Exercise-induced muscle damage and potential mechanisms for the repeated bout effect

Unfamiliar, predominantly eccentric exercise, frequently results in muscle damage. A repeated bout of similar eccentric exercise results in less damag e and is referred to as the 'repeated bout effect'. Despite numerous studie s that have clearly demonstrated the repeated bout effect, there is little consensus as to the actual mechanism. In general, the adaptation has been a ttributed to neural, connective tissue or cellular adaptations. Other possi ble mechanisms include, adaptation in excitation-contraction coupling or ad aptation in the inflammatory response. The 'neural theory' predicts that the initial damage is a result of high st ress a relatively small number of active fast-twitch fibres. For the repeat ed bout, an increase in motor unit activation and/or a shift to slow-twitch fibre activation distributes the contractile stress over a larger number o f active fibres. Although eccentric training results in marked increases in motor unit activation, specific adaptations to a single bout of eccentric exercise have not been examined. The 'connective tissue theory' predicts that muscle damage occurs when the noncontractile connective tissue elements are disrupted and myofibrillar in tegrity is lost. Indirect evidence suggests that remodelling of the interme diate filaments and/or increased intramuscular connective tissue are respon sible for the repeated bout effect. The 'cellular theory' predicts that muscle damage is the result of irrevers ible sarcomere strain during eccentric contractions. Sarcomere lengths are thought to be highly non-uniform during eccentric contractions, with some s arcomeres stretched beyond myofilament overlap. Loss of contractile integri ty results in sarcomere strain and is seen as the initial stage of damage. Some data suggest that an increase in the number of sarcomeres connected in series, following an initial bout, reduces sarcomere strain during a repea ted bout and limits the subsequent damage. It is unlikely that one theory can explain all of the various observations of the repeated bout effect found in the literature. That the phenomenon oc curs in electrically stimulated contractions in an animal model precludes a n exclusive neural adaptation. Connective tissue and cellular adaptations a re unlikely explanations when the repeated bout effect is demonstrated Drie r to full recovery, and when the fact that the initial bout does not have t o cause appreciable damage in order to provide a protective effect is consi dered. It is possible that the repeated bout effect occurs through the inte raction of various neural, connective tissue and cellular factors that are dependent on the particulars of the eccentric exercise bout and the specifi c muscle groups involved.