When we use the muscles that can voluntarily be controlled, such as those of our arms and legs, they alternately contract and relax as they move. Muscles that support our head, neck, and trunk contract similarly in a synchronized fashion to maintain our posture. A muscle (or even a few fibres of a muscle) that involuntarily contracts is called a spasm. If the spasm is forceful and sustained, it becomes a cramp. A muscle cramp is thus defined as an involuntarily and forcibly contracted muscle that does not relax. This causes a visible or palpable hardening of the involved muscle.