Physical training is shown to have potential to increase the mass of healthy bones. In contrast, immobilization, used as a treatment of soft tissue and bone injuries, is shown to result in atrophy of these tissues. In this unique 26-month follow-up of a 26-year-old female student, we determined the effects of a 1-year strenuous unilateral lower limb strength training and subsequent anterior cruciate ligament (ACL) rupture on bone mineral density (BMD) of lumbar spine and lower extremities (femoral neck, distal femur, patella, proximal tibia, calcaneus), measured repeatedly (11 times) by dual-energy X-ray absorptiometry (DXA). Also, the strength and functional characteristics of the lower extremities were repeatedly examined. This prospective study strikingly demonstrated the deleterious effects of an ACL rupture on the BMD of the affected limb in contrast to beneficial but modest effects of the preceding controlled training. The site-specific increases in the BMD were 1.8-3.1% during the training. The posttraumatic decrease in the BMD of the injured knee was rapid and corresponded to approximately 2 SD (approximately 20%) observed in the age- and sex-matched population. A year after the injury, the subject's knee function and muscular performance as well as the usage of the injured limb were completely recovered, but the site-specific BMDs were still approximately 1 SD below the subject's baseline BMD, although continuously increasing. This study suggests that considerable posttraumatic osteopenia at the affected bone sites occurs quickly regardless of appropriate preventive and rehabilitative actions, and the time needed for steady-state and recovery will be long.(ABSTRACT TRUNCATED AT 250 WORDS)