Muscles are the bundles of soft tissues in the bodies of animals. They are joined with bones and nerves. They help produce movement and keep the body parts in position. There are three types of muscles: skeletal muscles, smooth muscles and cardiac muscles. They constitute the major body mass of an organism. Skeletal muscles help to move external body, smooth muscles are responsible for the movement in stomach, intestines, arteries, and cardiac muscles are responsible of heartbeats. Muscles constitute the major body mass of an organism. Only skeletal muscles are voluntary, meaning you can control them consciously, for example, movement of a hand. Smooth and cardiac muscles act involuntarily, meaning you cannot control them, for example, movement of muscles of heart.