Everyone who has ever cramped mid-run has heard the same diagnosis from a well-meaning bystander: “You need more water. And salt.” It's the explanation on sports-drink labels, in locker rooms, and in most of the advice you'll find online. The trouble is that when researchers actually went looking for it, the link between dehydration and cramping turned out to be much weaker than the folklore suggests. Here's what the evidence says, where hydration still fits in, and what actually seems to help.
The classic theory: dehydration and lost electrolytes
The traditional explanation is intuitive. You sweat, you lose water and sodium, the fluid around your muscle cells shrinks and shifts, nerve endings get irritable, and the muscle fires on its own. The idea dates back to early twentieth-century reports of cramping among miners and stokers working in extreme heat, whose symptoms improved with salted drinks. It has stuck around for a century because it makes sense and because it is easy to sell a solution for.
The problem is that a plausible story isn't the same as a proven one. If dehydration and salt loss caused cramps, you'd expect cramping athletes to be more dehydrated and more sodium-depleted than non-cramping ones in the same event. Repeatedly, that's not what studies find.
What studies of marathoners and field workers show
Several studies have measured body weight, blood sodium, and hydration markers in endurance athletes before and after races, then compared those who cramped with those who didn't. In Ironman triathletes and ultramarathon runners, crampers and non-crampers had essentially the same weight loss and the same post-race blood sodium. Being more dehydrated didn't predict cramping; the strongest predictors were a history of previous cramps and running faster than usual relative to training.
The pattern holds outside sport too. Research on agricultural and construction workers in hot climates finds cramps are common but correlate poorly with measured hydration status. Whole-body dehydration also doesn't explain why cramps are so local: if your sodium were the issue, you'd expect your whole body to be affected, not just the calf you've been hammering for two hours. Lab experiments where volunteers were deliberately dehydrated by a few percent of body weight didn't make them noticeably easier to cramp with electrical stimulation, either.
The newer theory: neuromuscular fatigue
The explanation with the most support today is about nerves rather than fluids. Muscles are governed by two opposing reflex signals: one from muscle spindles that encourages contraction, and one from Golgi tendon organs that tells the muscle to relax when tension gets high. As a muscle fatigues, the excitatory signal ramps up and the inhibitory one fades. Push it far enough — especially in a shortened position, like a pointed foot — and the muscle tips into a sustained, involuntary contraction. That's a cramp.
This theory explains the things the dehydration model can't. It explains why cramps hit the specific muscles doing the work, why they cluster late in races and in athletes going harder than they trained for, why stretching the cramping muscle (which fires the inhibitory reflex) stops it within seconds, and why cramping happens in cool swimming pools where nobody is sweating much at all.
Where sodium may still matter
None of this means electrolytes are irrelevant. Some people are genuinely heavy, salty sweaters — you know the type by the white crust on their cap and shirt after a long session. In very long, very hot events, losing many grams of sodium over several hours can plausibly contribute, and sodium replacement seems to help a subset of these athletes even if it does nothing for most crampers. It's also worth knowing that some evidence suggests a strongly salty or spicy drink can cut short a cramp far faster than the salt could possibly be absorbed — which points, again, at a nerve reflex rather than a chemistry fix.
If you suspect you're in that heavy-sweater group, the honest move is to measure rather than guess. Weigh yourself before and after a typical hard session to estimate your sweat rate, then use the electrolyte replacement calculator to see roughly how much sodium that implies. For a refresher on what each mineral does, see our electrolytes guide.
Night cramps are a different thing
The calf cramp that jolts you awake at 3 a.m. isn't the same animal as the one that stops you at mile 22. Nocturnal leg cramps are common, more so with age and in pregnancy, and they don't track with dehydration either. They're associated with sleeping in positions that shorten the calf, prolonged sitting during the day, certain medications such as diuretics and statins, and conditions like peripheral artery disease and nerve disorders. Drinking a glass of water before bed is fine, but it isn't a treatment. Gentle calf stretching before sleep has modest supporting evidence; the old quinine remedy is no longer recommended because of side effects.
What actually helps prevent exercise cramps
Because fatigue is the best-supported cause, the most effective prevention is unglamorous: don't ask a muscle to do more than you've trained it for. Pace conservatively, particularly early in an event and on hot days. Build up volume and intensity gradually so race-day effort isn't a step change. Include some strength work for the muscles that cramp on you; a stronger muscle reaches fatigue later. And when a cramp does start, stop and passively stretch the muscle — pull your toes toward your shin for a calf — which is still the fastest, most reliable way to end one.
Hydration belongs on the list, but as one factor rather than the headline. Starting a session well hydrated and drinking to thirst keeps your cardiovascular system from working harder than it needs to, which delays fatigue, which delays cramping — an indirect benefit rather than a magic one. If you race in heat, our hot-weather guide covers sensible targets, and the runners' hydration post walks through timing before, during, and after a run. Chugging extra water beyond that won't cramp-proof you and can tip you toward the opposite problem, low blood sodium.
When to see a doctor
Occasional exercise cramps are normal and not dangerous. Talk to a doctor if cramps are frequent or severe enough to disrupt sleep or daily life, if they arrive without exertion, if they come with weakness, numbness, swelling, or skin changes in the limb, or if they started after a new medication. Persistent cramping can occasionally point to thyroid, kidney, liver, or nerve issues, or to electrolyte problems that have nothing to do with sweat.
Keep the one part you control simple
You can't control your reflexes, but you can control your training, your pacing, and whether you turn up to a session already behind on fluids. That last one is the easiest to get wrong on an ordinary Tuesday, not a race day, which is why Realwater keeps your daily intake visible as a sphere that fills as you log drinks. It won't stop a cramp — nothing bottled will — but it does take “did I drink enough today?” off the list of things to wonder about at the start line.
This article is for general information and isn't medical advice. If you have frequent, severe, or unexplained muscle cramps, or a condition affecting your kidneys, nerves, or electrolyte balance, talk to your doctor.
