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 anthropo metry 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 confi rmed, 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 fr acture cases. Using DXA data, hone structural geometry and cortical dimensi ons were derived at scan locations and muscle cross-sectional area was comp uted at the midthigh, Measurements were compared within gender between pool ed 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 g enders, but patterns differed, Female cases had thinner cortices and lower areal bone mineral density (BMD), whereas male cases had externally narrowe r bones but similar cortical thicknesses and areal BMDs compared with contr ols. In both genders, differences in fitness, muscle, and bone parameters s uggest poor skeletal adaptation in fracture cases due to inadequate physica l conditioning prior to training, To determine whether bent! and muscle str ength parameters differed between genders, all data were pooled and adjuste d 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 hut lower femur areal BMDs. Female bones, on average, wer e narrower and had thinner cortices (not significant in the femur, p = 0.07 ), Unlike the hone 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. (C) 2000 by Elsevier Science Inc. All rights reserved.