilities. During the active surveillance time period, all graduating recruits from the original study cohort of 513 were asked to report 2–4 d before graduation day to the MCRD San Diego Sports Medicine Clinic for evaluation.

For the female recruits at MCRD Parris Island, active surveillance for unreported injuries occurred during the 8 wk between April 20 and June 18, 1996, the respective graduation weeks for platoons 4012 through 4019. Five days before graduation day, the active surveillance questionnaire was administered to all graduating recruits. Recruits identified as needing a physician evaluation were seen within 3 d at the MCRD Parris Island Sports Medicine Clinic.

Injury data.

A military sports medicine physician or physician’s assistant diagnosed all musculoskeletal injuries. A lower extremity musculoskeletal injury was defined as any problem affecting bones, muscles, tendons, ligaments, and associated connective tissue that (a) involved the hip, leg, or foot and (b) was training-related. Injuries to the skin and subcutaneous tissues, such as blisters, abrasions, and cellulitis, were not included in the analyses.

Reported and unreported lower extremity musculoskeletal injuries were determined for all active surveillance subjects. A reported lower extremity musculoskeletal injury was defined as one for which a recruit voluntarily presented to a medical clinic with symptoms. Reported injuries were measured by review of each recruit’s medical record at the completion of his/her training. All reported injuries were treated and documented at military medical treatment facilities located at the training sites. An unreported lower extremity musculoskeletal injury was defined as one that (a) adversely affected the recruit’s physical performance, (b) had never been previously reported to a health care provider, and (c) was diagnosed during the study’s active surveillance procedures.

Physical training.

USMC recruit training consisted of standardized military instruction for both male and female recruits. For male subjects training occurred at MCRD San Diego during an 11-wk period, with 57 defined training days. For female subjects training occurred at MCRD Parris Island during a 12-wk period, with 64 defined training days. Men and women were trained separately by drill instructors of their own gender. For both male and female recruits, scheduled training events that required at least a moderate level of physical exertion (brisk walk) consisted of (a) close-order drill/marching, (b) general physical conditioning exercises, (c) military-specific activities, and (d) water survival training. General physical conditioning exercises included running, calisthenics, obstacle courses, and circuit courses. Military-specific activities consisted of mission-related evolutions such as rappelling, field movement exercises, and load-bearing hikes.

Overall, physical training was similar for male and female recruits, although some differences did exist. In general, the scheduling of specific training events varied between the sexes, and women performed some additional general physical conditioning and military-specific activities during their extra week of boot camp. During 11 wk of recruit training, men participated in approximately 140 scheduled hours of moderate to vigorous physical training, including approximately 42 h of close-order drill/marching, 28 h of general physical conditioning, 55 h of military-specific activities, and 14 h of water survival training. Women, during 12 wk of training, participated in approximately 152 scheduled hours of moderate to vigorous physical training, including approximately 40 h of close-order drill/marching, 44 h of general physical conditioning, 52 h of military-specific activities, and 16 h of water survival training. For both male and female recruits, the average weekly volume of scheduled moderate to vigorous physical training was 12.7 h.

Data analysis.

Active surveillance study recruits were categorized as having (a) no injuries, (b) at least one reported injury, or (c) at least one unreported injury. Injury rates were calculated separately for male and female subjects and compared using relative risks (RR) and 95% confidence intervals (CI). Incidence of reported injuries was expressed as the number of recruits with at least one reported lower extremity musculoskeletal injury per 100 recruits surveyed. Incidence of unreported injuries was expressed as the number of recruits with at least one unreported lower extremity musculoskeletal injury per 100 recruits surveyed. A “total true lower extremity musculoskeletal injury rate” was then calculated for male and female recruits and defined as the number of recruits with at least one injury (reported or unreported) per 100 recruits surveyed.

Cumulative incidence rates of the most common diagnosis-specific reported and unreported injuries were also determined for male and female subjects. Rates were expressed as number of diagnosis-specific injuries per 100 recruits surveyed.

RESULTS

Subjects.

Male subjects were 176 USMC enlisted recruits. Age and race data were available for 114 (64.8%) of the 176 subjects. The age range was 17 to 28 yr, with a mean of 19.4 yr and a SD of ± 1.7 yr. Most of the subjects were Caucasian (61.4%); 23.7% were Hispanic, 10.5% black, and 4.4% other.

Female active surveillance subjects were 241 USMC enlisted recruits, ages 17–31 yr, with a mean of 19.8 yr and a SD of ± 2.1 yr. Most were Caucasian (67.6%); 12.5% were Hispanic, 16.4% black, and 3.5% other.

Injury data.

Of the 176 male subjects, 96 (54.6%) had no injuries, 45 (25.6%) had at least one reported injury, and 42 (23.9%) had at least one unreported injury. Among the 241 female recruits, 112 (46.5%) had no injuries, 106 (44.0%) had at least one reported injury, and 28 (11.6%) had at least one unreported injury. In both gender groups, injured subjects tended to have either reported injuries only or unreported injuries only. Only seven (4.0%) men and five (2.1%) women had both reported and unreported injuries.

Figure 1 compares the reported, unreported, and total true incidence of lower extremity musculoskeletal injuries in male and female recruits. Forty-five male recruits (25.6%) had a total of 48 reported injuries compared with 106 female recruits (44.0%) with a total of 189 reported injuries. Female recruits were 1.72 times more likely to have a reported injury than their male counterparts (95% CI 1.29–2.30, P = 0.0002). In the unreported injury category, 42 male recruits (23.9%) had a total of 52 unreported injuries, and 28 (11.6%) female recruits had a total of 33 unreported injuries. Female recruits were 0.49 times as likely to have an unreported injury as were male recruits (95% CI 0.31–0.75, P = 0.0015). Stated another way, male recruits had 2.05 times the risk of having an unreported injury as female recruits. There was no statistically significant difference between the male (45.5%) and female (53.5%) total true injury rates (RR = 1.18, 95% CI 0.96–1.44, P = 0.1262).

Figure 1
Figure 1:
Incidence, with relative risks and 95% CI, of reported, unreported, and total lower extremity musculoskeletal injuries in USMC male and female recruits; * significant at α = 0.05 level. Since some recruits had both reported and unreported injuries, the incidence of total injuries is not the simple sum of reported and unreported rates.

Tables 2 and 3 present the cumulative incidence rates of the most common reported and unreported injury diagnoses in male and female recruits, respectively. For both men and women recruits, all of the most frequently occurring injuries, reported and unreported, were anatomically distributed from the knee downward. With the exception of ankle and foot sprains, the common injuries were all ones that are typically caused by repetitive activity overuse as opposed to acute trauma.

Table 2
Table 2:
Incidence of most common reported and unreported lower extremity musculoskeletal injuries in male recruits, MCRD San Diego.
Table 3
Table 3:
Incidence of most common reported and unreported lower extremity musculoskeletal injuries in female recruits, MCRD Parris Island.

Between genders, some differences in the incidence of reported and unreported diagnoses were noted. Patellofemoral syndrome, the number one reported (10.0% of subjects) and unreported (2.1%) diagnosis for female recruits, rarely occurred in the male recruits (1.1% reported, zero unreported). Ankle sprain, a frequent reported injury for both male (2.8%) and female (9.1%) recruits, was a common unreported injury for male (2.8%) but not for female (zero) recruits. Stress fracture also was a common unreported diagnosis for male (2.3%) but not for female (zero) recruits.

DISCUSSION

This study was the first to investigate gender differences in the reporting of lower extremity musculoskeletal injuries among U.S. military trainees. Our results showed that female USMC recruits, relative to male USMC recruits, were more likely to have a reported injury (44.0% vs 25.6%, RR = 1.72, 95% CI 1.29–2.30) and less likely to have an unreported injury (11.6% vs 23.9%, RR = 0.49, 95% CI 0.31–0.75) during boot camp training. When both reported and unreported injuries were measured, injury rates for both sexes were high (53.5% women, 45.5% men, RR = 1.18, 95% CI 0.96–1.44), but the difference between the rates was not statistically significant. Our findings indicate that gender differences in injury reporting is an important variable to consider when comparing exercise-related musculoskeletal injury rates in young adult men and women.

The overall incidence and patterns of reported injuries found in this study were similar to those described in other investigations of U.S. military trainees (11,13,17), suggesting that our study cohort was comparable to these groups. Higher stress fracture rates than those seen in our subjects typically have been found in other studies of male (11,13,17) and female (11,13,17,26,27) recruits. However, our cohort consisted only of recruits who successfully completed boot camp training by the end of the study. Since we did not capture stress fractures that resulted in medical attrition or delays in training, our findings likely underrepresented true stress fracture rates.

In both men and women subjects, the majority of reported and unreported diagnoses were overuse injuries, distributed from the knee downward. Similar anatomical patterns and types of injuries have been described in civilian runners (4,9,22,30), suggesting that repetitive, weight-bearing activities are etiologic factors for injuries in both male and female recruits. Although the most common reported and unreported injuries were similar between genders, there were also some striking differences. It is possible that the gender differences in injury diagnoses resulted from variations in physical training patterns for male and female recruits or from differences in diagnostic practices between clinicians at MCRD San Diego and MCRD Parris Island. However, the men’s and women’s recruit training programs were very similar, despite some differences in types and scheduling of events. Also, most of the medical practitioners who participated in the study had received similar sports medicine training.

Patellofemoral syndrome, the number one reported (10.0%) and unreported (2.1%) injury for female recruits, was rarely seen in male recruits (1.1% reported, zero unreported). Other studies of military recruits (11,27) and civilian athletes (7,8) have also suggested that women have an increased risk of patellofemoral joint problems in comparison with men. Gender differences in musculoskeletal anatomy, biomechanics, and physical conditioning are thought to contribute to the increased incidence of these injuries in women (7). However, few studies have investigated these hypotheses, and further work is necessary in this area.

Ankle sprains and stress fractures, two injuries that typically are considered more serious than the other types found in this study, were common unreported injuries in male recruits but not in female recruits (zero cases). In studies of gender differences in health care utilization, researchers generally have concluded that although women are more likely than men to use health care services for minor illnesses and injuries, both genders seek medical care at equal rates for conditions that are serious or that interfere with normal activities (28,31). Our findings do not appear to support these conclusions. It is possible that the male recruits in our study with stress fractures and ankle sprains did not perceive these as serious injuries. However, most stress fractures and many ankle sprains are associated with significant disability and prolonged rehabilitation periods. In military training populations, these injuries also frequently result in lost training days and medical attrition. Our findings indicate that male USMC recruits may be at increased risk of sustaining a relatively serious lower extremity musculoskeletal injury for which they will not seek medical care.

Our study results suggest that the increased risk of training-related musculoskeletal injuries found among female military trainees in previous investigations (5,6,10–13,17,25–27) may be explained by gender differences in injury reporting. Our findings are supported by other studies in civilian (14,31) and military populations (24), which have shown that young adult women utilize health care services at a higher rate than their male counterparts. Although none of these prior studies measured unreported illnesses and injuries, they clearly demonstrated that women seek medical care at higher rates than men.

Several hypotheses have been developed to explain gender differences in symptom reporting (2,14,31). A number of psychosocial and environmental factors are addressed in these hypotheses, including barriers to medical care (such as time available and job flexibility), occupational and lifestyle stresses, and gender socialization. It is unclear why male recruits in this study were less likely to report injuries than were female recruits; several factors may have contributed. USMC recruit training was very similar for men and women. Medical care was free and easily accessible. Therefore, occupational and lifestyle stresses and barriers to medical care were comparable for both male and female recruits. However, some may argue that women recruits may have felt more occupational stress since they were entering a field that has traditionally been held by men. Some researchers believe this “role strain” experienced by women in nontraditional occupations leads to increased help-seeking behaviors and symptoms (2). Long-term gender socialization also may have contributed to the differences in injury reporting. Boys are discouraged at an early age from reporting pain and illness (14,28,31), whereas girls are taught that being sick and taking care of one’s illness is acceptable (31). It is particularly likely that young men who join the USMC, a historically male-dominant and traditional organization, would hesitate to report injury or illness.

Interestingly, our study also showed injured individuals from both genders tended to have either reported injuries only or unreported injuries only. Only 4.0% of men and 2.1% of women had both reported and unreported injuries. The explanation for this finding is not readily apparent. It may be the result of differences in individual psychological characteristics, behavior patterns, or perceived environmental barriers to seeking care. In any case, these findings should be further investigated. Determination of the underlying causes may be necessary for the development of effective interventions aimed at reducing the high rates of unreported injuries in both sexes.

The intent of this study was not to determine whether female USMC recruits are at increased risk of musculoskeletal injury relative to male recruits. Rather, its purpose was to investigate whether gender differences in symptom reporting may be an important factor to consider when making such comparisons in similar military and civilian populations. However, our study findings provide directions for future investigations into gender differences in exercise-related injury risks. Our results showed that when gender differences in symptom reporting were controlled, female recruits were at similar risk of injury as their male counterparts. Previous research has identified several other factors that may have contributed to the higher injury rates found in prior studies of women military trainees, such as women’s poorer baseline fitness levels (5,6,10–12,17), male-specific military training, and poorly fitting footwear (17,27). Future studies should control for these factors as well as others associated with exercise-related injury risk, such as volume of physical training (3,4,9,15,21,22,30,32) and prior injury (21,22,30,32). Our results suggest that if all of these factors were controlled, studies may find that female military trainees are at lower risk of injury than male trainees.

In summary, our study has shown that female USMC recruits were more likely to voluntarily report a training-related musculoskeletal injury than their male counterparts. When unreported injuries were taken into account, the increased injury risk among female recruits disappeared, indicating that gender differences in symptom reporting is an important factor to consider when comparing injury rates between men and women from similar populations. In our study both sexes frequently failed to report injuries, male recruits more often than female recruits. In addition, a number of the unreported injuries among male recruits were considered relatively serious.

Coaches, fitness professionals, and military trainers should be made aware of the possibility of unreported injuries among their trainees. Trainees should be encouraged to report symptoms since untreated musculoskeletal injuries will likely result in a worsening of the problem or in future exercise-related injuries (20–22,30,32). The high injury rates, reported and unreported in both sexes, found in this study further demonstrate the critical need for the development of strategies that will prevent exercise-related musculoskeletal injuries in men and women.

The authors thank the U.S. Marines at MCRD San Diego and MCRD Parris Island, particularly LTC Harmon and LTC Salinas, for their interest in and support of this research. We thank the medical staffs at the MCRD San Diego and MCRD Parris Island Sports Medicine Clinics for their participation throughout this work. We also acknowledge the invaluable contributions of our on-site project coordinators Karen Maxwell Williams, Kelli Betsinger, and Mary Durm. Finally, we thank the research staff at NHRC, especially Michelle Stoia and LeAnn Gay, for their administrative and technical assistance.

This research was supported by the U.S. Naval Medical Research and Development Command under Work Unit Number 63706N M0095.008–6504, and the U.S. Army Medical Research and Material Command, Defense Women’s Health Research Proposal, Army Reimbursable 6436/MIPR 95MM5524. The views expressed in this study are those of the authors and do not reflect the official position of the Department of the Navy, the Department of Defense, or the U.S. Government.

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Keywords:

EXERCISE; PHYSICAL CONDITIONING; TRAINING; U.S. MARINE CORPS RECRUITS; ACTIVE SURVEILLANCE

© 1999 Lippincott Williams & Wilkins, Inc.