Electrolytes and cramps: how strong is the evidence really

Your calf seizes up late in a match, or a hamstring locks on the last kilometre of a race. The changing-room diagnosis arrives within seconds: not enough water, low magnesium, should have eaten a banana. It sounds sensible, and it has been repeated for decades — but the evidence behind it is far thinner than its popularity suggests.

Here is what we can say with reasonable confidence about exercise cramps, what is merely plausible, and where the story usually goes wrong.

What we know reasonably well

Cramp shows up when the effort exceeds what you are trained for. The evidence here is consistent: cramps come late in a session, in a fatigued muscle, and mostly in people going faster, longer or harder that day than they ever do in training. Race pace on training legs is the classic trigger.

The strongest single predictor is having cramped before. People who cramped in previous races or matches tend to cramp again — and that history predicts cramping better than any blood measurement does.

Cramp favours muscles that cross two joints — calf, hamstring, sometimes quadriceps and the small muscles of the foot — and it strikes when the muscle is already shortened. That is why a cyclist cramps at the top of the pedal stroke and a swimmer cramps in the arch of the foot.

Stretching stops a cramp that has already started. This is the most reliable finding in the whole area, and it is a clue in itself: a mechanically lengthened muscle settles immediately, long before anything you swallow could reach it.

In the moment, a cramp is a nerve event, not a chemistry event. Recordings show a burst of electrical activity in the muscle before it locks, and that activity comes down from the nervous system. What set it off is where the argument starts.

What is likely but not settled

Fatigue distorts reflex control. The leading explanation is that fatigue turns up the signals that excite a muscle and turns down the ones that inhibit it, so the muscle stays switched on. The laboratory half of that story holds together well; carrying it over to the pitch is not fully done. Treat it as the best current working model, not a proven fact.

Sodium in very long efforts in the heat. In people who sweat heavily and saltily, across efforts lasting several hours in warm conditions, some research hints that a bigger salt deficit lowers the cramp threshold. Other work finds nothing. This is genuinely unresolved — plausible enough that salt is worth trying, too thin to call it a requirement.

Strong flavours in the mouth. Sour or spicy drinks appear to shorten a cramp faster than absorption could possibly explain, which points to a reflex triggered by receptors in the mouth and throat. Interesting idea, but tested mostly on artificially induced cramps in small groups — weak to moderate evidence at best.

Strengthening the muscle that keeps cramping. If fatigue is the driver, a muscle that fatigues later should cramp less. The reasoning is sound; direct evidence is scarce. It is a similar situation to warm-ups, where the evidence on injury prevention shows how hard it is to prove that any single measure prevents something rare and unpredictable.

What gets repeated wrongly

"Cramp means you're dehydrated." When crampers and non-crampers in the same race are compared, differences in body mass lost and in blood results usually fail to appear. Severe dehydration is a problem in its own right, but as an explanation for the everyday training cramp it does not hold up.

"You're low on magnesium." Here the evidence is unusually clear, and it points the other way: in healthy adults, extra magnesium generally does not reduce how often or how hard you cramp, and much the same applies to night cramps in older people. If it seems to help you, expectation is the likeliest reason — not worthless, but not something to build a plan on.

"Eat a banana for the potassium." Sweat carries very little potassium, and one banana shifts blood levels neither quickly nor meaningfully. It is a perfectly good pre-training snack; as a treatment for a cramp already in progress it has nothing to offer.

Cramp and muscle soreness get mixed up. A cramp is a sudden, painful contraction lasting seconds to a couple of minutes. The ache that turns up a day or two later is a different phenomenon with a different cause and a different fix.

Caffeine takes the blame unfairly. The claim that caffeine dries you out and causes cramp has no solid basis; at the doses people actually use before training the extra fluid loss is small and easily replaced.

Night cramps are not the same thing. Leg cramps that wake you from sleep follow a different pattern and have different causes, so what applies to training in later decades does not transfer to them automatically.

What this means for your training and eating

Look at the dose of effort before the shaker. If you only cramp on race day and never in training, the gap between training pace and race pace is the prime suspect. Building weekly volume by 5–10% and putting one session a week near race intensity does more than any powder.

Get used to the heat. If you compete in warm conditions, 5–10 consecutive days of 60–90 minutes of easy work in the heat changes both the volume and the composition of your sweat. It is one of the few measures with both a clear mechanism and a practical payoff.

Fluid: enough, not more. A workable target is to keep body-mass loss during a session under roughly 2%. Drinking well past thirst "just in case" carries its own risk and does not protect you from cramp.

Salt: a reasonable try, not a guarantee. For efforts beyond 90 minutes in the heat, 300–600 mg of sodium per hour from drinks or salty food is the usual practical range. It is cheap and low-risk in healthy people — just don't file it under "solved".

Fuel and fatigue. Cramp likes a tired muscle, so anything that delays fatigue helps indirectly: 30–60 g of carbohydrate per hour on efforts longer than about 90 minutes, and enough sleep in the weeks before a hard event.

Strength in the shortened position. If the same muscle keeps going, train it through full range with an emphasis on the lowering phase: calf raises with a complete stretch at the bottom, and 2–3 sets of Nordic curls, twice a week, for 6–8 weeks.

When one strikes. Stop, lengthen the muscle and hold for 20–30 seconds, then repeat. Gently contracting the opposing muscle helps. Carry on more conservatively afterwards — once a muscle has cramped, it tends to do it again in the same session.

When to see a doctor

• Cramps that appear at rest and have little to do with training.
• Cramps alongside weakness, numbness, altered sensation or visible muscle wasting.
• Dark urine with severe, persistent pain and swelling after a very hard session — that needs urgent assessment.
• Cramps that began after starting a new medication; do not change any prescription on your own.
• Known kidney or thyroid disease, or diabetes, together with new and frequent cramping.

In short

• Cramp is most likely a story about fatigue and nerve control, not an empty mineral tank.
• Strongest evidence: cramp follows unaccustomed effort, hits two-joint muscles, and stops with stretching.
• Magnesium generally does not help healthy people; bananas and a bit more water do not fix a cramp mid-effort.
• Sodium in long, hot efforts is worth trying but remains unproven.
• The best returns come from being prepared for the pace that breaks you, heat acclimatisation, adequate fuelling, and strong muscles through full range.