The perception of muscle soreness is probably due to the activation of free nerve endings around muscle fibers. These nerve endings serve as receptors of noxious stimuli associated with muscle damage. Modulation of soreness may take place at the peripheral receptor sites or at a central or spinal level. This multilevel modulation may explain the large intersubject variation in the perception of muscle soreness. The type of exercise that produces the greatest degree of soreness is eccentric exercise, although isometric exercise may also result in soreness. Eccentric exercise has been shown to produce muscle cellular damage and decrements in motor performance as well. Although training is considered to prevent muscle soreness, even trained individuals will become sore following a novel or unaccustomed exercise bout. Thus, training is specific to the type of exercise performed. Our laboratories have shown that the performance of a single exercise bout will have an effect on a subsequent similar bout given up to 6 weeks later. Thus, when a second bout of downhill running was given to subjects 6 weeks after the first bout, with no intervening exercise, less soreness developed, and muscle damage was estimated to be reduced. The explanation for this long-lasting prophylactic or "training effect" is currently under investigation in our laboratories.