Muscle Cramps: What Causes Them and What Actually Stops Them
Night cramps and exercise cramps are extremely common and almost universally attributed to dehydration or low electrolytes. The evidence for that explanation is considerably weaker than its popularity suggests.
Two different things
Exercise-associated muscle cramps occur during or soon after exertion.
Nocturnal leg cramps occur at rest, usually at night, mostly in calves and feet, mostly in older adults.
They probably have different mechanisms and respond to different things.
The dehydration and electrolyte theory
The intuitive explanation is that sweating depletes fluid and sodium, triggering cramps. Problems with it:
- Cramps usually affect only the exercising muscle, not the whole body, which is hard to explain through a systemic blood change
- Studies comparing athletes who cramp with those who do not often find no difference in hydration status or blood electrolytes
- Cramps occur in cool conditions and in people who are not dehydrated
- They can be relieved within seconds by stretching, which a true electrolyte deficit could not explain
There is a genuine exception: profuse salty sweaters in prolonged heat can develop significant sodium depletion, and in that specific group sodium replacement helps.
The altered neuromuscular control theory
The better-supported explanation is that muscle fatigue alters the balance between two reflexes: excitatory signals from muscle spindles increase and inhibitory signals from Golgi tendon organs decrease, leaving the motor neuron hyperexcitable. The muscle locks.
This fits the observations: cramps occur in fatigued muscles, in a shortened position, late in an event, when exercising harder than trained for, and they respond immediately to stretch, which stimulates the inhibitory reflex.
Medical causes worth excluding for persistent cramps
- Medications: diuretics, statins, some asthma drugs, nifedipine, some antipsychotics
- Dehydration and electrolyte abnormalities, genuinely, in people on diuretics, with vomiting or diarrhoea, or on dialysis
- Peripheral arterial disease — calf pain on walking that stops with rest is claudication, not cramp
- Peripheral neuropathy, including diabetic
- Lumbar spinal stenosis
- Hypothyroidism
- Chronic kidney or liver disease
- Pregnancy, where cramps are very common
- Low magnesium, calcium or potassium, where present
- Restless legs syndrome, which is a different sensation: an urge to move relieved by moving, rather than a painful contraction
- Motor neurone disease, rarely, where cramps are widespread and accompanied by weakness, twitching and wasting
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Stopping a cramp
Stretch the affected muscle. This works within seconds and is the most reliable intervention.
- Calf: pull the toes up toward the shin with the leg straight, or stand and lean into a wall with the heel down
- Hamstring: straighten the leg and lean forward from the hips
- Quadriceps: stand and pull the heel to the buttock
- Foot arch: pull the toes back and massage the arch
Follow with gentle massage and warmth. Walk around for a minute or two. The muscle may feel sore for a day.
Preventing exercise cramps
The evidence favours training rather than supplements:
- Train for the actual demand. Cramps cluster at intensities and durations beyond what the muscle is conditioned for
- Progress gradually; most cramping happens in the first races or matches of a season
- Pace realistically, since cramps correlate with going out faster than usual
- Regular stretching of the cramp-prone muscle
- Eccentric strengthening of that muscle
- Hydrate sensibly and, for prolonged exertion in heat with heavy salty sweating, include sodium in drinks
- Adequate carbohydrate, since depletion contributes to fatigue
- Acclimatise to heat gradually
Pickle juice has some trial evidence for stopping cramps quickly, but the effect appears within 30 to 90 seconds, too fast for absorption, and is thought to be a reflex triggered in the mouth and throat rather than an electrolyte effect. Any strong-tasting acidic liquid may do the same.
Preventing night cramps
- Stretch the calves before bed, holding 30 seconds, several repetitions. This has the best evidence of any preventive measure
- Keep bedding loose at the foot of the bed so the foot is not held pointed
- Stay reasonably hydrated through the day
- Regular gentle activity, such as cycling or walking
- Review medications with your doctor, particularly diuretics
- Check for the medical causes above if frequent
- Magnesium has limited evidence overall, though it may help in pregnancy and in documented deficiency
- Quinine is effective but is no longer recommended for cramps in most countries because of serious haematological side effects. Tonic water contains too little to work
See a doctor if
Cramps are frequent and severe, occur with muscle weakness or wasting, come with swelling, redness or skin changes, follow a change in medication, occur with numbness or pain on walking that stops with rest, or come with dark urine after exertion, which suggests rhabdomyolysis and needs urgent assessment.
This is general information. Frequent or severe cramps, or cramps with any other neurological or vascular symptom, deserve a medical review rather than a supplement.