![]() A bone that is porous following a rest period may fail earlier than a fully adapted bone. The first phase of remodelling is bone resorption, which weakens the bone through increased porosity. Remodelling of the equine metacarpus is reduced during race training and accelerated during rest periods. Fatigue injuries develop when microdamage accumulates faster than remodelling can repair. Bone also repairs fatigued matrix through remodelling. Bone adapts to increased loading by modelling to maintain the strains within the bone at a safe level. This is important in horses as loads within the limb increase with increasing speed. Fatigue life decreases exponentially with increasing load. The term ‘fatigue life’ refers to the numbers of cycles of loading that can be sustained before failure occurs. Fatigue is a process that has undergone much study in material science in order to avoid catastrophic failure of engineering structures. Microcracks in the subchondral bone at sites where intra-articular fractures and palmar osteochondral disease occur are similar to the fatigue damage detected experimentally after repeated loading of bone. The highest joint loads occur in the fetlock, which is also the most common site of subchondral bone injury in racehorses. Many bone injuries are a consequence of repeated high loading during fast work, resulting in chronic damage accumulation and material fatigue of bone. Musculoskeletal injuries are a common cause of lost training days and wastage in racehorses.
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