To measure the physiological changes in bone in response to strenuous exercise we performed a prospective study of male army recruits over 10 weeks of basic training. Measurements performed at the start and completion of training consisted of ultrasound (US) measurements of the heel: velocity of sound (VOS in m/seconds) and broadband ultrasound attenuation (BUA in dB/MHz) and bone turnover markers; osteocalcin (OC), bone-specific alkaline phosphatase (BALP), and tartrate-resistant acid phosphatase (TRAP). Forty subjects were recruited for the study and 26 completed training. Over the 10-week study period there was a significant 1.7% fall in mean VOS [mean paired difference (mpd) 27.2 m/second, SEM 9.5 (95% CI 7.5-46.8) P = 0.009] and a nonsignificant 3.4% increase in BUA (P = 0.159). There were significant falls in markers of bone formation OC [11.6%, mpd 0. 11 microg/liter (95% CI 0.07-0.14) P < 0.001] and BALP [13.3%, mpd 3. 49 U/liter (CI 0.80-6.18) P = 0.013] and a nonsignificant 9.5% fall in TRAP a marker of bone resorption. The 10 recruits subsequently injured had a significantly lower VOS on entry [mean difference 24.2 m/seconds (95% CI 4.6-43.7) P = 0.017] and nonsignificantly raised BUA and baseline levels of all bone markers. The ultrasound changes may be accounted for by increase in trabecular separation and a fall in trabecular connectivity due to microfracture. The decrease in bone markers implies a fall in bone turnover.

During a four-month period we observed 12 stress fractures of the inferior pubic ramus in 11 military recruits undergoing basic training. Eleven of these were in women. This high number was considered to be caused by the introduction of mixed training of male and female recruits; this forces women to increase their stride length when marching. The presenting symptom was chronic groin pain which failed to settle with rest, and the clinical diagnoses were confirmed by radiographs in all but one patient who was diagnosed by 99mTc bone scanning. Since the required stride length has been reduced there have been no new cases of stress fracture of the pelvis.

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