Muscles contract and relax to create movement. Skeletal muscles, which are attached to bones, are controlled voluntarily by the nervous system. When the brain sends signals through nerves, muscles receive the message and shorten, pulling bones and causing movement.

Smooth muscles (in organs) and cardiac muscle (in the heart) function involuntarily. Muscles work in pairs—when one contracts, the other relaxes. Regular use strengthens them, and muscles also generate heat and maintain posture.