Why Do Muscles Cramp? The Science Explained
Anyone who has felt a calf muscle seize mid-run, or woken up at 2am with a foot locked in a painful spasm, knows exactly what a muscle cramp feels like. What is less well known is why it happens and the honest answer is that scientists are still refining their understanding.
This article breaks down what current research actually says causes a muscle cramp, why the old "you're just dehydrated" explanation does not hold up as well as it used to, and what you can realistically do about it.
The Short Answer
There is no single cause of muscle cramping. Cramps during exercise are now more strongly linked to muscle fatigue and nerve signalling issues than to dehydration. Cramps at night or at rest tend to have a different set of triggers, including age, pregnancy, and certain medications. Below, we cover both.
Exercise Cramps: The Theory Is Changing
For decades, the go-to explanation for cramping during sport was dehydration and electrolyte loss. It is an intuitive idea you sweat, you lose salt and fluid, your muscles misfire. It is also, according to a growing body of research, an incomplete one.
In 1997, sports physician Martin Schwellnus proposed an alternative explanation called altered neuromuscular control. The idea is that cramp happens when a fatigued muscle's nerve signalling becomes imbalanced too much "contract" signal, not enough "relax" signal rather than from a shortage of fluid or salt.
A 2011 study following 210 triathletes at an Ironman event tested this directly. Researchers compared blood electrolyte levels and hydration status between athletes who cramped and those who did not, and found no meaningful difference between the two groups. What did predict cramping was a faster race pace and a history of cramping in previous races both pointing toward fatigue and individual susceptibility rather than fluid balance.
This does not mean hydration is irrelevant to performance generally. It does mean it is probably not the main reason your calf locks up at kilometre 30.
Night Cramps Are a Different Story
If you get cramps while sitting on the couch or asleep in bed, exercise fatigue is not the explanation. According to Mayo Clinic, most night leg cramps have no single identified cause, and are generally attributed to tired muscles and nerve issues rather than dehydration.
A few factors are known to raise the odds:
● Age around one in three adults over 60 experience night cramps occasionally
● Pregnancy common in the second half, likely related to added strain on the lower legs
● Long periods sitting or standing, especially on hard floors
● Underlying conditions such as kidney disease, diabetic nerve damage, or poor circulation
● Certain medications, including diuretics
What About Magnesium and Other Supplements?
Low magnesium, potassium, calcium, or sodium can cause cramping in cases of a genuine deficiency, and this is a recognised medical cause. For people without a diagnosed deficiency, the evidence for supplementing is weaker than the popularity of the advice suggests. A 2017 randomized clinical trial published in JAMA Internal Medicine tested magnesium supplementation against a placebo specifically for night leg cramps, and found no meaningful difference between the two groups.
This is a useful reminder that a cramp remedy which sounds biologically plausible is not the same thing as one that has been tested and confirmed to work.
When to See a Doctor
Most cramps are unpleasant but harmless. Get it checked out if:
● A cramp lasts longer than ten minutes or is unusually painful
● Cramps happen frequently or are getting worse over time
● There is swelling, skin discolouration, or noticeable warmth in the limb
● You started a new medication around the same time the cramps began
● You have a condition such as diabetes or kidney disease alongside the cramping
What Actually Helps
The right response depends on which type of cramp you are dealing with.
● For exercise cramps: build gradual heat exposure into training, and pace effort deliberately rather than starting faster than your fitness supports
● For night cramps: gentle stretching before bed and staying generally active during the day are commonly recommended, even though the supporting evidence is mixed
● For a cramp already underway during exercise, some athletes reach for a small, fast-acting dose of pickle juice its proposed mechanism is a nerve reflex rather than rehydration, which is why it can act within about a minute. Our full dosage guide explains how much to take.
You can read that guide here: How Much Pickle Juice Should I Drink for Cramps?
Frequently Asked Questions
What is the number one cause of muscle cramps?
There isn't one single cause. Exercise cramps are now linked more to fatigue and nerve signalling than dehydration, while night cramps are usually put down to a mix of age, nerve issues, and sometimes an underlying condition or medication.
Does drinking more water stop cramps?
It might help some people, but several large studies on athletes haven't found a consistent link between hydration levels and cramp risk. Staying hydrated is still a good idea for general health and performance just do not expect it to be a guaranteed cramp fix.
Why do I only get cramps at night?
Night cramps seem to follow a different pattern to exercise cramps, and are thought to relate to tired muscles and nerve signalling during sleep. Age, pregnancy, and certain medications all raise the likelihood.
Should I take magnesium for cramps?
If a blood test shows you are genuinely deficient, yes, under medical guidance. If you're not deficient, a 2017 clinical trial found magnesium performed no better than a placebo for night cramps, so it is worth talking to a doctor before relying on supplements alone.
This article has been reviewed and fact-checked against peer-reviewed research and guidance from Mayo Clinic and Cleveland Clinic to ensure quality, accuracy, and trustworthiness. It is general information and does not replace advice from your own doctor.
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