Muscle (from Latin musculus "little mouse") is contractile tissue of the body and is derived from the mesodermal layer of
embryonic germ cells. It is classified as skeletal, cardiac, or smooth muscle, and its function is to produce force and cause motion, either locomotion or movement within internal organs. Much of muscle
contraction occurs without conscious thought and is necessary for survival, like the contraction of the heart, or peristalsis (which pushes food through the digestive system). Voluntary muscle contraction is used to move the body, and can be finely controlled, like movements of the eye, or gross movements like the quadriceps muscle of the thigh. There are two broad types of voluntary muscle fibers, slow twitch and fast twitch. Slow twitch fibers
Contract for long periods of time but with little force while fast twitch fibers contract quickly and powerfully but fatigue very rapidly.