It can happen for no reason, taking you completely by surprise. And it can be excruciating. Suddenly, a muscle contracts violently, as if it had been prodded with a jolt of electricity.
A seized calf muscle or a hamstring can be frightening, swimmers fear they will drown. Cyclists nearly fall off their bikes, runners drop to the ground, grimacing, gritting their teeth.
The contraction is so strong that you could not will yourself to ball your muscle that tightly, and your muscle is likely to feel sore the next day.
This happens when you get a cramp, an experience so common among endurance athletes, researchers say, that almost everyone who has tried endurance sports has felt a muscle cramp or has a friend who has had one.
Cramps affect 39 percent of marathon runners, 79 percent of triathletes, and 60 percent of cyclists at one time or another, said Dr. Martin P. Schwellnus, a professor of sports medicine at the University of Cape Town. Cramps can occur during exercise, immediately after, or as long as six hours later.