Exercise immunology: a skeletal muscle perspective.

When exploring the possible mechanisms by which adaptation to physical exercise can occur, the immune system appears to be a likely candidate of importance, because physical exercise will exert both systemic and local effects on the immune system. This review will focus on the immune system in the perspective of skeletal muscle tissue adaptation to physical exercise. The involvement of circulating and tissue bound leukocytes, cytokines, hormones and growth factors will be discussed. A few new hypotheses are presented: 1) The primary mechanism governing skeletal muscle adaptation to physical exercise is suggested to be of non-inflammatory origin; 2) it is suggested that IL-10 may function as one of the signals transmitted by skeletal muscle cells when substrate levels are low; 3) creatine kinase could have immuno-modulatory actions, thereby serving as a messenger molecule between skeletal muscles and the immune system. Based on the intricate and complex array of events governing life and death in other cells types, a simple explanation of the issues of skeletal muscle development, regeneration and adaptation is unlikely. There are large individual differences in the immune response to the same relative or absolute exercise intensity, suggesting that interpreting the average effect of exercise on a group of individuals may not reveal the cause and effect of any given immunological variable. More sophisticated analyses of collected data is suggested in future studies and perhaps some re-evaluation of existing data is also needed. If we can understand the mechanisms behind muscle adaptation, we can also optimize training programs and athletic performance.