Optimum stiffness for leg bones

A limb bone will be lighter if it is made thinner. However, thinner bones are more flexible (for given length and shape of cross‐section) so their muscles must shorten more to move the distal end of the bone against a resisting force. To shorten more, a muscle needs longer muscle fibres and so must be heavier. Thus a particular thickness for the bone will enable the combined mass of bone plus muscle to be minimized. Peak stresses due to bending moments, in bones that were optimized according to this criterion, would be about ±70 MPa. Stresses of about this magnitude have been found in leg bones of various mammals, in strenuous activities such as running and jumping. However, similar stresses might be predicted on grounds of strength requirements, to give adequate factors of safety.