Exercise effects on bone mass in postmenopausal women are site-specific and load-dependent

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Abstract

It is considered that skeletal mass in humans may respond to loading or the number of loading cycles. The aim of this study was to examine the effect of a 1 year progressive resistance training program on the bone mass of 56 postmenopausal women. Assignment was by block randomization to one of two resistance training groups: a strength trained group (3 × 8 repetition maximum) or an endurance group (3 × 20 repetition maximum). The resistance exercises were selected to stress the ipsilateral forearm and hip region. The exercising side was randomly assigned with one side exercised while the alternate side acted as the nonexercise control. Bone mineral density (BMD) was measured every 3 months at the radial forearm and four hip sites using the Hologic QDR 2000 bone densitometer. A linear regression function was fitted for each individual's bone density results, and the slope was compared for the exercise and control side using paired t-tests. The bone mass increase with the strength regimen was significantly greater at the trochanteric hip site (control -0.6 + 2.2%, exercise 1.7 + 4.1%,p < 0.01), at the intertrochanteric hip site (control -0.1 + 2.1%, exercise 1.5 + 3.0%,p < 0.05), Ward's triangle (control 0.8 + 5.2%, exercise 23 + 4.0%,p < 0.05), and at the ultradistal radial site (control -1.4 + 2.3%, exercise 2.4 + 4.3%. p < 0.01). There was no significant increase in BMD with the endurance regimen except at the radius midsite (control -1.0 + 23%, exercise 0.1 + 1.4%, p < 0.01). In both the endurance and the strength group, muscle strength, tested by a one-repetition maximum (IRM) test, increased significantly for all 10 exercises (p < 0.01) and to a similar degree in the two groups. In the strength group but not the endurance group there were significant correlations between the slope of the change in BMD and the percentage increase in strength as follows: trochanter with leg press; intertrochanter with leg press (p < 0.05); and Ward's triangle with hip extension and hip adduction (p < 0.05). Thus these results support the notion of a site-specific response of bone to maximal loading from resistance exercise in that although the trochanter and intertrochanteric bone density was elevated by the resistance exercises undertaken, there was no effect on the femoral neck value. Postmenopausal bone mass can be significantly increased by a strength regimen that uses high-load low repetitions but not by an endurance regimen that uses low-load high repetitions. We conclude that the peak load is more important than the number of loading cycles in increasing bone mass in early postmenopausal women.

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