Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin  that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.

A muscle is a contractile tissue in an animal's body used especially for movement.