You tell your muscles what to do without thinking about it. But sometimes they do their own thing -- they might pulse or contract and can’t relax. Twitches and spasms are most common in the thighs, calves, hands, arms, belly, ribcage, and the arches of your foot. They can involve part of a muscle, all of it, or a group of muscles. Doctors aren’t always sure why they happen, but a few common triggers can set them off.