Actually, muscle activity has a beneficial effect in the prevention of many chronic diseases, and a better understanding of the mechanism of activity-dependent muscle changes may help to identify potential therapeutic targets. The field of study is no longer confined to basic science, but its applications are spreading from sport sciences to clinical medicine and are likely to expand in the near future. ![]() The molecular mechanisms of the heterogeneity and plasticity of muscle fibers have been the object of intensive investigation, and several signaling pathways have been identified that appear to mediate the emergence of specific muscle phenotypes. This property is defined muscle plasticity or malleability. In addition, the structural and functional properties of the fibers, which are generally referred to as fiber phenotype, can change in response to hormonal and neural influences, nerve-activity being a major determinant of the fiber type profile. The heterogeneity of the muscle fibers is the base of the flexibility which allows the same muscle to be used for various tasks from continuous low-intensity activity (e.g., posture), to repeated submaximal contractions (for example, locomotion), and to fast and strong maximal contractions (jumping, kicking). The selective recruitment of motor units enables a muscle to respond in the best manner to the functional demands. To form a muscle, many (from tens to hundreds) motor units are assembled together, and each brings its own specific and distinct contribution. The functional unit of the motor system, the motor unit, is composed of a motor neuron and a bunch of muscle fibers with similar, if not identical, structural and functional properties. ![]() Mammalian skeletal muscles are heterogeneous in nature. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. ![]() The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors.
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