Bone is a metabolically active tissue with continuous remodelling occurring throughout life. Although it has been noted for some time that athletes have a greater bone mineral density (BMD) than the normal population” 2, the role of exercise in bone mineralization is only recently being understood. The effects of exercise on BMD appear to be related to the mechanical stresses applied with maximal BMD in those regions of maximal stress. For example, previous cross-sectional studies involving tennis players have demonstrated cortical hypertrophy and increased BMD in the playing arms. Others have shown increased BMD in the os calcis of runners and the lumbar spine of weightlifters. There is also evidence that BMD is a function of muscle strength. Snow-Harter examined BMD in relation to muscle strength in young to middle-aged men and concluded that muscle strength did indeed make important contributions to BMD. Doyle demonstrated a significant correlation between the weight of the psoas muscle and the ash-weight of the third lumbar vertebral body in cadavers, thus indicating the close relationship that exists between functional loading and the adaptive response of bone.
Longitudinal studies in women have shown significant increases in BMD with exercise. In an investigation involving pre-menopausal women, Gleeson. reported a significant difference in the percentage change of lumbar BMD in exercising women versus controls. Many interventional exercise studies have demonstrated increased BMD in the lumbar spine, femur tibia, calcaneum and radius; other studies however have not supported these findings. A study by Smidt found no difference in BMD of the lumbar spine or upper femur following year of high intensity trunk exercise in postmenopausal women. The extent of weight bearing seems to play a role although, in one recent non-weight bearing exercise study,Bloomfield were still able to demonstrate an increase in bone mineral without the effects of gravity.