Eccentric action of muscles: physiology, injury, and adaptation.

Eccentric muscle action deserves special consideration from the standpoint of physiology, adaptation, and training. The function of muscles as shock absorbers or springs seems to be quite different from other actions described in classical descriptions of muscle biology. This uniqueness certainly requires a more careful understanding of muscle as a unit consisting of myofibers and fascia which may work together or in opposition in response to chronic or acute stimuli. In addition, the stretch-shortening cycle is a special case of its own. However, from the standpoint of maximum human performance, there remain tremendous gaps in our understanding of the role of eccentric muscle action and its use in athletic training. How much is good? Does microfibrosis represent a problem of overtraining and eventually limit performance, or is it advantageous for success? Is the body-builder really developing muscle or connective tissue separating muscles? How does eccentric muscle action sometimes produce pain but not always? It would appear that much work is needed before a complete understanding of eccentric muscle action is obtained. This brief review has been designed to encourage research, argument, and discussion.