Why Do I Cramp During Marathons? The Science Your Watch Can't Tell You

You trained for months. Your long runs went well. You nailed your taper. And then, somewhere around mile 18, your calves seized up and your race fell apart.

Muscle cramping is the single most common performance-wrecking issue in marathon running — and the advice you've probably read about it is either outdated, oversimplified, or flat-out wrong.

The three theories of why you cramp

Theory 1: Electrolyte depletion

When you sweat, you lose sodium, potassium, magnesium, and chloride. Sodium gets the most attention because you lose the most of it — anywhere from 200 mg to over 1,500 mg per litre of sweat, depending on your individual physiology.

When sodium levels drop too far, your muscles lose the ability to properly regulate contraction and relaxation.

The part most articles miss: two runners side by side in the same race at the same pace can lose dramatically different amounts of sodium per hour. One might lose 400 mg per litre; the other might lose 1,200 mg. This is why the generic advice of "take an electrolyte tablet every hour" works for some runners and does nothing for others.

Theory 2: Dehydration and fluid loss

When you lose more fluid through sweat than you take in, blood volume drops. Your heart works harder to deliver oxygen to working muscles, core temperature rises, and fatigued muscles are more prone to cramping.

Your actual needs depend on your sweat rate (0.5–2.5+ L/hr), temperature and humidity, your pace, and your body size. A runner with a sweat rate of 1.8 L/hr in warm conditions needs a completely different hydration plan than a runner losing 0.8 L/hr on a cool morning.

Theory 3: Neuromuscular fatigue

This is the theory that's gained the most traction in recent sports science. The idea: cramps aren't primarily about what's in your blood — they're about your nervous system.

When a muscle is overworked beyond what it's been trained to handle, the normal feedback loop between your spinal cord and your muscles breaks down. The signals that tell a muscle to relax get overridden by signals telling it to contract.

This explains why cramps tend to happen in the final third of a race, in muscles you've loaded most heavily, on courses with more hills than you trained for, and when you go out too fast. It also explains why some well-hydrated, sodium-loaded runners still cramp. If the muscle is overtaxed, no amount of electrolytes will override a neurological misfiring.

What actually causes your cramps

The honest answer: it's almost always a combination of all three.

A runner who is slightly dehydrated, running low on sodium, and pushing into fatigue territory is far more likely to cramp than a runner with just one factor in play. Think of it as a three-legged stool — knock out one leg and you might stay upright; knock out two and you're going down.

This is exactly why standard advice fails. "Drink more water" addresses one leg. "Take salt tablets" addresses another. Neither addresses the fatigue component — and none of it is personalised to your body.

A practical framework for preventing cramps

Step 1: Know your sweat rate

Weigh yourself before a 60-minute run at target race pace. Run at your goal race effort. Don't drink anything. Weigh yourself again immediately after.

Every gram of weight lost = 1 ml of sweat. If you lost 0.8 kg, your sweat rate is roughly 800 ml/hour in those conditions. This only tells you fluid loss, not sodium concentration — but it's the essential starting point.

Step 2: Build a fluid plan based on your data

Replace 60–80% of sweat loss. You don't need to replace 100% — your body can handle a mild fluid deficit. Over-drinking is its own risk.

For a runner with a sweat rate of 1.2 L/hr: target 720–960 ml per hour, roughly 180–240 ml every 15 minutes. At most marathon aid stations (every 2–3 km), that's 1–2 cups per station.

Step 3: Get your sodium right

The general recommendation is 300–600 mg of sodium per hour. If you're a salty sweater (white residue on your kit, stinging eyes, salty taste on your skin), you may need 1,000+ mg per hour.

Sources: electrolyte drinks (check the label — many are surprisingly low), salt capsules (200–250 mg per capsule), and some gels contain sodium.

Step 4: Respect the fatigue factor

No hydration plan eliminates cramps if your muscles aren't prepared for the effort. Train at or near race pace in your long runs. Include race-specific terrain. Don't go out faster than your training supports. Taper properly to arrive with fresh legs.

Step 5: Test everything in training

Your long runs are dress rehearsals. Test the products, the timing, the quantities. If something causes GI distress at mile 16 of a training run, it will cause GI distress at mile 16 of your race.

The danger of drinking too much

Hyponatremia — dangerously diluted blood sodium from drinking too much plain water — affects slower marathon runners in particular, because they have more time to drink at aid stations. Symptoms include nausea, confusion, headache, and in severe cases it can be life-threatening.

More water is not always better. Your hydration plan should be based on your actual sweat rate, not a "drink as much as possible" mentality.

Where your wearable data fits in

Your Apple Watch or Garmin collects heart rate, pace, cadence, training load — all useful. But there's a critical gap: your watch doesn't know what's happening with your hydration.

Heart rate can hint at dehydration (cardiac drift — a rising heart rate at the same pace), but it can't tell you how much sodium you've lost or how much fluid you need to replace.

Sweatr pulls your wearable data — heart rate, pace, training load, environmental conditions — and combines it with your personal sweat profile to calculate exactly what you need to drink and when. Not averages. Not ranges. A specific, personalised plan for your body, your race, and the conditions on the day.

The bottom line

Cramping during a marathon isn't caused by one thing. It's the collision of electrolyte loss, dehydration, and neuromuscular fatigue — and the exact mix is different for every runner.

The runners who don't cramp at mile 18 aren't lucky. They're prepared. And preparation starts with data — your data.

Sweatr calculates your personalised hydration and fueling plan using your Apple Watch, Garmin, and Strava data — so you know exactly what to drink, when to drink it, and how much sodium you need. No guesswork. No generic averages. Join the waitlist to be first in line.