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.

If this sounds familiar, you're not alone. 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most common performance-wrecking issue in marathon running — and the advice you've probably read about it is either outdated, oversimplified, or flat-out wrong.

Let's fix that.

The Three Theories of Why You Cramp

For decades, runners have been told that cramps come from dehydration or a lack of electrolytes. Drink more water. Take more salt. Problem solved.

Except it's not that simple. Sports science has evolved, and the picture is more nuanced than any single explanation. There are three main theories, and the reality is that all three play a role.

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 200mg to over 1,500mg 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 result: involuntary, painful cramping.

Here's the thing most articles miss: your sodium loss rate is highly individual. Two runners side by side in the same race, at the same pace, in the same heat, can lose dramatically different amounts of sodium per hour. One might lose 400mg per litre of sweat. The other might lose 1,200mg.

This is why the generic advice of "take an electrolyte tablet every hour" works for some runners and does nothing for others. The dose needs to match your loss rate, not an average.

Theory 2: Dehydration and Fluid Loss

When you lose more fluid through sweat than you take in, your blood volume drops. Your heart has to work harder to deliver oxygen to your muscles. Your core temperature rises. Your muscles fatigue faster — and fatigued muscles are more prone to cramping.

The general guideline is to consume 400–800ml of fluid per hour during a marathon. But again, this is a range, not a prescription. Your actual needs depend on:

• Your sweat rate — which varies from 0.5L to 2.5L+ per hour
• The temperature and humidity — hotter and more humid conditions dramatically increase sweat loss
• Your pace — faster running generates more heat and more sweat
• Your body size — larger runners with more surface area tend to sweat more

A runner with a sweat rate of 1.8L/hour in warm conditions needs a completely different hydration plan than a runner losing 0.8L/hour on a cool morning. Using the same plan for both is a recipe for one of them cramping.

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 — pushed 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 that tell it to contract. The result is a cramp.

This explains why cramps tend to happen:

• In the final third of a race (when fatigue is highest)
• In muscles you've loaded most heavily
• On courses with more hills than you trained for
• When you go out too fast

It also explains why some well-hydrated, sodium-loaded runners still cramp. If the muscle is simply overtaxed, no amount of electrolytes will override a neurological misfiring.

So 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 who has just one of those factors 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 the standard advice fails. "Drink more water" only addresses one leg. "Take salt tablets" only addresses another. Neither addresses the fatigue component — and none of them are personalised to your body.

Why Generic Hydration Advice Doesn't Work

Here's the uncomfortable truth about most hydration advice you'll find online:

It's based on averages. And averages don't sweat.

The recommendation to "drink 400–800ml per hour" is a population-level guideline. It's a reasonable starting point, but it's not a plan. It doesn't account for:

• Your personal sweat rate (which you probably don't know)
• Your individual sodium concentration (which varies 3–4x between athletes)
• The specific conditions on race day (temperature, humidity, wind, sun exposure)
• Your pace and effort level (which determines heat production)
• Your body weight and composition

This is the gap that frustrates athletes. You can read ten articles about marathon hydration and still not know exactly how much you should drink, how much sodium you need, or when you should take it.

Your Garmin or Apple Watch tracks your heart rate, pace, distance, and even estimates your training load. But it doesn't tell you how much you're sweating right now, how much sodium you're losing, or what to drink at the next aid station.

The data is incomplete. And incomplete data leads to incomplete plans.

How to Actually Prevent Cramps: A Practical Framework

Let's turn this science into action. Here's a framework you can use starting with your next long run.

Step 1: Know Your Sweat Rate

This is the single most impactful thing you can do. Your sweat rate tells you how much fluid you're losing per hour at a given intensity and temperature.

The DIY method:

1. Weigh yourself (in minimal clothing) before a 60-minute run
2. Run at your target race pace in conditions similar to race day
3. Don't drink anything during the run
4. Weigh yourself again immediately after
5. Every gram of weight lost = 1ml of sweat

If you lost 0.8kg, your sweat rate is roughly 800ml/hour in those conditions.

The limitation: This only tells you fluid loss. It doesn't tell you sodium concentration, which varies hugely between athletes and has a massive impact on cramping risk.

Step 2: Build a Fluid Plan Based on Your Data

Once you know your sweat rate, you can build a plan that replaces 60–80% of your losses. You don't need to replace 100% — your body can handle a mild fluid deficit. Over-drinking is its own risk (more on that below).

For a runner with a sweat rate of 1.2L/hour:

• Target intake: 720–960ml per hour
• That's roughly 180–240ml every 15 minutes
• At most marathon aid stations (every 2–3km), that's 1–2 cups per station

Step 3: Get Your Sodium Right

The general recommendation is 300–600mg of sodium per hour during a marathon. But if you're a salty sweater (white residue on your kit, stinging eyes, salty taste on your skin), you may need significantly more — up to 1,000mg+ per hour.

Sources of sodium during a race:

• Electrolyte drinks (check the label — many are surprisingly low in sodium)
• Salt capsules or tablets (200–250mg per capsule is typical)
• Some gels contain sodium (check the label)

Step 4: Respect the Fatigue Factor

No hydration plan eliminates cramps if your muscles aren't prepared for the effort. This means:

• Train at or near race pace in your long runs
• Include race-specific terrain (hills, if your race has them)
• Don't go out faster than your training supports
• Taper properly to arrive with fresh legs

Step 5: Test Everything in Training

This is non-negotiable. Your long runs are dress rehearsals. Every element of your hydration and fueling plan should be tested multiple times before race day.

Test the products. Test the timing. Test 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

While most cramping content focuses on dehydration, there's a less-discussed risk that affects slower marathon runners in particular: hyponatremia.

Hyponatremia occurs when you drink so much water that your blood sodium levels become dangerously diluted. Symptoms include nausea, confusion, headache, and in severe cases, it can be life-threatening.

This tends to happen when:

• Runners drink at every aid station regardless of thirst
• Plain water is consumed without sodium replacement
• Slower runners spend more time on the course and consume more total fluid

The takeaway: 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 a huge amount of data during every run — heart rate, pace, cadence, temperature, training load. This data is genuinely 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.

Training load data can indicate fatigue risk, but it doesn't connect that fatigue to a fueling or hydration plan.

This is where the next generation of athlete tools comes in. 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.

It's the bridge between the data your watch collects and the decisions you need to make at each aid station.

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.

Generic hydration advice can't solve a personalised problem. You need to know your sweat rate, understand your sodium needs, and build a plan that's specific to you.

Your watch gives you half the picture. A personalised hydration plan gives you the rest.

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. Try it free on the App Store.