Detraining reverses positive effects of exercise on the musculoskeletal system in premenopausal women

We studied the effects of a 6-month withdrawal of exercise after 12 months of progressive impact (jump) plus lower body resistance training on risk fa ctors for hip fracture in premenopausal women (age, 30-45 years). Twenty-ni ne women completed the 12-month training and detraining programs and were c ompared with 22 matched controls. Bone mineral density (BMD) at the greater trochanter, femoral neck, lumbar spine, and whole body and body compositio n (% body fat) were measured by dual energy X-ray absorptiometry (DXA; Holo gic QDR-1000/W). Knee extensor and hip abductor strength were assessed via isokinetic dynamometry (Kin-Com 500H); maximum leg power was tested using a Wingate Anaerobic Power test; and dynamic postural stability was measured on a stabilimeter (Biodex). All measurements were conducted at baseline, 12 months and 18 months with an additional midtraining measurement of BMD. Ex ercisers trained three times per week in a program of 100 jumps and 100 rep etitions of resistance exercises at each session. Intensity was increased u sing weighted vests to final values of 10% and 13% of body weight (BW) for jump and resistance exercises, respectively. Differences between groups fro m training were analyzed by repeated measures analysis of covariance (ANCOV A), adjusted for baseline values. Detraining effects were analyzed by compa ring the changes from training with the changes from detraining using repea ted measures analysis of variance (ANOVA). Baseline values were not signifi cantly different between exercisers and controls. Percent change over the t raining period was significantly greater in the exercise group than in the control group at the greater trochanter (2.7 +/- 2.5% vs. 0.8 +/- 0.8%, res pectively; p < 0.01) and approached significance at the femoral neck (1.2 /- 3.2% vs. -0.3 +/- 1.9%, respectively; p = 0.06). Significant improvement s also were observed in exercisers versus controls for strength and power w ith exercisers increasing 13-15% above controls, whereas stability was not different between groups. After 6 months of detraining, BMD and muscle stre ngth and power decreased significantly toward baseline values, whereas cont rol values did not change. We conclude that the positive benefits of impact plus resistance training on the musculoskeletal system in premenopausal wo men reverse when training is withdrawn. Therefore, continued training, perh aps at a reduced frequency and intensity, is required to maintain the muscu loskeletal benefit from exercise that may lower fracture risk in later life .