More than 640 muscles are arranged in crisscrossing layers between the skin and bones of the skeleton. Most are attached to bones by tough fibrous straps called tendons. Muscles, tendons, and bones work together like a system of levers.
For example, when the biceps muscle in the upper arm contracts, it pulls on the bone in the forearm and bends the arm at the elbow.
Muscles allow us to walk, run, jump, and wiggle our fingers. They also move blood around the body and food through our intestines and allow us to speak.
Muscles make the body move. All muscles are made up of tiny fibers that can contract, making a muscle shorter so that it pulls on a part of the body. The largest muscles in the body are connected to the bones. Most of these skeletal muscles are under voluntary control, which means we can move them at will. Other muscles, such as those in the heart and stomach, are involuntary - they work without our having to think. Altogether, there are billions of muscles in the human body, including microscopic muscles on every hair and blood capillary.
There are three types of muscle in the body: skeletal, smooth, and cardiac (heart) muscle.
Skeletal muscles are attached to the bones. They are made up of very long, threadlike cells that contract powerfully and quickly, for short periods of time.
Smooth muscle occurs in the stomach, intestines, blood vessels, and eyes. It is made up of short, spindle-shaped cells that can contract for longer periods than other types of muscle.
Cardiac muscle is found only in the heart. It consists of short muscle cells that contract rhythmically without tiring.