What actually causes a muscle cramp?

A muscle cramp is a sudden, involuntary, painful contraction of a muscle, and the best-supported explanation for why one starts is not a shortage of water or salt but a temporary failure of the nerves that are meant to switch the muscle off [s1]. The dehydration-and-electrolytes story is the one everybody knows; it is also the part of the science that holds up worst, and understanding why explains some of the stranger things that stop a cramp.

Cramps come in more than one kind — the ones that seize a calf at night, the ones that strike during hard exercise — and no single mechanism has been proven to cover all of them. But the research on exercise-associated cramps is where the two competing theories have actually been tested against each other, and the verdict there is instructive.

The theory everyone repeats

The familiar idea is that sweating during exercise strips the body of water and electrolytes, and that the depleted muscle cramps as a result. In a 2009 review in the British Journal of Sports Medicine, the sports physician Martin Schwellnus laid out what the evidence for that idea actually consists of [s1]. Support for the "electrolyte depletion" and "dehydration" hypotheses, he found, comes mainly from anecdotal clinical observation, from case series totalling 18 cases, and from one small case-control study of just 10 people [s1]. Set against that, four prospective cohort studies — the stronger design, following athletes forward in time — did not support the hypotheses at all [s1].

There is also a mechanistic problem. If cramping were caused by whole-body dehydration or salt loss, it should affect the whole body, yet a cramp seizes one muscle, often one that has just been working hard, while the rest stays relaxed [s1]. A systemic deficit is a poor fit for a local event.

The theory that fits better

The alternative Schwellnus favours is "altered neuromuscular control" [s1]. In brief: a muscle is kept in check by a balance of nerve signals — excitatory input from muscle-spindle receptors that sense stretch, and inhibitory input from Golgi tendon organs that sense tension. When a muscle is fatigued, that balance can tip toward runaway excitation, and the motor neurons feeding the muscle begin firing in a sustained burst. The muscle locks. On this account a cramp is a control problem in the nervous system, not a supply problem in the muscle, and the evidence for it comes from human cramp models, epidemiological studies of cramping athletes, and animal experiments [s1].

The distinction is not academic, because the two theories predict different remedies. If cramps were dehydration, fluid and salt should relieve them quickly. If they are a nerve reflex gone wrong, something that acts on the nerves should work — even something that never reaches the muscle.

The pickle-juice experiment

That prediction was tested directly, and the result is one of the cleaner demonstrations in the field. Researchers made male volunteers hypohydrated — deliberately dehydrated to about 3% of body weight, with blood concentration (plasma osmolality) raised to about 295 mOsm/kg — and then induced a cramp in a small foot muscle by electrically stimulating the nerve that supplies it [s2]. Just after the cramp began, each subject swallowed about 1 mL per kg of body weight (73.9 mL on average) of either deionised water or pickle juice [s2].

Pickle juice stopped the cramp faster. Cramp duration was 49.1 seconds shorter after pickle juice than after water — 84.6 seconds versus 133.7 seconds [s2]. The decisive detail is what did not happen: blood composition barely changed in the five minutes after swallowing, so the pickle juice could not have worked by topping up fluid or electrolytes — there was no time for it to be absorbed [s2]. The researchers concluded that the relief came from a reflex triggered in the mouth and throat that dampens the firing of the motor neurons driving the cramp [s2]. A remedy that never reached the cramping muscle relieved the cramp, which is hard to reconcile with the electrolyte story and easy to reconcile with the nerve one.

Where quinine fits, and why it is restricted

The one drug with a long history for cramps is quinine, and its story is a caution about trading a modest benefit for a real risk. A Cochrane review of 23 trials in 1,586 people found that, compared with placebo, quinine reduced the number of cramps over two weeks by 28%, cut cramp intensity by 10%, and reduced the number of days with cramps by 20%, though it did not significantly shorten how long each cramp lasted [s3]. The most common dose studied was 300 mg a day [s3]. So the benefit is real but small.

The harm is the problem. More people had minor adverse events on quinine than placebo, mainly gastrointestinal, and the review notes that quinine overdose reported elsewhere can cause potentially fatal effects [s3]. Rarely, quinine can trigger a severe drop in platelets (thrombocytopenia) that causes dangerous bleeding. On that basis the US Food and Drug Administration has repeatedly warned that quinine is not approved for and should not be used to treat or prevent leg cramps, because the risk of serious and life-threatening reactions outweighs the benefit for that use [s4]. This is reported here as a harm, not a suggestion: whether any drug is appropriate for cramps is a decision for a clinician who knows the person.

What it means for a reader

Cramping looks like a plumbing problem and behaves like a wiring one [s1]. That reframing explains why the standard advice to drink more and take salt has thin evidence behind it for exercise cramps, and why measures aimed at the nerve-muscle system — such as stretching the cramping muscle, which loads the tendon receptors that inhibit contraction — tend to give the fastest relief [s1]. It also explains the pickle-juice curiosity without any magic: a strong taste sets off a protective reflex [s2]. None of this settles the cause of every night-time calf cramp, which remains incompletely understood, and it is distinct from the ordinary soreness that follows hard exercise, covered in our piece on what causes muscle soreness. Persistent, severe or frequent cramps can occasionally signal something else — nerve, circulation or metabolic problems, or a drug side effect — and are worth a clinician's attention rather than a sports-drink aisle.