Exercise-associated muscle cramps
Evidence: contested
The cause of exercise-associated cramp is genuinely unresolved, and the trial base is thin. Field evidence has largely refuted the older dehydration-and-electrolyte theory, and the leading alternative, a fatigue-driven failure of neuromuscular control, is better supported without being proven. The popular fixes, magnesium and electrolytes, have little or no evidence behind them.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
An exercise-associated muscle cramp is a sudden, involuntary, painful contraction of a muscle during or soon after exercise, usually in the calf, hamstring or quadriceps, and usually late in a hard or long run once the muscle is tired. They are common, they can wreck a race, and after decades of study why they happen is still argued over.
Two competing theories
The long-standing folk explanation blames dehydration and the loss of electrolytes, especially sodium, in sweat. The competing explanation, which now has the better of the argument, is that cramp is a failure of neuromuscular control: as a muscle fatigues, the balance between its excitatory and inhibitory reflexes tips toward sustained, involuntary firing. On the weight of evidence the best-supported cause is this fatigue-driven loss of control, and the dehydration-and-electrolyte theory is largely refuted (Miller et al. 2022).
The strongest evidence against the electrolyte theory comes from the field. In a 161 km ultramarathon run in the heat, total sodium intake from food and supplements was unrelated to who cramped. Intake was much the same in those who cramped and those who did not, and it was equally unrelated to dehydration, low blood sodium and nausea (Hoffman & Stuempfle 2015). If sodium loss drove cramp, the runners taking in the least should have cramped the most, and they did not.
The evidence is not all one way, which is why the question stays open. In a laboratory study that induced cramp in the calf during exercise in the heat, a carbohydrate-electrolyte drink more than doubled the exercise time before cramp began, compared with a dehydrated condition. Even so, 69% of participants still cramped when hydrated and supplemented, so electrolyte loss cannot be the sole cause (Jung et al. 2005). The fair reading is that hydration and electrolytes may shift the threshold at which a tired muscle cramps without being the root cause.
What does not work
Magnesium is the supplement most heavily marketed as a cramp cure, and the evidence does not support it. A Cochrane review concluded that magnesium is unlikely to help ordinary cramps at any dose or route tested, and it found no trials at all in people who cramp while exercising, so there is no direct evidence for exercise-associated cramp (Garrison et al. 2020). Electrolyte and magnesium products are sold against a mechanism the field evidence does not bear out.
What the neuromuscular theory predicts, and what helps
If cramp is a reflex problem rather than a chemistry problem, then interrupting the reflex should stop it, and it does. The first response to an acute cramp is to stretch and gently hold the cramping muscle. Lengthening it raises tension in the tendon, which triggers an inhibitory reflex that quietens the overactive muscle, and this is the practical cornerstone.
Pickle juice gives the neatest support. In dehydrated men, drinking a small amount shortened an electrically induced cramp by about 49 seconds compared with water, and the relief began within about a minute. That was far too fast for the fluid to have been absorbed, and blood electrolytes barely moved (Miller et al. 2010). The researchers concluded the effect could not be rehydration or electrolyte replacement, and proposed a reflex triggered in the mouth and throat that damps down the nerve signals driving the cramp. This was a small study of induced cramps rather than spontaneous ones, so treat it as a mechanism worth knowing rather than a guaranteed remedy. It still points at the nervous system rather than the sweat glands.
Prevention
Because fatigue is the common thread, the most defensible prevention is to prepare the muscle for the demand. Build training up gradually so the muscles that will work hardest are conditioned for it, and avoid a sudden jump in pace, distance or terrain that tires them faster than they can cope with. Runners who cramp late in races often find that starting a little more conservatively, so the muscle fatigues later, helps. A normal intake of fluid and sodium is worth the usual attention for its own sake, but it is not a reliable cramp shield (Hoffman & Stuempfle 2015). A cramp that is frequent, severe, or strikes at rest rather than under load is worth a medical opinion, since a few have causes unrelated to exercise.