Publisher Summary Muscle provides several striking examples of the way a tissue can adapt to different types of physical signals. Adaptation to a different work regime is brought about by changes in fiber type and cross-sectional area, which result in altered fatigue resistance and power output. This process involves quantitative and qualitative changes in gene expression including the myosin heavy chain isogenes, which encode different types of molecular motors. Using a similar process some species of fish respond to seasonal temperate change and are able to rebuild their myofibrillar systems for warm and cold temperature swimming by selective myosin gene expression. In mammals, it is apparent that muscle tissue is influenced by mechanical signals and that there must be local as well as systemic regulation of tissue mass and phenotype. The nature of the chemical link between the physical signal and the up-regulation of certain muscle genes involved in muscle mass determination has been elucidated by the cloning of a new growth factor that is only expressed in muscles subjected to stretch and/or exercise and that is designed for an autocrine/paracrine action.