A spasm feels like tightness or a knot in a muscle. It may hurt when you use the muscle. It may be hard to use the muscle. A cramp during exercise may be extremely painful.
A spasm usually occurs from overusing muscles or from an injury. If you are dehydrated during strenuous activity, you are likely to have cramps. Muscle spasms, especially in the neck, also may occur when you are under lots of stress.