What causes a muscle cramp, and why is it still debated?

Nobody knows for certain. A cramp is a sudden, involuntary, painful contraction of a skeletal muscle, and it can be set off by several things, but for the cramps that strike during or after exercise the cause is still not fully understood. That is surprising, because they are extremely common, even among elite athletes.

Cramps are usually short. They are generally temporary and non-damaging, and they go away on their own within seconds or sometimes minutes. They are narrower than spasms: a spasm can be any involuntary abnormal contraction, while a cramp is sustained and painful. The muscles hit most often are the calves, the thighs and the arches of the foot. In the United States a cramp is also known as a Charley horse.

Researchers have two main hypotheses for exercise cramps. One is that heavy sweating drains the body of sodium and other electrolytes, which may shrink the fluid spaces between cells and make cramping worse. This is widely believed. The other is altered neuromuscular control: fatigue tips the balance of signals from sensors in the muscle and tendon towards excitation, which alters control from the spinal cord.

Neither has settled the matter. The cause is hypothesised to be altered neuromuscular control, dehydration or electrolyte depletion, and cramping can result from many underlying conditions.

For cramps at night, the picture is just as hazy. The precise cause is unclear. Contributing factors include dehydration, low levels of certain minerals, though the evidence has been mixed, and reduced blood flow through muscles in prolonged sitting or lying down.