One of the main functions of the skeleton is to support the body against gravitational forces and to provide a resistant attachment for muscles to enable locomotion. The natural stimulus for maintenance of bone strength is therefore mechanical loading (1). Animal models have suggested that only a few repetitions of site-specific loading of bone are necessary to stimulate increased formation of bone.
WHAT IS OSTEOPOROSIS? From middle age onwards the skeleton loses bone mineral and becomes more fragile. This is a normal part of ageing but it happens faster in some individ- uals than others putting them at greater risk of suffering osteoporotic fracture. Women are more vulnerable because of the decrease in oestrogen levels which occurs at the time of menopause and usually causes an accelerated loss for the fol- lowing few years. A third of all women and one in twelve men will suffer an osteoporotic frac- ture at some time in their lives.
Assessment The standard measurement of bone fragili- ty is to measure the bone mineral density (BMD) using a low dose x-ray technique called dual energy x-ray absorptiometry or more commonly referred to as DEXA scan- ning. Bone mineral density is closely related to bone strength. The WHO classi- fication of osteoporosis is a value more than two standard deviations below the mean for young normal women.
Potential benefits of exercise Epidemiology and fracture risk Large community surveys, based on ques- tions about previous patterns of physical activity, show that those with sedentary lifestyles have a greater risk of osteo- porotic fracture than the physically active (2). This remains so even after allowing
for other influences such as family histo- ry and smoking. The definition of an ‘active lifestyle’ in these surveys is based on self-reported accounts of occupational, sporting and leisure activities which are combined to produce a global score of physical activity. This is not helpful for arriving at an exercise prescription and as yet there have been no interventions with fractures as an outcome. However it seems likely that a variety of brief energetic weight-bearing activities will improve or maintain bone strength. The positive influence of exercise on muscle strength and balance will reduce the risk of falls and so will help to prevent fractures by a second route.
Randomised controlled trials Studies of various forms of exercise have