A muscle cramp is an involuntary contraction of a muscle without relaxation afterward. Normal movement involves alternation of contraction and relaxation of the muscles of the body. Muscles that stabilize the body, like those of the head, trunk, and neck, work similarly. When a muscle contracts involuntarily, it is referred to as a spasm; a forceful and prolonged spasm becomes a cramp.
Symptoms of a muscle cramp include local pain at the site of the cramp, which can be severe, and firmness or tenderness of the involved muscle. Any muscle can develop a cramp, but the most common sites for muscle cramps are in the legs. The most commonly involved muscle groups are the back of the lower leg/calf, the back of the thigh (hamstrings), and the front of the thigh (quadriceps).