Stress fracture in military recruits: gender differences in muscle and bone susceptibility factors.

A total of 693 female U.S. Marine Corps recruits were studied with anthropometry and dual-energy X-ray absorptiometry (DXA) scans of the midthigh and distal third of the lower leg prior to a 12 week physical training program. In this group, 37 incident stress fracture cases were radiologically confirmed. Female data were compared with male data from an earlier study of 626 Marine recruits extended with additional cases for a total of 38 stress fracture cases. Using DXA data, bone structural geometry and cortical dimensions were derived at scan locations and muscle cross-sectional area was computed at the midthigh. Measurements were compared within gender between pooled fracture cases and controls after excluding subjects diagnosed with shin splints. In both genders, fracture cases were less physically fit, and had smaller thigh muscles compared with controls. After correction for height and weight, section moduli (Z) and bone strength indices (Z/bone length) of the femur and tibia were significantly smaller in fracture cases of both genders, but patterns differed. Female cases had thinner cortices and lower areal bone mineral density (BMD), whereas male cases had externally narrower bones but similar cortical thicknesses and areal BMDs compared with controls. In both genders, differences in fitness, muscle, and bone parameters suggest poor skeletal adaptation in fracture cases due to inadequate physical conditioning prior to training. To determine whether bone and muscle strength parameters differed between genders, all data were pooled and adjusted for height and weight. In both the tibia and femur, men had significantly larger section moduli and bone strength indices than women, although women had higher tibia but lower femur areal BMDs. Female bones, on average, were narrower and had thinner cortices (not significant in the femur, p = 0.07). Unlike the bone geometry differences, thigh muscle cross-sectional areas were virtually identical to those of the men, suggesting that the muscles of the women were not relatively weaker.

Running mileage, movement mileage, and fitness in male U.S. Navy recruits.

PURPOSE: The purpose of this study was to determine whether there is a relationship between overall fitness improvement and varying amounts of running and movement mileage. METHODS: Subjects were male U.S. Navy recruits (N = 1703, 25 divisions), ages 17-35 yr (mean age = 20.1 +/- 2.9 yr), who attended boot camp from April 1996 through August 1996. During the first week of training, recruits performed a 1.5-mile run to determine baseline fitness levels. The results from the initial run were compared with a final 1.5-mile run conducted 6 wk later. RESULTS: Based on an age-adjusted fitness scale for a 1.5-mile run time, about one third of the recruits began recruit training in "Excellent-Superior" condition (N = 558), one third began in "Good" condition (N = 582), and one third began in "Poor-Fair" condition (N = 563). Running mileage among divisions ranged from 11.5 to 43.5 miles for the entire 7-wk training period (mean = 22.7 +/- 7.2 miles; 8-22 run days, mean = 13 +/- 4 d). In addition to running, the divisions accumulated many movement miles (110-202 miles; mean = 145 +/- 26 miles) while marching in formation. Recruits who began training in Poor-Fair condition improved the most with an average decrease in run time of 1:55 +/- 1:06 min (15.6% improvement). The Good group improved by 47 +/- 37 s (7.3% improvement), and the Excellent-Superior group improved by 17 +/- 32 s (2.9% improvement). CONCLUSION: The magnitude of fitness improvement, as measured by run time improvement, was directly related to baseline fitness level but not related to movement mileage or high-intensity run mileage accrued during training.

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

PURPOSE: This study determined gender differences in voluntary reporting of lower extremity musculoskeletal injuries among U.S, Marine Corps (USMC) recruits, and it examined the association between these differences and the higher injury rates typically found among women trainees. METHODS: Subjects were 176 male and 241 female enlisted USMC recruits who were followed prospectively through 11 wk (men) and 12 wk (women) of boot camp training. Reported injuries were measured by medical record reviews. Unreported injuries were determined by a questionnaire and a medical examination administered at the completion of training. RESULTS: Among female recruits the most commonly reported injuries were patellofemoral syndrome (10.0% of subjects), ankle sprain (9.1%), and iliotibial band syndrome (5.8%); the most common unreported injuries were patellofemoral syndrome (2.1%), metatarsalgia (1.7%), and unspecified knee pain (1.7%). Among male recruits iliotibial band syndrome (4.0% of subjects), ankle sprain (2.8%), and Achilles tendinitis/bursitis (2.8%) were the most frequently reported injuries; shin splints (4.6%), iliotibial band syndrome (4.0%), and ankle sprain (2.8%) were the most common unreported diagnoses. Female recruits were more likely to have a reported injury than male recruits (44.0% vs 25.6%, relative risk (RR) = 1.72, 95% confidence interval (CI) 1.29-2.30), but they were less likely to have an unreported injury (11.6% vs 23.9%, RR = 0.49, 95% CI 0.31-0.75). When both reported and unreported injuries were measured, total injury rates were high for both sexes (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. CONCLUSIONS: Our results indicate that the higher injury rates often found in female military trainees may be explained by gender differences in symptom reporting.