A spasm is a sudden involuntary contraction of a muscle, a group of muscles, or a hollow organ such as the heart.

A spasmodic muscle contraction may be caused by many medical conditions, including dystonia. Most commonly, it is a muscle cramp which is accompanied by a sudden burst of pain. A muscle cramp is usually harmless and ceases after a few minutes. It is typically caused by ion imbalance or muscle overload.

A muscle spasm is a sudden, involuntary contraction of one or more muscles. A spasm results from an abnormally sustained muscle contraction and is often painful. Various muscles may develop spasms, including the small intrinsic hand muscles of a musician to the larger “charley horse” calf muscles of a runner.