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This Article Abstract Full Text (PDF) Free All Versions of this Article: 34/1/108    most recent 0363546505278703v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Request Reprints Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Shaffer, R. A. Articles by Macera, C. A. Search for Related Content PubMed PubMed Citation Articles by Shaffer, R. A. Articles by Macera, C. A. Related Collections Female Athletes Fracture Overuse

Predictors of Stress Fracture Susceptibility in Young Female Recruits

Richard A. Shaffer, PhD, MPH^*,, Mitchell J. Rauh, PhD, PT, MPH^,,, Stephanie K. Brodine, MD^,, Daniel W. Trone, MA^*,||,¶ and Caroline A. Macera, PhD^,

From the ^* Behavioral Science & Epidemiology Program, Musculoskeletal Injury Epidemiology, Naval Health Research Center, San Diego, California, the Graduate School of Public Health, San Diego State University, San Diego, California, the Graduate Program in Orthopaedic & Sports Physical Therapy, Rocky Mountain University of Health Professions, Provo, Utah, GEO-CENTERS INC, Newton Centre, Massachusetts, and the ^|| University of California, San Diego/San Diego State University, Joint Doctoral Program in Public Health Epidemiology, San Diego, California

^¶ Address correspondence to Daniel W. Trone, MA, Behavioral Science & Epidemiology Program, Musculoskeletal Injury Epidemiology, Naval Health Research Center, PO Box 85122, San Diego, CA 92186-5122 (e-mail: trone{at}nhrc.navy.mil).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Background: Stress fractures account for substantial morbidity for young women undergoing U.S. Marine Corps basic training.

Hypothesis: Certain pretraining characteristics identify women at increased risk of stress fractures during boot camp.

Study Design: Cohort study; Level of evidence, 2.

Methods: Data collected included baseline performance on a timed run (a measure of aerobic fitness), anthropometric measurements, and a baseline questionnaire highlighting exercise and menstrual status among 2962 women undergoing basic training at the Marine Corps Recruit Depot, Parris Island, in 1995 and 1996.

Results: One hundred fifty-two recruits (5.1%) had 181 confirmed lower extremity stress fractures, with the most common sites being the tibia (25%), metatarsals (22%), pelvis (22%), and femur (20%). Logistic regression models revealed that having low aerobic fitness (a slower time on the timed run) and no menses during the past year were significantly associated with the occurrence of any stress fracture and with pelvic or femoral stress fracture during boot camp.

Conclusion: These findings suggest that stress fractures may be reduced if women entering Marine Corps Recruit Depot training participated in pretraining activities designed to improve aerobic fitness. Furthermore, women reporting no menses during the previous year may need additional observation during training.

Clinical Relevance: Consistent with previous studies, we found that low aerobic fitness was the only modifiable risk factor associated with stress fractures during boot camp.

Key Words: injuries • menstrual status • military • women

	INTRODUCTION

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Musculoskeletal injuries are a serious problem for recruits participating in military training but may be especially important for women. Prospective studies of military populations participating in different entry-level programs have consistently reported higher injury rates among women than among men.^{4,7,16,19,20,34} In particular, estimates of stress fracture rates of 5% to 12% have been reported among women undergoing various entry-level military training programs, rates that are about 1.5 to 5 times higher than those reported for men undergoing similar training.^3,16 More important, pelvic and femoral stress fractures, more common among women than among men in military settings, can be particularly devastating because they typically require a more prolonged rehabilitation period (approximately 4 months compared with 1 to 2 months for tibial, fibular, or metatarsal stress fractures)^5,15 and have a higher frequency of complications, including delayed union and nonunion.^27,28

Despite the general knowledge that stress fractures are one of the leading causes of lost training time, medical expenses, attrition, and decreased readiness in military recruits training, little is known about the risk factors for stress fractures, especially among women. Low levels of aerobic fitness before recruit training have been consistently identified as a risk factor among women in previous military studies.^4,16,35 Although less studied, other pretraining risk factors for injury in recruits that have been observed among women are menstrual dysfunction^9,35 and white race.²¹ Potential risk factors such as age^14,35 and body mass^21,22 appear equivocal in young military training populations.

Knowledge of risk factors for stress fractures is essential if they are to be prevented. Intrinsic risk factors have been incorporated into a predictive model for musculoskeletal injury among men undergoing Marine Corps recruit training.³³ The analysis of these risk factors indicates that as much as 60% of the stress fractures during training can be predicted from various measures of fitness, body structure, injury history, and exercise history.³³ The strongest predictor, the baseline fitness of the recruit, is modifiable and suggests that improvement of aerobic fitness before training may reduce stress fractures during training.

The objective of the present study was to identify pre-training risk factor models of overall lower extremity stress fracture and for the more severe pelvic or femoral stress fracture among female Marine Corps recruits. These models were constructed from items in a questionnaire, which focused on the exercise and menstrual status before recruit training, as well as an aerobic fitness measure and assessment of body composition.

	MATERIALS AND METHODS

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Subjects
Of the 3249 female Marine Corps recruits aged 17 to 33 years who arrived at the Parris Island Marine Corps Recruit Depot (MCRD) from March 1995 to September 1996, 2962 recruits (91.2%) volunteered to take part in this study. All potential participants were briefed on the study, and those who volunteered received the privacy act statement and consent form according to the guidelines of the Institutional Review Board of the Naval Health Research Center in San Diego, California.

Outcome Data
All volunteers were observed throughout basic training for occurrence of lower extremity musculoskeletal injuries, with specific attention to stress fractures. Stress fracture data were gathered from reviewing each subject’s medical record at the completion of training or time of separation from the MCRD. The injury data extracted from the medical records included date of visit and onset of injury, injury site, specific final diagnosis, total visits for the diagnosis, and the nature and duration of restricted duty due to injuries. The senior medical officer at the Branch Medical Clinic routinely confirmed all stress fractures among the subjects either radiographically or scintigraphically. Stress fractures were defined as partial or complete fatigue fractures of insidious onset in nondiseased bone. Diagnosis of stress fracture was based on (1) the clinical presentation of localized pain of insidious onset, without prior acute trauma, aggravated by repetitive weightbearing activities and relieved with rest; and (2) a confirmatory (+) radiograph or bone scan or both at a site consistent with the clinical presentation. A positive (+) radiograph finding was defined as presence of periosteal reaction, endosteal callus formation, and/or a fracture line in an otherwise normal bone. A positive bone scan result was defined as the presence of 3+ to 4+ intensity localized fusiform uptake at the site of pain. For analysis purposes, a woman was considered as a single case having a stress fracture if she had 1 or more confirmed stress fractures.

Aerobic Fitness Measurement
Performance on a three-quarter-mile or 1-mile timed run, a component of the Marine Corps Initial Strength Test that was conducted before the start of training, determined the entry level of aerobic fitness. Because of the change in distance for the timed run, which was beyond the control of the investigators, the run times were categorized into quartiles within each distance, with quartile 1 consisting of the fastest runners (referent group) and quartile 4 consisting of the slowest runners.

Anthropometric Measurements
The recruits’ weight and height were measured by a standard, calibrated physician’s beam scale and stadiometer. Body mass index (BMI) was calculated from weight in kilograms and height in meters as weight/height². For weight, height, and BMI variables, values 1 standard deviation from the mean were categorized as at increased risk tertiles, with the middle group as the referent group. This method was chosen over using standard BMI cut points because of the homogeneous nature of the recruits.

Questionnaire Measurements
Basic information on age and race/ethnicity was obtained from a questionnaire administered to the recruits before the start of training. Age was used as a continuous variable. Black women were considered the referent group because the previous literature suggests they are at lower risk of bone injury than are white or Hispanic women.^17,21 The questionnaire also addressed history of previous stress fractures as clinically diagnosed by a medical provider and other lower extremity (hip to toe) overuse injuries. Those who did not report a history of stress fracture(s) or other lower extremity overuse injury were considered the referent groups, respectively. The questionnaire also obtained information on self-assessed physical activity and aerobic fitness practices before MCRD training. Self-rated fitness level of excellent or very good was the referent group compared with those who rated their fitness as good, fair or poor. Several questions were asked to assess the type of physical activity the women participated in during the 2 months before entering MCRD training. These questions assessed exercise or sports participation, frequency of working up a good sweat when participating in an exercise or sport, and running behavior, including mean frequency (per week), mileage (per run), and distance (per session). For all variables, the group with the highest level of activity was the referent group.

From the questionnaire, menstrual status was determined, and 3 variables were created: (1) delayed menarche was defined as women with an age of menarche of 16 or older; (2) menstrual activity during the past year, with 10 to 12 menses considered as the referent group; and (3) secondary amenorrhea, defined as 6 or more consecutively missed menses during the past year.^18,29 Other information on oral contraceptive use and pregnancy status during the past 12 months was also available.

Analysis
Means and standard deviations for continuous variables, such as age, height, weight, BMI, and mean age of menarche, were calculated by stress fracture status to document personal characteristics. Statistical comparisons of mean values were performed with t tests.

Risks of stress fracture injury were calculated as the incidence (percentage) of recruits with at least 1 lower extremity stress fracture (or pelvic or femoral stress fracture) divided by the total number of subjects. Odds ratios (ORs) comparing the proportion of recruits in a high-risk group with the proportion of recruits in a baseline or referent group were calculated for each of the potential risk factors. Women who reported being pregnant during the 12 months before training (n = 146) were excluded from the univariate analyses involving women’s reports of menstrual history during the past 12 months before basic training.

For multivariate analyses, the measure of association was the adjusted ORs, which were generated from a multiple logistic regression analysis. The items included in the logistic regression model analyses were selected from those with significant univariate associations and age and race/ethnicity, which are known to possibly confound the risk relationship.¹⁷ Again, women who reported being pregnant during the preceding 12 months before training (n = 146) were excluded from all multivariate analyses involving women’s reports of menstrual history during the past 12 months before basic training.

	RESULTS

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Stress Fractures
During the 13 weeks of Marine Corps basic training, 152 of the 2962 women (5.1%) suffered a total of 181 stress fractures. Sixty-nine (2.3%) of the women incurred a pelvic or femoral stress fracture. The most common sites of stress fractures were the tibia (24.9%), metatarsals (22.1%), pelvis (21.6%), and femur (19.9%) (Table 1).

Women who reported a pregnancy in the 12 months before training (n = 146) were excluded from the analyses on menstrual function (Table 5). Among measures of menstrual status and oral contraceptive use, only women who reported no menses or secondary amenorrhea during the past year were at increased risk of stress fracture. Women who reported having no menses during the 12 months before training were more than 5 times more likely (OR = 5.64; 95% CI, 2.2–14.4) to incur a stress fracture and more than 8 times more likely (OR = 8.54; 95% CI, 2.8–25.8) to incur a pelvic or femoral stress fracture than were women who reported eumenorrhea (10–12 menses during the past 12 months). Women who were considered to have secondary amenorrhea (6 or more consecutive missed menses during the past year) were more than twice as likely (OR = 2.53; 95% CI, 1.1–6.0) to incur a pelvic or femoral fracture than were women considered eumenorrheic. No significant associations were found for oral contraceptive use.

	DISCUSSION

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The type of stress fracture injury (more than 40% of the fractures were pelvic or femoral fractures) was more severe in this cohort of female Marine Corps recruits than has been reported in civilian athletes or male military recruits. Pelvic stress fractures are relatively rare in civilian male and female athletes, representing 1% to 2% of all stress fractures,²⁵ and are uncommon among male recruits undergoing similar physical training.^2,3

We found that low aerobic fitness, as measured by the timed run, was strongly associated with consequent stress fracture injury. As the run time increased (slower runners), the risk of stress fracture increased. Recruits who had the slowest times (quartile 4) were more than 3 times more likely to incur a stress fracture than were those with the fastest times (quartile 1). This finding is consistent with 3 other studies that have reported that slower run times are associated with greater risk of lower extremity injury among women undergoing military training.^4,7,16 In contrast, although Jones et al found lower self-rated physical fitness levels to be associated with injury among men, they did not find the same association among women.¹⁶ It seems logical that low aerobic fitness, as measured by a timed run, would be associated with a higher risk of injury during Marine Corps training because recruits must repeatedly perform activities, such as walking, marching, or running, which might increase the overuse mechanism on the musculoskeletal system. Those who were more aerobically fit may be protected from injury because they may have performed similar types of activities that allowed the body to adapt to the increasingly intense demands on the musculoskeletal system that occur during military training.

We found that women who did not run or reported running less than a mean of 1.5 miles per run were also at increased risk of overall stress fracture, although this finding was not significant after controlling for run time. This finding is similar to that of a study of female Marine Corps officer candidates, which also found a higher incidence of stress fracture for women who ran 2.8 or fewer miles per week before entering officer training.³⁵ Finally, self-reported infrequent bouts of working up a sweat during physical activity and self-rated poor or fair fitness level (after being controlled for run time in the final logistic model) were not significantly associated with stress fracture. Although the findings of these 2 self-perceived fitness variables confirm those previously reported in female Navy recruits,¹⁸ they are in direct contrast of their predictability of increased stress fracture risk reported in male U.S. Marine Corps recruits.³³ These findings suggest that although objective measures such as run time or previous aerobic or high activity levels are consistent in predicting stress fracture during basic training for both genders, subjective self-perceived measures of aerobic fitness may not be as sensitive in predicting stress fracture injury for women as they are for men.

We found the lowest rates of stress fractures are among black women undergoing military training, which is consistent with previous reports.^8,13,18,21 We also found that Hispanic women were twice as likely to suffer a stress fracture as compared with black women, although this risk was not significant in the final adjusted model. Another study of pelvic stress fractures among naval female recruits also observed Hispanic women to have a significantly higher incidence of pelvic stress fracture than do black women.¹⁸ Previous studies have also reported a higher incidence of stress fracture among white and Asian women compared with black women.^8,13,18,21 Although we also observed higher incidence rates of stress fracture among Asian and white women when compared with black women, these rates were not statistically significant. As suggested by others, we surmise that the lower rate or apparent protective factor against stress fractures among black women may be related to their higher bone density^5,18 or to different biomechanical features that may protect against stress fracture development.⁵ Further research is needed to understand the relationship between race/ ethnicity and stress fracture risk.

Our data confirm previous collegiate athlete and military studies that suggest a history of amenorrhea is a risk factor for stress fractures.^{1,9,12,23,26,35} The mechanism likely involves decreased bone density secondary to hypoestrogenemia.^10,11,26,31 However, the severity of amenorrhea needed to place a recruit at increased risk for stress fracture in our study is worthy to note. In the final logistic model, we found that only women who reported no menses during the past year had a greater likelihood of stress fracture (or pelvic or femoral stress fracture) than did women who reported 10 to 12 menses. However, we did not observe a linear trend by the number of menses during the past year. In contrast, Barrow et al¹ found amenorrheic runners (those reporting 0 menses during the past year) were significantly at increased risk for stress fracture as compared to eumenorrheic runners but reported a linear trend between menstrual irregularity and risk of stress fracture. Others have also shown that athletes or recruits reporting lesser menstrual irregularity (9 or fewer menses during the prior 12 months) were at increased risk for stress fracture.^23,35 Thus, although our final model did not lend support for lesser menstrual irregularity, some of our other menstrual history findings provided some probable evidence that some level of menstrual irregularity may increase the likelihood of stress fracture. Although not significant in the final model, female recruits who reported secondary amenorrhea during the year before training were at higher risk for pelvic or femoral stress fracture. This finding is in contrast to that of Kelly et al,¹⁸ who found no association between secondary amenorrhea and pelvic stress fracture using the same definition (6 consecutive missed menses in the past year). Finally, the relationship between oral contraception pill use and the likelihood of stress fracture also remains unclear. Like others,^6,9 we did not find an association between oral contraceptive pill use and stress fracture, which is in contrast to others who have reported a protective effect of oral contraceptive use against stress fracture.^1,26 The differences in these studies suggest the need for additional studies on the relationship of oral contraceptive pill use and the incidence of stress fracture.

We found that in this young population, age was not associated with stress fracture. Although some studies have found a younger age to be a possible risk factor for stress fracture,^21,35 others have found older age to increase the risk of stress fracture.^14,21 Thus, age as a risk factor for stress fracture remains equivocal and deserves further investigation.

In our study, other potential risk factors such as height, body weight, and BMI were not significantly associated with stress fracture risk. Although others have found risk associations between stress fracture and shorter stature¹⁶ or higher BMI,²² our findings are consistent with other studies that did not report such associations.^6,21,35 However, these factors appear to remain as equivocal risk factors for stress fracture and require further investigation.

Studies of risk factors for injury among recreational and competitive runners have shown prior injury to be related to subsequent injury.^24,30 In military studies, however, the relationship between prior injury and the risk of stress fracture during basic training appears equivocal.^32,33 In our study, even with a large sample size, we did not find an association between prior injury or prior stress fracture injury with stress fracture occurrence during training. Shaffer et al³³ reported a lower risk of stress fracture occurrence among male recruits who reported a prior injury with full recovery as compared with male recruits who reported a prior injury without full recovery or no prior injury. They suggested that prior injury may serve as an indicator of past physical activity and that past activity is protective against stress fracture. Several factors, such as a difference in how men and women recruits entering basic training report prior injuries or differing types and severity levels of prior injury, may contribute to the differences in our findings. We also speculate that there may be some natural selection of those who join the military. That is, those with a prior stress fracture or similar significant injury may be less likely to join the military for fear of reinjury. In summary, these studies indicate that the association between prior injury and stress fracture occurrence during basic training deserves further investigation.

The prospective design, the large sample studied, and the standardized Marine Corps training program provided us with a unique opportunity to examine the association between important characteristics and stress fractures resulting from a controlled, rigorous training that involves repetitive exercise using the lower extremities. However, this study is subject to the weaknesses of survey recall bias by study participants. To minimize recall bias, information on physical activity, menstrual and prior injury history was collected before the diagnosis of a stress fracture injury. We acknowledge that asking women to recall their menstrual histories is not as accurate as prospective record keeping. However, as the recruits were asked to report only their menstrual history during the past year and the age of menarche as a significant aspect of their health history, we felt their reports were reasonably valid. Although these findings may not generalize to all active women, they provide useful information for the assessment of stress fracture risk in women undergoing Marine Corps basic training.

In summary, this prospective study of 2962 women during Marine Corps basic training documents stress fracture as a significant source of morbidity. Several risk factors were significantly associated with subsequent occurrence of overall stress fracture, including baseline aerobic fitness (slow run time) and menstrual dysfunction (no menses during past year). The findings were similar for pelvic or femoral stress fracture. We suggest that the identification of a risk profile for injury, specifically stress fractures, is a necessary step toward the development of an effective intervention. The risk profile data developed in the current study could be an integral part of a future intervention study, such as gradual increases in physical activity during training, encouragement of sport or physical activities that emphasize both aerobic and lower extremity muscle strengthening components before entering a military training program, and increased pretraining screening for recruits with a history of menstrual dysfunction. For the latter, as the causes of menstrual dysfunction may be multifactoral, early medical intervention to determine the cause(s) of the menstrual dysfunction may help to return the recruit to normal menstrual patterns before and during basic training.

	ACKNOWLEDGMENTS

 
Report No. 04-30 was supported by the Office of Naval Research, Arlington, Virginia, under Work Unit No. 60213, document NC 2275 M0001802WR20006.

	FOOTNOTES

 
The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Department of the Navy, the Department of Defense, or the U.S. Government.

No potential conflict of interest declared.

	REFERENCES

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