Muscle spasms and cramps are spontaneous, often painful muscle contractions.
Most people are familiar with the sudden pain of a muscle cramp. The rapid, uncontrolled contraction, or spasm, happens unexpectedly. Sometimes it can happen during or following athletic activity or a workout. It can also happen with either no stimulation or some trivially small one. The muscle contraction and pain last for several minutes, and then slowly ease. Cramps may affect any muscle, but are most common in the calves, thighs, feet, and hands. While painful, they are harmless, and in most cases, not related to any underlying disorder. Nonetheless, cramps and spasms can be manifestations of many neurological or muscular diseases.
The terms cramp and spasm are often used inter-changeably. They can be somewhat vague because they are sometimes used to also include types of abnormal muscle activity other than sudden painful contraction. These include stiffness at rest, slow muscle relaxation, and spontaneous contractions of a muscle at rest (fasciculation or clonism). Fasciculation is a type of painless muscle spasm, marked by rapid, uncoordinated contraction of many small muscle fibers that people often describe as a sort of "muscle fluttering." For a physician, a critical part of diagnosis is to distinguish these different meanings and to allow the patient to describe the problem as precisely as possible.
Elliot Greene, Rebecca J. Frey PhD, The Gale Group Inc., Gale, Detroit,