Weight-bearing exercise and ground reaction forces: a 12-month randomized controlled trial of effects on bone mineral density in healthy postmenopausal women

Abstract

The effects of brief daily exercise on bone mineral density (BMD) were assessed in a randomized controlled trial in 44 healthy postmenopausal women using weight-bearing exercise in a regimen adapted from osteogenic protocols reported in animal studies. BMD was assessed masked using dual energy X-ray absorptiometry at 0, 6, and 12 months. The sites assessed were the proximal femur (neck, Ward's triangle, and trochanter) and the lateral spine (L2-3) to assess the effects of the exercise, and the radius (ultradistal and 33% distal) as a marker for systemic effects. The test group was required to perform 50 "heel drops" daily at home (raising the body weight onto the toes and then letting it drop to the floor keeping the knees and hips extended) and to attend a weekly class of mixed exercises, which included some high-impact activity. The control group also attended a weekly exercise class run by the same teacher, which included only low-impact activity, and did flexibility exercises at home daily. The ground reaction forces (as a ratio of body weight) during heel drops were 2.5 to 3.0 N/N, with a rate of rise of 50-100 kN/sec. A patient with an instrumented femoral implant allowed comparison of compressive axial forces in the shaft of the proximal femur with the ground reaction forces, and these appeared to be transmitted undamped to the shaft of the femur. Initial analysis of BMD in the women showed no significant increases after 12 months of exercise at any site in either group, and the groups did not differ significantly from each other in this respect. Proximity to menopause was not associated with rapid bone loss, and in those who were more than 6 years postmenopausal, there was evidence for a maintenance effect of the exercise in the test group. Compliance (83%) and increases in leg extensor power (15%) were similar in both groups, and when they were combined, BMD was maintained at the trochanter but fell significantly at the radius (p < 0.001).