Muscle is the soft tissue found in most animals. Muscle cells contain actin and myosin filaments, which slide over each other and contract, changing the length and shape of cells. Muscles produce strength and movement. They are mainly responsible for maintaining and changing posture, movement and internal organ movement, such as the contraction of the heart and the movement of food through the digestive system through peristalsis.

Muscle tissue is derived from the mesoderm of germ cells in a process called myogenesis. There are three types of muscles: skeletal or striated, cardiac and smooth. Muscle movements can be classified as voluntary or involuntary. The unconscious contraction of the heart and the smooth muscle is called involuntary, while the skeletal muscle contracts on command. Skeletal muscle can be divided into fast muscle fiber and slow muscle fiber.

Muscle is primarily powered by oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, especially rapid twitching of fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules, which are used to drive the movement of myosin heads.

The term "muscle" derives from the Latin term "small muscle," meaning "little mouse," either because of the shape of certain muscles, or because the contracted muscles look like mice moving under the skin.