The purpose of this article is to understand of the muscle adaptation based on myosin heavy chain. Especially, skeletal muscle dadptation in related to aging, unloading, training will discussed. MHC expression is highly plastic in muscles of adult mammals in accordance with the environmental conditions. These changes is called muscle plasticity. The plasticity is the atility of muscle cell to alter either the quantity of protein or the type of protein. MHC is both an important structural and regulatory protein comprising the contractile apparatus.