Almost certainly not salt or dehydration, which is what most people are told. The better-supported explanation is neuromuscular fatigue — running further or faster than you are conditioned for. The practical consequence is that the fix is training and pacing, not another electrolyte tablet.
Why the salt explanation is weak
The dehydration and electrolyte-depletion hypotheses are old, intuitive and widely repeated. The evidence behind them is thinner than their popularity suggests: it rests largely on anecdotal clinical observation, case series, and one small case-control study.
Against that, prospective studies following marathon runners, 56 km race participants, Ironman triathletes and 161 km ultramarathon competitors found no clinically meaningful relationship between cramping and electrolyte changes. Be precise about what that means, because it gets overstated in both directions: two of those cohorts did find statistically lower post-race sodium in crampers — 139.8 against 142.3 mmol/L in one, 140 against 143 in another — but both sat inside the normal range, which is why the authors judged the difference not clinically significant.
One honest caveat, which the researchers themselves raise: in most of those studies blood was drawn after the cramp had resolved, sometimes hours later. A negative finding under those conditions is less conclusive than it looks. And a study in American football players — padded, helmeted, training twice a day in August heat, with sodium losses no distance runner approaches — found cramp-prone players’ blood sodium drifted down (137.8 to 135.7 mmol/L) where controls’ did not. That was 6 cramp-prone players against 8 controls. So “electrolytes are irrelevant” overstates it. “Electrolytes are the main cause” overstates it a great deal more.
So what does cause cramp?
The altered neuromuscular control theory holds that cramp comes from an imbalance between excitatory drive from the muscle spindles and inhibitory drive from the Golgi tendon organs, driven by fatigue and overload. It fits the observations that the salt theory cannot:
- Cramp hits the muscles you are working, not the whole body — which a systemic electrolyte problem would.
- It arrives late in a race, when you are tired, and at paces faster than you trained for.
- Passive stretching relieves it — stretching increases Golgi tendon organ inhibition.
- The strongest predictors of cramping in prospective work are a history of cramping and running faster than usual, not hydration status.
What actually helps?
Train the distance you intend to race. This is the intervention with the clearest logic behind it. Cramp is a conditioning signal.
Pace the first half conservatively. Running above your trained intensity is the reliable way to produce cramp in the last third.
Stretch it when it happens. Passive stretch is the immediate treatment, and it works.
Pickle juice, oddly, has some support — and not for the reason you would think. In a laboratory study it cut cramp duration by about 37% (85 ± 19 seconds against 134 ± 16). The mechanism is not electrolyte replacement: it acts far too fast for that. The researchers suggested it triggers receptors in the mouth and throat that reduce alpha motor neuron firing. The caveats matter more than the headline, though: this was 12 men, deliberately dehydrated by about 3% of body mass, with cramp induced electrically in the flexor hallucis brevis — a small muscle in the foot, not the calf or hamstring runners actually cramp. A mechanism study, not a race result.
Salt tablets are not established. In a 161 km ultramarathon, 93.9% of runners used sodium supplements — and use was essentially identical between those who cramped (94.8%) and those who did not (93.3%). Cramping or near-cramping affected about 14% of finishers. Near-universal supplementation simply did not separate the crampers from the rest. If you sweat heavily over long hot efforts, sodium still matters for other reasons — see how to drink on a long run — just not as cramp insurance.
When cramp is a medical question
Cramp that happens at rest, at night, repeatedly and severely, or alongside muscle weakness, dark urine or feeling generally unwell is not exercise-associated cramp and needs a doctor. Dark cola-coloured urine after hard exercise in particular needs seeing the same day.
References
Schwellnus MP. Cause of exercise associated muscle cramps (EAMC) — altered neuromuscular control, dehydration or electrolyte depletion? Br J Sports Med, 2009;43(6):401–8. · Maughan RJ, Shirreffs SM. Muscle cramping during exercise: causes, solutions, and questions remaining. Sports Med, 2019;49(Suppl 2):115–24. · Hoffman MD, Stuempfle KJ. Muscle cramping during a 161-km ultramarathon. Sports Med Open, 2015;1:24. · Horswill CA, Stofan JR, Lacambra M, et al. Sodium balance during U.S. football training in the heat: cramp-prone vs. reference players. Int J Sports Med, 2009;30(11):789–94. · Nelson NL, Churilla JR. A narrative review of exercise-associated muscle cramps. Muscle Nerve, 2016;54(2):177–85. · Miller KC, Mack GW, Knight KL, et al. Reflex inhibition of electrically induced muscle cramps in hypohydrated humans. Med Sci Sports Exerc, 2010;42(5):953–61.