Heat And Muscle Cramps: Why Losing Salt May Not Be The Real Culprit

A runner’s calf locks up in the final miles of a summer race, and the explanation seems obvious: too much sweat, too little salt. Many athletes and clinicians accept that story, according to a 2022 review, and federal workplace guidance still describes heat cramps as the result of low salt levels in muscles.

That explanation is incomplete at best, a growing body of research suggests. Lab experiments and race-day studies keep finding cramps without the dehydration that supposedly causes them, and in some cases dehydration without the cramps.

A single muscle seizing late in a hard effort fits a different suspect: fatigue scrambling the nerve signals that tell a muscle when to contract and when to relax. Heat still matters, mostly by wearing the body down faster.

Why Are Scientists Doubting The ‘Heat Cramps’ Explanation?

Because cramps keep appearing without the dehydration and salt loss that are supposed to cause them.

In a 2010 Medicine & Science in Sports & Exercise trial, 10 men pedaled with one leg in a 106°F room, in 30-minute bouts, until they had sweated off about 3% of their body weight. That took roughly two hours. Researchers designed the setup to dry the men out without tiring the muscle being tested. Before and after, they measured how easily a small muscle under the big toe cramped when stimulated with mild electrical pulses. A lower pulse rate needed to set off a cramp means a more cramp-prone muscle. Losing that much water and salt did not change the cramp threshold by a statistically meaningful amount. A 2013 follow-up in the British Journal of Sports Medicine pushed 10 men to an average body-weight loss of 4.7%, along with about 4 grams of sodium lost in sweat, and again found no change.

A 2022 review in the Journal of Athletic Training, written by cramp researchers in the U.S. and South Africa, called “heat cramps” an inaccurate name. Painful, localized exercise cramps happen in all kinds of weather, according to the review. They aren’t tied to body temperature, aren’t brought on by simply sitting in heat, and don’t go away right away when the person is cooled down.

Real races point the same way. In a prospective study published in the British Journal of Sports Medicine, researchers weighed 210 Ironman triathletes and drew their blood before and after the race. Athletes who cramped (43) and those who didn’t (166) showed no statistically meaningful differences in weight lost or in blood salt levels. When the researchers analyzed all the factors together, only two stood out: faster overall race times and a history of cramping in the athlete’s last 10 races. A 2019 Sports Medicine review also pointed to a marathon run at 50°F to 54°F in which 15 of 92 runners, about 1 in 5, still cramped.

So Where Does Heat Fit Into Muscle Cramps?

Mostly by speeding up fatigue and strain.

In one American football season, cramps were most common during the hottest months, according to the Journal of Athletic Training review, which ties hot, humid conditions to dehydration, earlier fatigue and a body not yet adjusted to the heat. CDC guidance calls dehydration a primary contributor to heat exhaustion and warns that it can drag down work performance even when people don’t notice.

A leading explanation for the cramps themselves is the altered neuromuscular control hypothesis. Every muscle runs on a balance of “go” signals that tell it to contract and “stop” signals that tell it to relax. According to the theory, hard, repetitive work, especially in a tired muscle, tips that balance toward “go,” so the nerve keeps firing and the muscle locks. Muscles that cross two joints, like the calves and hamstrings, often contract while already shortened, which weakens one of the body’s built-in brakes. That fits when and where most cramps happen: in the hardest-working muscles, usually near the end of an event. It would also explain why stopping or stretching helps, since a stretched muscle gets a stronger “stop” signal.

Researchers haven’t settled the question. That idea grew out of animal experiments, and much of the supporting lab work uses electrically triggered cramps in small muscles, like the one under the big toe, which differ from the muscles that usually cramp during sports. Scientists still don’t know whether cramping requires crossing some fatigue threshold, and well-trained athletes still cramp, so fatigue can’t be the whole story. Prior cramping, pace, poor conditioning, recent injury, short sleep, lack of heat acclimatization and fluid balance all appear on the review’s list of factors that may add up.

When Might Salt And Water Still Matter?

In some situations.

In a BMJ Open Sport & Exercise Medicine study, Australian researchers had 10 young men run downhill in a 95°F to 97°F climate chamber until they lost 2% of their body weight. That alone didn’t change how easily an electrically stimulated calf muscle cramped. What they drank afterward did. Plain water made the calf easier to cramp over the next hour, while an oral rehydration drink containing salts and sugar made it harder. Researchers suspect plain water diluted the body’s remaining electrolytes. They also note that their drink contained sugar, so they can’t rule out that ingredient. A small all-male sample and lab-induced cramps further limit how far the result applies.

A 2005 Journal of Athletic Training trial at the University of Alabama produced mixed results that are still useful. Researchers had 13 college-aged men with a history of exercise cramps do a calf-tiring routine in a hot room twice: once drinking a sports drink with added salt to match their sweat losses, and once with no fluid at all. More men cramped with the drink (nine) than without it (seven), so the drink didn’t prevent cramps. But among the seven who cramped both times, the drink more than doubled how long they lasted before cramping, from 14.6 to 36.8 minutes on average. As the 2022 review points out, the drink contained water, sugar and salt together, so the study can’t say which ingredient made the difference.

Can Muscle Cramps In The Heat Be Prevented?

Partly, though the evidence points well beyond salt.

For a single cramping muscle, gentle static stretching until the cramp lets go is the 2022 review’s strongest recommendation, along with slowing down or stopping. Stretching works faster than drinking, since fluids take at least 13 minutes to be absorbed. Once the cramp passes, water or a sports drink is reasonable.

For prevention, the review recommends training at the intensity and in conditions, including heat, that athletes will face in competition, building in rest so muscles can recover from damage, getting enough sleep and setting realistic goals for pace. It also warns against overcorrecting: an overemphasis on fluids and electrolytes to prevent cramps contributed to the deaths of two high school athletes from dangerously low blood sodium in 2014.

Some cramps signal something more serious. CDC guidance says to get medical help for heat cramps that don’t ease within an hour, or when the person has heart problems or follows a low-sodium diet. According to the 2022 review, cramps that come with dizziness, collapse, vomiting, dark urine or confusion point to a more serious condition that needs immediate care, and the CDC lists confusion and loss of consciousness as signs of heat stroke, a call-911 emergency.

That summer-race cramp, in other words, probably isn’t a salt deficit announcing itself. Heat can set the stage and sweat losses may play a part, but a single muscle seizing late in a hard effort points more often to overworked muscle control than to an empty salt shaker.

Disclaimer: This article summarizes several published studies and reviews on exercise-associated muscle cramps. Most of the experiments were small, enrolled only men, and measured cramps triggered by electrical stimulation in a lab, which may not behave like cramps that strike during sports. Race-day studies such as the Ironman cohort are observational and can identify links, not prove cause and effect. This article is for general information and is not medical advice. Anyone with frequent, severe or unexplained cramps, or cramps accompanied by dizziness, vomiting, dark urine or confusion, should seek care from a medical professional.