To the Editor: Professor Marcus' editorial on the importance of mechanical loading for skeletal integrity was welcome and thought provoking; I hope you will accept this reply as a continuation of much needed debate about some of the discrepancies in the literature. Professor Marcus comments on our recent paper comparing response to the same jumping exercise in pre- and postmenopausal women1 and offers, as an explanation for the lack of response in the postmenopausal women, a lack of progression of the intensity of the exercise. However, there was no progression in either group beyond the first few weeks, so while I agree that progression is desirable, if it is safe, this does not explain the discrepancy. He claims that the ground reactions recorded in this jumping exercise of 3–4 body weights (BW) would be an inadequate osteogenic stimulus because jogging, which achieves ground reactions of similar magnitude, is not effective for the hip. This is not so; jogging and varied weight-bearing exercise do have significant positive effects on BMD at the hip, as reported in controlled trials, most of them randomized, in perimenopausal women,2 postmenopausal women,3-6 and men.7 The ground reactions were not measured in these studies but are unlikely to have exceeded 4 BW. The older women in our study were exercising with an average ground reaction of 4 BW, whereas the younger group achieved only 3 BW. This was because the jump height was similar but the body mass of the younger women was significantly less (6%, p < 0.05). Jumping is an energetic activity not normally performed by middle-aged women; the ground reactions would certainly have been unusual and greater than habitually incurred, probably more so than in the young women. (Women in either group who went jogging were screened out). Professor Marcus argues that it was the bone strains caused by muscle tension rather than by transmitted force from the ground reaction that stimulated bone in the younger group. Muscle tension in this exercise is related to the ground reaction and as great in magnitude although it has a slower rise time. It is undoubtedly important, but it was less in the younger women than in the older. The jumping exercise was not maximal, so although the younger women had slightly greater maximal leg extensor power (5% NS), they would have used less muscle tension to achieve the same jump height because of their lower body mass. (This muscle tension would have been even less relative to their maximal capability). Therefore, for the young women both muscle tension and ground reaction were less than in the older women in absolute and relative terms, but it was sufficient to improve their BMD at the hip. The view that the younger group had “sufficiently high muscle strength at entry that a constant stimulus sufficed” and that “skeletal loading in the older women likely remained subthreshold” runs counter to the facts of the study and also to the mechanical theory of osteogenesis which predicts that strains that are unusual in direction and greater than habitual in magnitude will be effective. The explanation has to lie elsewhere and to be related to age rather than hormonal status. Our jumping regimen differed from others in its brevity, taking only 10 minutes including warm-up and stretches, whereas other regimens which improved BMD at the hip, including a jumping regimen in perimenopausal women,8 have taken 30–50 minutes. Perhaps older women need an additional permissive stimulus inherent in more prolonged bouts. Professor Marcus refers to the excellent paper by Kerr et al.9 which showed that heavy weights lifted a few times, some freely, were effective, whereas lighter weights lifed more often were not, although the improvement in muscle strength was similar. (They did not report on cycling). This is in keeping with the need for a substantial overload and explains why so many less intensive studies that improved muscle strength did not improve bone. The desirability of a simple public health message is unquestioned. However, a common thread may be developing. There is evidence that feasible weight-bearing exercise such as intermittent jogging and stair climbing is as effective as weight training for the hip and more so for the spine4, 5; it will also help maintain balance and prevent falls. The recent recommendations for reducing risk of cardiovascular disease suggest that several short bouts of weight-bearing exercise are as effective as one long one, an approach that will also be better for arthritic joints. Four for the price of one is not so bad.