Weight training decreases vertebral bone density in premenopausal women: a prospective study.

The effect of exercise on bone mass is unclear. To determine the skeletal effect of weight-bearing exercise in premenopausal women, we prospectively evaluated the effects of a weight-training program on lumbar spine bone mass in 10 women (mean +/- SEM, 36.2 +/- 1.3 yr) and compared the results with those in 7 sedentary women (40.4 +/- 1.6 yr). None of the women had previously participated in a weight-training program, and all ingested a 500-mg calcium supplement each day throughout the study. Axial loading and balance of large muscle groups were emphasized. Individual strength increased by 57 +/- 8% over 9 months. Despite the increase in muscle strength, lumbar spine bone density in the exercising women decreased by 2.90% at 4.5 months and 3.96% at 9 months (P = 0.01). In contrast, there was no change in lumbar density in the controls over the 9-month period. We conclude that short term weight training at this frequency and intensity decreases vertebral bone mass in premenopausal women.