It's termed as muscle spasm. What the person feels is a twitch usually described as muscle cramp. It occurs when a person starts doing exercises in ranges where he's not used to or a person who has a sedentary lifestyle starts doing strenuous activity without warm-ups. Every muscle has an outer, middle and inner range of movement. So begin with the outer range (starting range of crunches) and slowly progress to middle and inner ranges. Counter stretching or relaxing usually releives the spasm. Mineral sports drinks is also recommended for active persons who experiences spasm. As it's a purely muscular condition you can consult with a physiotherapist.