Why bones bend but don't break.

The musculoskeletal system is adept at dissipating potentially damaging energy that could accelerate fracture consequent to multiple loading cycles. Microstructural damage reduces bone's residual properties, but prevents high stresses within the material by dissipating energy that can lead to eventual failure. Thus skeletal microdamage can be viewed as an adaptive process to prevent bone failure by dissipating energy. Because a damaged bone has reduced strength and stiffness, it must be repaired, so bone has evolved a system of self-repair that relies on microdamage-stimulated signaling mechanisms. When repair cannot occur quickly enough, low energy stress fractures can occur. The regulating effects of muscle also prevent failure by controlling where high stresses occur. Acting synergistically, muscle forces dissipate energy by appropriately regulating accelerations and decelerations of the limbs during movement. When muscles become fatigued, these functions are constrained, larger amounts of energy are imparted to bone, increasing the likelihood of microstructural damage and fracture. Thus, healthy bones are maintained by the ability of the musculoskeletal system to dissipate the energy through synergistic muscular activity and through the maintenance of microstructural and material properties that allow for crack initiation, but also for their repair.