Why Cramps Happen: What the Science Says (and Doesn't Say) About Sodium

The Runner Who Does Everything Right and Still Seizes Up

I have an athlete who takes his sodium religiously. He weighs his sweat, pops salt tablets on every long run, drinks exactly by protocol. And yet at kilometer 30 of his marathon, his hamstrings lock up like someone cinched a tourniquet around them. If cramps were purely an electrolyte problem, he shouldn't be on the ground grabbing his leg.

This story repeats itself across recreational and elite runners alike, and it's why the "electrolyte imbalance" theory as the sole explanation for cramping has lost ground in the scientific literature over the last two decades. It's not that sodium doesn't matter — it matters a great deal, just for other things. It's that sodium is probably not the primary cause of the cramp that stops you cold in the last half hour of a race.

The Old Theory, and Why It Doesn't Explain Everything

For years, the standard explanation was simple: you sweat, you lose sodium and other electrolytes, the muscle loses the ionic environment it needs to contract and relax normally, and a cramp follows. It's intuitive. The problem is that studies comparing runners who cramp with runners who don't — in the same race, under the same heat, with similar sweat losses — found no consistent differences in blood sodium levels between the two groups. Plenty of runners who cramp end up with perfectly normal electrolytes. And plenty who don't cramp lose more sodium than the ones who do.

That doesn't flip the idea that hydration and sodium are irrelevant — an athlete's overall metabolic context still matters for performance, thermoregulation, and recovery — but it does push us to look for a more complete explanation of the specific mechanism behind exercise-associated muscle cramps.

The Neuromuscular Fatigue Hypothesis

The explanation with the strongest mechanistic support today is called altered neuromuscular control theory. The core idea: the cramp doesn't originate in the muscle from a lack of salt — it originates in the nervous system from overload.

Here's a simplified version of how it works. Inside the muscle, two types of receptors regulate muscle tone: muscle spindles, which detect stretch and send signals that tend to increase motor neuron activity (more contraction), and Golgi tendon organs, which detect tension and send inhibitory signals (less contraction, protecting the muscle from tearing). Under normal conditions, these two systems stay in balance.

When a muscle becomes severely fatigued — think the last hour of a marathon, or a set of hill repeats at the end of a long session — excitatory activity from the muscle spindles increases while inhibitory activity from the Golgi tendon organs decreases. The result is a motor neuron firing out of control, and the muscle locks into a sustained, involuntary contraction: the cramp.

This explains several things the sodium theory never could:

• Why cramps almost always show up in the final stretch of a race or workout, when neuromuscular fatigue is at its peak — not necessarily when dehydration is at its worst.
• Why cramps are specific to the muscle working hardest (the calf of the runner with a short stride, the hamstring of the one who overstrides), rather than a systemic phenomenon, which is what you'd expect from a purely electrolyte-driven problem.
• Why passively stretching the affected muscle relieves the cramp almost instantly — stretching activates the Golgi tendon organs and restores the inhibitory signal that calms the hyperactive motor neuron.
• Why highly trained athletes in very long races still cramp even with flawless hydration protocols.

So Is Sodium Useless?

That's not the right conclusion, and I want to be careful here because I don't want anyone to stop hydrating well. Sodium remains critical for plasma volume, cardiovascular function, and thermoregulation during prolonged exercise. An athlete with severe hyponatremia can absolutely develop serious neuromuscular symptoms, and that's a distinct and serious medical condition. What the current evidence suggests is that for the "common" distance-runner cramp — the one that shows up in the final kilometers with no signs of hyponatremia — the dominant mechanism appears to be neuromuscular, not electrolyte-driven.

This connects to a broader point about how prolonged exercise stresses multiple systems simultaneously. Recent research on exercise biomarkers [PMID:41922043] emphasizes that monitoring physiological stress during endurance effort — not just electrolytes, but also markers of neuromuscular fatigue and muscle damage — gives a much fuller picture of why an athlete falls apart late in a long race than looking at any single isolated parameter. The body doesn't break down because of one variable; it breaks down from the accumulation of several.

Similarly, work on the cardiac effects of prolonged endurance exercise [PMID:40667749] reminds me of something I apply constantly in planning for my masters athletes: the cardiovascular system and the neuromuscular system don't age or fatigue at the same rate, and each imposes different limits on how much volume and intensity a runner can tolerate in a long session or a distance race. Understanding that fatigue has multiple origins — cardiac, metabolic, neuromuscular — changes how I design the buildup toward a goal race.

What This Means for Your Training

If cramping is primarily a neuromuscular control problem under fatigue, the prevention strategy shifts focus. Here's what I work on with my athletes:

1. Train the specific duration, not just weekly volume. If your goal race lasts 4 hours, your legs need to experience accumulated fatigue near that time range at some point in the training block — maybe once or twice, not every week. The neuromuscular system adapts to tolerate duration-specific fatigue, just as the aerobic system adapts to tolerate volume.

2. Work on strength and neuromuscular control, not just mileage. Eccentric exercises for hamstrings and calves, hip stability work, and general strength twice a week help the motor unit tolerate fatigue longer before it loses the fine control that prevents cramping. This is part of why I build CFSC-style strength work into nearly every endurance plan, regardless of the athlete's level.

3. Don't abandon sodium, but stop treating it as the single fix. Keep hydrating according to your sweat rate, keep replacing electrolytes on long or hot efforts. But if you're still cramping with solid hydration, the problem is probably the dose of neuromuscular fatigue your body hasn't yet learned to tolerate — not the pinch of salt you missed.

4. Practice race pace, not just training pace. Many cramps show up because an athlete runs the first several kilometers faster than sustainable, racking up neuromuscular fatigue ahead of schedule. Disciplined pacing — something I repeat constantly in my pacing guides — delays the point at which the nervous system loses fine control over the muscle.

5. If you cramp, stretch — don't immediately reach for more salt. Passive stretching activates the Golgi tendon organs and usually cuts the cramp short within seconds. It's an immediate mechanical response to the actual problem.

A Note on Nervous System Recovery

It's worth mentioning that the central nervous system also benefits from the same practices that improve recovery overall. Research on brain-derived neurotrophic factor (BDNF) and exercise [PMID:25455510] shows that regular aerobic training supports neural plasticity, and more recent work on the AMPK/SIRT1/BDNF axis in exercise and fasting models [PMID:40765038] suggests that metabolic regulation and nervous system health are more tightly linked than we used to think. None of this prevents cramping directly, but it reinforces something I say often: the nervous system is a tissue that trains, fatigues, and recovers like any other, and it deserves the same respect in planning that we give the cardiovascular system.

What to Carry Into Your Next Long Run

Cramps aren't a sign of weakness, nor are they purely a sign of poor electrolyte preparation. Most of the time, they're a signal that your neuromuscular system hit a fatigue limit it isn't yet fully prepared for. The good news is that limit is trainable: through duration-specific work, strength training, smart pacing, and patience in your progression. Sodium helps, but it isn't the whole answer, and understanding that will save you frustration the next time you do everything "right" and still feel your leg lock up in the final stretch.

References

• Wang X, Shi J. Different Fasting Methods Combined With Running Exercise Regulate Glucose Metabolism via AMPK/SIRT1/BDNF Pathway in Mice. Compr Physiol. 2025. [PMID:40765038]
• Szuhany KL, Bugatti M, Otto MW. A meta-analytic review of the effects of exercise on brain-derived neurotrophic factor. J Psychiatr Res. 2015. [PMID:25455510]
• Frandsen J, Aaroe M, et al. Cardiac Effects of Prolonged Endurance Exercise in Young and Older Athletes. Scand J Med Sci Sports. 2025. [PMID:40667749]
• Siebers M, Bizjak DA, et al. Exercise biomarkers. Adv Clin Chem. 2026. [PMID:41922043]