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Each of your muscles is made up of thousands of thin, long, cylindrical cells called muscle fibers. The muscle fibers' highly specialized structure enables the muscles to relax and contract to produce movement. Muscles vary greatly in their shape and size, depending on their function.

Back view of muscles
Click image for larger view

Source: AMA's Current Procedural Terminology, Revised 1998 Edition. CPT is a trademark of the American Medical Association.