Optimum strengths for bones liable to fatigue and accidental fracture

Limb bones are liable to fail by fatigue, due to stresses imposed repeatedly in activities such as running. They may also be broken by larger stresses which occur occasionally in accidents. Too weak a bone will probably fail but too strong a bone is unduly heavy. A mathematical model predicts optimum strengths for bones subject to the hazards both of fatigue and of accidents. When accidents are mild and predictable, fatigue is the more important hazard. When they are large and unpredictable they are the more important hazard and stronger bones are generally preferred, but in extreme cases it may become advantageous to dispense with the bone. There is a restricted range of circumstances in which fatigue and accidents are both important hazards. The strengths of many limb bones seem surprisingly low, in the light of the theory and of current knowledge of the fatigue properties of bone.