Muscle is generally divided into three subtypes-skeletal, cardiac, and smooth-but because this edition focuses on the musculoskeletal system, this article concentrates on skeletal muscle. We review ultrastructure and function and then look at the latest scientific ideas concerning the physiological basis of muscle contraction. It is important to appreciate the different muscle types and how they act with respect to muscle growth and adaptation. Finally, what happens to muscle cells when they are damaged and the reparative response is considered.