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A muscle is a group of specialized cells that contract in response to nerve impulses. There are dozens of non-differentiated muscles in the body, and three muscular organs - the heart, stomach and intestines.
Muscles are generally used to move bones and are almost always anchored to a pair of bones. Muscles can only contract and therefore appear in pairs in order to pull a given bone in both directions. For example, the bicep moves the forearm towards the bicep, while the triceps moves the forearm away from it. There are some specialized muscles, like the diaphragm (which allows the chest cavity to expand to allow the lungs to expand), which does not have another paired muscle.
Muscle cells respond to stress and more of them grow when the muscle is constantly being used. However, muscles can also be torn or strained, and the ligaments that attach them to the bone can also be torn or severed. Overuse of a muscle can also result in pain. Muscles can also fire randomly (a spasm) which also usually results in pain.