The muscle's wide array of functional characteristics is mediated by the existence of fiber types, differing one from the other in terms of contractile and metabolic protein isoforms content. Numerous classification systems have evolved, describing the muscle's physical, architectural and metabolic characteristics. Nevertheless, it appears that those characteristics assemble in more or less independent modules, excluding the development of a generalized classification of muscle fiber types. The discovery of several mechanisms allowing for the muscle to adapt to training or other changes opens new interventional perspectives regarding sport performances, as well as the maintenance of an adequate musculature in patients.