Growing evidence has shown that skeletal muscle secretes several bioactive proteins from within the cell into extracellular fluid. The secretion of several proteins, whose levels increase in response to exercise, can regulate the functions of several organs via autocrine and paracrine actions, and mediate exercise-induced benefits such as metabolic improvement, anti-inflammation, and muscle building; this is known as the myokine theory. In addition, we found a novel muscle-secreted protein, secreted protein acidic and rich in cysteine (SPARC), a secreted matricellular glycoprotein. The muscle-secreted protein SPARC can support underlying mechanisms of epidemiological studies that suggest that regular exercise can prevent the incidence of colon cancer. Many different types of studies have suggested that many other proteins secreted from muscle have yet to be identified. In addition to the proteins, non-coding small RNA in exosome and metabolites which generate in process of nutrients metabolism with muscle contraction are also suggested to be secretory bioactive factors. These secretory factors may be biomarkers that reflect muscular function and beneficial adaptation achieved by exercise training, and could estimate adequate condition of exercise to obtain its beneficial effects
