What term describes a sudden, involuntary contraction of muscles?

The term that best describes a sudden, involuntary contraction of muscles is spasm. A spasm refers to a brief, involuntary twitch or contraction of a muscle or group of muscles, which can occur unexpectedly and can be quite painful. Spasms can happen in various muscles throughout the body and are often related to factors such as overuse, dehydration, or electrolyte imbalances.

While cramp is a similar term often used interchangeably with spasm, it specifically refers to a painful, prolonged contraction of a muscle, typically occurring in the legs or feet, and often associated with physical exertion. Tonicity pertains to the state of muscle tension or tone and does not refer directly to involuntary contractions. Tremor describes a rhythmical, oscillating movement of body parts, typically involving smaller muscle groups, and does not convey the same concept as a sudden contraction. Thus, spasm is the most accurate term to describe the phenomenon in question.