Recent development of research in exercise-induced muscle damage and plasticity of skeletal muscle

摘要

Skeletal muscles adapt structurally and physiologically to mechanical stimuli generated in muscle contractions, and two common consequences of such adaptation are strength gain and muscle hypertrophy, which are typical examples of skeletal muscle plasticity. Injurious physical, chemical, or biological stressors damage skeletal muscles. The severity of muscle damage varies from micro injury of a small number of muscle fibres to a disruption of a whole muscle depending on the cause of damage. Skeletal muscle fibres have a capability to regenerate, and satellite cells within the injured muscle fibres are activated, proliferate and differentiate to form myoblasts, which could fuse to form myotube in the regeneration process. Satellite cells are the source to maintain myonuclei to cytoplasmic volume ratio by adding myonuclei to the increasing area and length of muscle fibres, thus play a major role in muscle hypertrophy. It has been documented that activation of satellite cells is essential for skeletal muscle hypertrophy. Thus, it seems reasonable to assume that muscle damage increases hypertrophic responses. Several studies have reported superiority of resistance training consisting of eccentric contractions to concentric contractions for muscle hypertrophy. Since eccentric contractions, especially performed in "accustomed" exercise, result in muscle damage, one might think that muscle damage is responsible for muscle hypertrophy. However, it does not appear that muscle damage is a pre-requisite for muscle hypertrophy, although it is true that eccentric contractions are more potent stimulus for muscle hypertrophy than concentric contractions. It is important to note that protein synthesis should exceed protein degradation for a muscle to be hypertrophied, and muscle damage is the process of protein degradation. If a muscle is damaged regularly, muscle hypertrophy might not be induced. Therefore, damaging muscles does not necessarily induce muscle hypertrophy, and the benefit of eccentric contractions as stimulus for muscle hypertrophy should be considered separately from muscle damage. Muscles become less susceptible to muscle damage, when, the same or similar exercise is repeated, which is another example of plasticity. Because of this adaptation, it does not appear that muscle fibres are extensively damaged even in strenuous exercise training. The presentation will update the information about exercise-induced muscle damage, and discuss how muscle damage is associated with muscle plasticity.