After performing an unaccustomed eccentric exercise and exhibiting severe soreness, the muscle rapidly adapts to reduce further damage from the same exercise. This is called the “repeated-bout effect”.
As a result of this effect, not only is the soreness reduced, but other indicators of muscle damage, such as swelling, reduced strength and reduced range of motion, are also more quickly recovered from. The effect is mostly, but not wholly, specific to the exercised muscle: experiments have shown that some of the protective effect is also conferred on other muscles.:69
The magnitude of the effect is subject to many variations, depending for instance on the time between bouts, the number and length of eccentric contractions and the exercise mode. It also varies between people and between indicators of muscle damage.:69 Generally, though, the protective effect lasts for at least several weeks. It seems to gradually decrease as time between bouts increases, and is undetectable after about one year.:70
The first bout does not need to be as intense as the subsequent bouts in order to confer at least some protection against soreness. For instance, eccentric exercise performed at 40% of maximal strength has been shown to confer a protection of 20 to 60% from muscle damage incurred by a 100% strength exercise two to three weeks later.:73 Also, the repeated-bout effect appears even after a relatively small number of contractions, possibly as few as two. In one study, a first bout of 10, 20 or 50 contractions provided equal protection for a second bout of 50 contractions three weeks later.:70
The reason for the protective effect is not yet understood. A number of possible mechanisms, which may complement one another, have been proposed. These include neural adaptations (improved use and control of the muscle by the nervous system), mechanical adaptations (increased muscle stiffness or muscle support tissue), and cellular adaptations (adaptation to inflammatory response and increased protein synthesis, among others).:74