What Are the Factors Which Influence the Strength of Our Bone?

It is a mistake to think of the skeletal system as an inert framework which merely supports and protects the body's tissues and organs. The strength (or integrity) of bone is adaptable and is largely determined by the mechanical stresses to which it is subjected. This and other factors which also influence the strength of bone are discussed below.

Weight bearing activity
Weight bearing activities are those which place repetitive compressive forces on bone. Regular weight bearing exercise stimulates an increase in the density and strength of bone whilst, in contrast, periods of inactivity lead to a reduction in bone density. The reduced density of mineral salts leaves bone weaker and more prone to fracturing.

Studies on various athletic groups have revealed that those involved in resistance training and running have greater bone densities than untrained people. Not surprisingly, swimmers, whose training does not involve weight bearing activity, have bone densities no greater than those of sedentary individuals. This 'invisible' benefit of load bearing activity should be considered when planning an exercise program, especially when the program is designed for middle aged and elderly women who are particularly prone to the development of osteoporosis, which is simply the weakening of bone tissue.

Whilst regular weight bearing exercise has a positive effect, excessive activity may result in injury. For example, stress fractures of the foot can occur in marathon runners as a result of excessive training. It is likely that when the training load is increased too rapidly the bones are not allowed sufficient time to adequately adapt to the higher levels of stress.

Hormonal environment
Both the male and female sex hormones (testosterone and estrogen) influence the strength of bone. Whilst both hormones are particularly important stimulators of the skeletal growth spurt at puberty each has a positive effect on bone density throughout life. Growth hormone has similar effects on bone growth and strength in both sexes.

For women the drop in estrogen levels at menopause contributes to the increased risk of osteoporosis in middle age. Indeed, hormone (estrogen) replacement therapy is sometimes prescribed by medical practitioners in order to maintain skeletal strength after menopause.

Dietary factors
Vitamins C and D, and the minerals calcium and phosphorous are important factors which influence bone integrity. For example, vitamin D increases the absorption of calcium from the gut and directly effects several steps in the bone formation process. Vitamin C, on the other hand, is required for the formation of collagen, the protein component of bone and many other tissues in the body. It must be said that diet is not deficient in either vitamins C or D, so supplementing the diet with extra quantities of these vitamins would be unlikely to result in the development of stronger bones.

The bone's calcium store is not 'fixed' at any stage of development. As mentioned previously, a drop in the level of calcium within the body's fluids causes the breakdown of mineral salts and the release of calcium from bone. In this sense, calcium's role in contributing to the integrity of bone is of secondary importance to the many physiological functions that it also performs. Calcium is, for example, essential for muscular contractions and the normal functioning of the nervous system.