Why Do Muscles Cramp? Fatigue, Salt and an Unsolved Puzzle

A calf locks up mid-run or a foot seizes in the night. Muscle cramps are common, painful and still surprisingly mysterious. A 2019 review in the journal Sports Medicine by Ronald Maughan and Susan Shirreffs weighs the evidence.

What a cramp is

The review defines a muscle cramp as a temporary but intense and painful involuntary contraction of skeletal muscle. Cramps linked to exercise are called exercise-associated muscle cramps, and surveys cited in the review report them in 67 percent of triathletes, 18 to 70 percent of marathoners and endurance cyclists, and 30 to 53 percent of American football players.

The two main explanations

The first proposed cause is a disturbance of water and salt balance. Support comes mostly from large industrial studies from the 1920s and 1930s: at the Hoover Dam construction site and in steel mills, adding salt to drinking water almost completely abolished cramp cases among men doing hard work in extreme heat. The second is the altered neuromuscular control theory: fatigue in a working muscle disturbs spinal reflexes that normally keep contraction in check, and the muscle fires uncontrollably. Laboratory studies support this - dehydrating volunteers did not change the electrical stimulation needed to trigger a cramp.

Why the answer is still open

Studies of marathon and Ironman athletes found no link between cramping and blood electrolyte levels, which argues against the simple dehydration story. But the review notes that blood samples are usually taken long after the cramp resolves, and may not reflect what is happening inside the muscle. The authors conclude that different cramps probably have different mechanisms, so no single prevention or cure works for all.

Who is most at risk

In a study of 210 Ironman triathletes cited in the review, the independent risk factors were a history of cramping and racing at a higher intensity than usual. Other reported risks include older age, a longer running history, higher body mass index and irregular stretching habits.

What actually helps

The review finds a long history of folk remedies; some may reduce how often cramps strike or how hard they hit, but none is consistently effective. The strongest preventive evidence remains the industrial-era finding that replacing both water and salt matters, not just water alone.

A note

Cramps that are frequent, severe or unrelated to exercise can have medical causes and deserve a doctor's attention. This is general information, not medical advice.

Sources

• Muscle Cramping During Exercise: Causes, Solutions, and Questions Remaining, Maughan and Shirreffs, Sports Medicine, November 2019: https://pmc.ncbi.nlm.nih.gov/articles/PMC6901412/

Facts checked: October 18, 2026.