A cramp is when a muscle suddenly gets tight and will not relax. Many people think leg cramps happen because they need more water or salt.

But a study of 210 triathletes found something different. Athletes who got cramps did not lose more water, and they had similar blood minerals. Instead, cramps were linked to racing at a fast pace and to having cramps before. Scientists now think tired muscles get mixed-up signals from the nerves.

Leg cramps have long been blamed on dehydration and a lack of minerals such as sodium. However, a study of 210 Ironman triathletes in South Africa challenged this idea. The researchers weighed the athletes and tested their blood before and after the race. The 43 athletes who cramped did not differ from the others in weight loss or blood minerals.

Instead, cramping was linked to racing at a fast pace and to a history of cramps. Another study found that pickle juice stopped cramps faster than water, but the effect appeared very soon after drinking, which is too fast for salt to be absorbed. Scientists now think cramps may come from a problem in how nerves control tired muscles.

Muscle cramps during exercise are usually blamed on dehydration and lost electrolytes, but the evidence increasingly points elsewhere. In a prospective study of 210 Ironman triathletes in South Africa, researchers measured body weight and blood electrolytes such as sodium before and after the race. The 43 athletes who developed cramps showed no greater weight loss or electrolyte changes than the 166 who did not.

What predicted cramping was a fast racing pace and a history of cramps. These findings fit the hypothesis that fatigue upsets the balance between excitatory signals to the muscle and the inhibitory feedback that normally limits contraction. Dehydration may still matter sometimes, though. A 2010 experiment added an odd clue. In mildly dehydrated men, pickle juice relieved electrically induced cramps faster than water.

The effect appeared very soon after drinking, and blood electrolytes barely changed, which points to a reflex triggered in the mouth or throat. Cramps produced by electric stimulation may not behave like natural ones, however. On treatment, a 2020 Cochrane review found that magnesium is unlikely to prevent cramps meaningfully in older adults, and a 2021 review found that stretching might ease the pain of night cramps without clearly making them less frequent.