MUSCLE JERK is a term used to describe an involuntary SPASM or twitch of a muscle. The crossword solutions for this term are SPASM (5 letters) and TIC (3 letters). SPASM is a sudden, involuntary contraction of a muscle or group of muscles, while TIC is a sudden, repetitive, nonrhythmic motor movement or vocalization. Both terms refer to the same phenomenon, which is an uncontrolled, sudden movement of a muscle or group of muscles.