Involuntary jerk is a term used to describe an uncontrolled muscle SPASM, usually in the arms or legs. It is also known as a myoclonic jerk and is caused by a sudden, involuntary contraction of a muscle or group of muscles. The term can also be used to describe a sudden, unexpected movement of the body, such as a startle response. The crossword solutions for involuntary jerk are START (5 letters) and SPASM (5 letters). START is an action word that means to begin or set in motion, while SPASM is a noun that refers to an involuntary contraction of a muscle or group of muscles.