Running cramps are painful, involuntary muscle contractions that happen during or shortly after exercise. They are not always caused by dehydration, low electrolytes, magnesium deficiency or “lactic acid”; current evidence suggests that exercise-associated muscle cramps usually reflect a combination of individual risk factors, often involving neuromuscular fatigue.
Key takeaways
- There is no single cause of every running cramp.
- Fatigue, pace, training history, previous cramp and environmental conditions can all matter.
- Gentle static stretching is the best-supported immediate treatment for an exercise-associated cramp.
- Drinking more water or taking more salt is not a universal prevention strategy.
- Magnesium has not been tested in randomised trials for exercise-associated cramps.
- Severe, widespread or recurrent cramps—especially with other symptoms—need medical assessment.
What is an exercise-associated muscle cramp?
An exercise-associated muscle cramp, often shortened to EAMC, is a painful and involuntary contraction of skeletal muscle during or soon after exercise. In runners it commonly affects hard-working muscles such as the calf, hamstring or quadriceps.
This is different from a side stitch and from abdominal cramping caused by runner's stomach. It is also different from the night-time leg cramps covered by general health advice. Identifying which problem you have matters because the causes and responses are not identical.
What causes running cramps?
For years, running cramps were commonly blamed on one of three things: dehydration, salt loss or lactate. The modern picture is less tidy.
A 2022 evidence review concluded that exercise-associated cramps appear to arise from a combination of intrinsic and extrinsic factors rather than one universal cause. The authors recommended identifying the risk factors specific to the athlete instead of giving everybody the same instruction to drink more fluid.
Neuromuscular fatigue
The leading explanation is altered neuromuscular control as a muscle fatigues. In simple terms, the balance between signals that tell a muscle to contract and signals that help it relax becomes disturbed. This helps explain why cramps often appear late in a race, during an unusually hard effort or when a runner asks a muscle to work beyond its recent preparation.
Risk is therefore not just about distance. A fast 10K, a hilly half marathon or a marathon run above a sustainable pace can all expose a mismatch between the demand placed on a muscle and what it is conditioned to tolerate.
Previous cramp and individual susceptibility
A runner with a history of cramping may be more susceptible than somebody following the same pace and hydration plan. Training history, injury, muscle damage, sleep, pacing and other personal factors can interact. This is why copying another runner's salt intake is rarely a complete solution.
Heat, fluid and sodium losses
Dehydration and electrolyte losses can still be relevant, particularly during long, hot events or in runners with high sweat and sodium losses. But prospective research has not supported them as the sole explanation for most exercise-associated cramps.
Use fluid and sodium to meet genuine hydration needs—not as a guaranteed anti-cramp treatment. Our guide to electrolytes for runners explains when they are useful and why more is not automatically better.
Are running cramps caused by lactic acid?
The familiar claim that lactate simply “builds up” and causes cramp is not supported by current exercise-cramp reviews. Cramps often occur as fatigue develops, but fatigue is not the same thing as lactate being trapped in a muscle.
This matters because the wrong explanation leads to the wrong fix. A product claiming to “flush lactic acid” is not a substitute for appropriate pacing, training specificity and an individual review of why cramp occurred.
What should you do during a running cramp?
- Slow down or stop safely. Continuing to force the same pace can deepen the cramp and alter your gait.
- Gently stretch the affected muscle. The 2022 evidence review identifies gentle static stretching as the standard immediate treatment. Do not bounce or force the joint.
- Massage may help. NHS guidance for leg cramps also recommends stretching and massaging the tightened muscle.
- Resume cautiously. Once the contraction has eased, walk and reassess. If the muscle starts to seize again, ending the session may be the sensible decision.
- Replace fluid according to the situation. If you have been running for a long time in heat and sweating heavily, normal fluid and sodium replacement may be appropriate. Avoid panic-drinking large volumes.
Painkillers will not stop an active cramp quickly. The NHS notes that paracetamol or ibuprofen take too long to help while a cramp is happening. Runners should also avoid treating ibuprofen as a default race strategy; see our guide to ibuprofen and running.
How can runners reduce their cramp risk?
There is no guaranteed prevention protocol, but an individual plan is more defensible than blindly adding salt or supplements.
Train for the demands of the event
If cramp repeatedly arrives on hills, late in long runs or at race pace, examine whether training has prepared the affected muscles for that exact demand. Progressive long runs, sensible strength work, terrain-specific preparation and realistic pacing may help reduce fatigue arriving earlier than expected.
Review pacing
A cramp late in a race may be a sign that the early pace exceeded current conditioning. Compare the cramp point with pace, elevation and effort rather than looking only at what you drank.
Build a personal hydration plan
Practise during training. Duration, weather, sweat rate and individual tolerance should shape fluid and electrolyte choices. For many easy runs in mild UK weather, normal meals and drinking to thirst may be enough. Longer or hotter sessions can require a more deliberate plan.
Track the pattern
Record the muscle affected, distance, pace, terrain, weather, fluid intake and what training preceded the episode. A repeated pattern is more useful than a one-off assumption about magnesium.
Does magnesium prevent running cramps?
There is no good evidence that taking magnesium prevents exercise-associated cramps. A 2020 Cochrane review found that magnesium was unlikely to provide a clinically meaningful benefit for idiopathic cramps in older adults, and it found no randomised controlled trials evaluating magnesium for exercise-associated muscle cramps.
A diagnosed magnesium deficiency is a separate medical issue. Cramp during a race does not diagnose one. For a fuller review, see magnesium for runners.
When should cramp be medically assessed?
Stop exercising and seek urgent medical help if severe or widespread cramping occurs with concerning systemic symptoms such as confusion, collapse or feeling seriously unwell. Recurrent cramps, episodes lasting unusually long, cramps with swelling or numbness, or symptoms that occur outside exercise should be discussed with a GP.
A clinician can consider medicines, circulation, nerve problems, metabolic conditions and other causes that a hydration plan cannot address.
Where RunStrong fits—and where it does not
RunStrong is not a treatment or proven prevention for running cramps. It does not provide electrolytes or a meaningful nutritional dose of magnesium. If your session requires carbohydrate, fluid or sodium, you still need a suitable fuelling and hydration plan.
RunStrong contains Carnipure® L-Carnitine L-Tartrate, Curcumin C3 Complex®, vegan Vitamin D3, Iron Bisglycinate and BioPerine®. Its role is daily nutritional support between runs, not an emergency fix for an acute muscle contraction.
Frequently asked questions
Why do I cramp near the end of races?
Late-race cramp often fits a fatigue-related pattern, particularly when pace, hills or distance exceed recent preparation. Hydration may contribute for some runners, but it is not the only possible cause.
Should I take salt tablets for running cramps?
Not automatically. Sodium can be useful when sweat losses and event conditions justify it, but extra salt is not a universal cramp cure. High-dose sodium strategies should be individualised, particularly if you have a relevant medical condition.
Does magnesium stop calf cramps?
Magnesium has not been tested in randomised trials specifically for exercise-associated cramps. Do not treat a calf cramp as proof of deficiency.
Should I stretch a cramp?
Yes—gentle static stretching is the best-supported immediate treatment. Stop or slow down first, stretch without bouncing and resume only when the muscle has settled.
Are stomach cramps while running the same thing?
No. Abdominal pain, side stitches and gastrointestinal cramping have different mechanisms from an involuntary calf, hamstring or quadriceps contraction.
Sources
- Miller KC et al. An Evidence-Based Review of the Pathophysiology, Treatment, and Prevention of Exercise-Associated Muscle Cramps. Journal of Athletic Training. 2022. PMID: 34185846
- Schwellnus MP. Cause of exercise associated muscle cramps—altered neuromuscular control, dehydration or electrolyte depletion? British Journal of Sports Medicine. 2009. PMID: 18981039
- Garrison SR et al. Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews. 2020. PMID: 32956536
- NHS: Leg cramps
This article provides general information and does not replace medical advice. Persistent, severe or unusual cramps should be assessed by a qualified healthcare professional.