What causes muscle cramps during exercise?
Probably not dehydration or lost salt. Triathletes who cramped showed no greater body-mass loss and no clinically significant electrolyte differences from those who did not[3], and a review found the dehydration and depletion hypotheses offer no plausible mechanism for how cramps present or resolve[1]. In 210 Ironman triathletes the real predictors were racing faster than usual and a history of cramping[2]. Cramp behaves like a fatigue and neuromuscular-control problem, which is why stretching the cramping muscle stops it[1].
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The salt-and-water story, and what happened when it was tested
Almost everyone has been told the same two things: you cramp because you are dehydrated, or because you have lost salt. Both are testable, and both have been tested.
A case-control study compared Ironman triathletes who cramped during the race with matched finishers who did not. There was no significant difference in body mass lost, and while post-race sodium differed slightly between the groups, 140 against 143 mmol/L, both values sat inside the normal clinical range. The authors concluded that acute exercise-associated cramping is not associated with greater body-mass loss or clinically significant differences in serum electrolytes[3].
A review of the whole question was blunter. The dehydration and electrolyte depletion hypotheses rest largely on anecdotal clinical observation, case series and one small case-control study, and do not offer a plausible mechanism that explains how cramps actually present or how they respond to treatment[1]. Dehydration did not even change the stimulation frequency needed to trigger a cramp in the laboratory[1].
What does predict a cramp
A prospective cohort study followed 210 Ironman triathletes and looked for the variables that separated crampers from the rest. The predictors were running or racing faster than usual, and having cramped before — not dehydration and not change in serum sodium[2].
That matches what people describe if you let them describe it. The cramp arrives late in the session, in the muscle doing the most work, on the day they pushed harder than they had trained for, and usually in someone who has cramped in the same muscle before.
The better-supported explanation
The hypothesis the evidence has been accumulating behind is altered neuromuscular control: as a muscle fatigues, the balance of excitatory and inhibitory input to its motor neurons shifts, and the muscle starts firing when it should be quiet[1]. Cramp as a fatigue phenomenon rather than a plumbing problem.
This explains the things the salt story cannot. It explains why the cramp happens in the muscle you were using and not all over your body, why stretching that muscle — which loads its tendon and raises inhibitory feedback — stops it within seconds, and why the people who cramp are the ones who went faster than their training had prepared them for[2].
It also means fuelling and pacing are the levers. A muscle that runs out of available carbohydrate fatigues sooner, and the macro split guide covers where carbohydrate sits in a daily target — relevant for anyone training hard in the evening on a low-carbohydrate day, or trying to play football after a full day of fasting.
What to do while it is happening
Stop the effort and stretch the cramping muscle, holding it until it releases. For a calf, that means straightening the knee and pulling the toes up toward the shin. This is the intervention that reliably works, and the neuromuscular explanation is why[1].
One curious finding is worth knowing precisely because of what it rules out. In a laboratory study, cramps were induced electrically in hypohydrated volunteers and then treated: cramp duration was 84.6 seconds after drinking pickle brine against 133.7 seconds after water, roughly 49 seconds shorter[4]. The authors attributed the relief to a reflex originating in the mouth and throat that inhibits firing in the cramping muscle, not to electrolyte replacement[4] — and a companion study showed that a small volume of the same fluid takes around half an hour to leave the stomach, so absorption cannot explain relief that arrives in under two minutes[5].
Two honest caveats. Those cramps were electrically induced in a laboratory, not spontaneous cramps in a race, and the subjects were deliberately dehydrated[4]. Treat it as a curiosity that points at a mechanism, not as a protocol.
Heat, hydration and the things that still matter
Living in Cairo or the Gulf, it is tempting to read all of the above as "hydration does not matter". It does — for performance, for thermoregulation and for safety in summer heat. What the evidence does not support is hydration as the treatment for a cramp, or salt as a preventive. Heat matters here mostly because it makes any given pace harder, and harder pace is what the cohort study actually implicated[2].
The practical prevention list is therefore shorter and duller than the supplement aisle suggests: build up to the pace you intend to hold, do not let race day be the first time you go that fast, and respect a muscle with a history of cramping by training it rather than salting it[2].
When a cramp is a medical question
Exercise-associated cramps are not the only kind, and the differences matter. See a doctor if cramps happen at rest or wake you at night regularly, if they began after starting a new medication, if one limb cramps persistently on its own with swelling, colour change or pain when walking, or if cramps come with muscle weakness rather than just pain. Several common prescriptions can provoke cramps, and that is a conversation with the person who prescribed them, not something to solve with a sports drink.
Sources
- Schwellnus MP. Cause of exercise associated muscle cramps (EAMC) — altered neuromuscular control, dehydration or electrolyte depletion? British Journal of Sports Medicine 2009;43(6):401–408. ↩
- Schwellnus MP, Drew N, Collins M. Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes. British Journal of Sports Medicine 2011;45(8):650–656. ↩
- Sulzer NU, Schwellnus MP, Noakes TD. Serum electrolytes in Ironman triathletes with exercise-associated muscle cramping. Medicine & Science in Sports & Exercise 2005;37(7):1081–1085. ↩
- Miller KC, Mack GW, Knight KL, Hopkins JT, Draper DO, Fields PJ, Hunter I. Reflex inhibition of electrically induced muscle cramps in hypohydrated humans. Medicine & Science in Sports & Exercise 2010;42(5). doi:10.1249/MSS.0b013e3181c0647e ↩
- Miller KC, Mack GW, Knight KL. Gastric emptying after pickle-juice ingestion in rested, euhydrated humans. Journal of Athletic Training 2010;45(6):601–608. ↩