Anatomies: A Cultural History of the Human Body by Hugh Aldersey-Williams Page B

elbow and the skin with no muscular protection – is a consequence of our evolution into bipeds. If we still walked on all fours, the forelimb would be angled so that the elbow bent towards the back, not outward, and it would be better protected. Our knees suffer too, as we learn when we reach a certain age, and this too is a consequence of evolution, and our using two feet to bear the weight formerly borne on four. The Achilles heel, however, cannot really be counted as a weak point: anybody would succumb if shot in the heel with a poisoned arrow as Achilles was in legend. This Victorian metaphor is thought to originate instead with Samuel Taylor Coleridge’s reference to ‘Ireland, that vulnerable heel of the British Achilles’.
    The physics is remarkable enough, but bone is also living tissue. It must perform its structural function at the same time as it grows with the rest of the body. Bones develop in response to stress. Tiny cracks form when they are subjected to forces during normal exercise. These cracks send chemical messages instructing new bone tissue to form. However, bone will fail if pushed only a little way beyond its normal performance limit – to about 120 per cent, compared to 200 per cent for materials like steel. ‘The body is neither over- nor under-engineered, because all bones have this 120 per cent factor,’ Chris tells me. ‘It’s actually quite natural that you become optimal.’ In other words, a bone does not become ‘too strong’ unless there is some exertion that is making it so, in which case it becomes simply strong enough . Conversely, a bone does not usually weaken beyond a safe level unless through lack of use. When sportsmen speak of ‘giving it 110 per cent’ they are talking more sense than perhaps they realize.
    Because of gravity, the body needs to save weight as it grows, as Haldane has explained with reference to the giants in The Pilgrim’s Progress . It achieves this goal in part by growing bone faster lengthways than across its width (at the price of some reduction in comparative strength in the adult bone). Something clearly guides bone to grow where it is most needed. Whatever this mechanism is – and we will come to it in a moment – it is highly dynamic and responsive to the bodily world around it. It has long been known that bones can be made to increase in size and strength if they are repeatedly stressed. The bones in the spear-carrying arm of a Roman soldier are larger than the bones in the opposite arm, and the same goes for the racquet-wielding arms of tennis players today. Especially in the case of athletic activities taken up in youth, such as ballet or gymnastics, the bones can also be shaped in response to exercise before they harden.
    This process allows us to tell much about our ancestors from their surviving bones. We conceitedly believe we are taller than our ancestors because we eat so well. In fact, evidence from skeletons of Homo erectus and early Homo sapiens shows that they were taller than we are, owing to the strenuous work necessary to survive. From the size of the rough areas on the bones to which muscle attaches, it is known that they were proportionately fitter and heavier too. There is nothing to prevent our regaining these superhuman proportions if only we are prepared to put in the effort – our shrinking stature is not an evolutionary change, but a response to our environment.
    Until recently, very little was known about this kind of bone growth. Normal bone growth during development is well understood; it involves the division of cartilage cells on fronts located at the ends of the long bones and their subsequent hardening into bone. But the way that bones respond to use or disuse during life has been something of a puzzle, despite the obvious importance of knowing more about it. Bone can lose up to a third of its mass during the short time a broken leg is in plaster, for example; fortunately, this mass is as quickly replenished