King's College London

But what causes them? There are many theories about what can lead to muscle cramps from dehydration and electrolyte imbalances to overactivity of our muscles.

Dr Richard Bruce, Programme Lead for Sports & Exercise Medical Sciences discusses some of the myths about why we get cramps and what we know about their cause:

“During the England Cricket team’s recent tour of Sri Lanka, there were a number of incidents of muscle cramp among the batsmen (of both teams) in the hot and humid conditions. Over the years I have heard commentators suggest this was evidence of a lack of fitness, dehydration and loss of electrolytes, or in the case of Jonny Bairstow’s recent return from injury “a consequence of… unaccustomed prolonged activity”. However, the causes of muscle cramps remain poorly understood, and the confidence of many commentator’s statements is certainly not reflected amongst most exercise scientists.

Our poor understanding is partly because of the unpredictable occurrence of cramp which makes it relatively difficult to be studied experimentally. However, muscle cramping during or soon after exercise has been of interest to scientists for over a century, ever since it was first reported in labourers and miners working in hot and humid conditions. These anecdotal observations led many to suppose that ‘dehydration’ and/or ‘electrolyte depletion’ (i.e. from sweating) directly caused the involuntary muscles contractions during a cramp. However, this is not supported by the limited available evidence, as athletes suffering from muscle cramps are almost never dehydrated or have disturbances in blood electrolyte (e.g. sodium) concentrations.

Despite this, the main risk factors for cramps are well established. These include the occurrence of previous cramps, a family history (interestingly suggesting a genetic cause?), and increased exercise intensity/duration. From these observations and new experimental evidence, there is a burgeoning hypothesis about the origin of cramps which relates to muscle fatigue. Repetitive intense contractions which result in muscle fatigue may increase the ‘excitability’ of the motor neurones (nerves) that control that fatigued muscle. Excitably refers to how easily/readily they will signal the muscle to contract, and if the excitability is too great, they can activate the muscle involuntarily.

This hypothesis is attractive as it may also be able to tie in certain anecdotal observations. Exercising for long periods in hot/humid conditions, or with deconditioned muscles will likely increase the risk of developing muscle fatigue – and hence cramps. However, much further work is needed to clearly demonstrate the link between fatiguing muscle contractions and alterations in motor neurone excitability. Further work is also needed to understand the factors underlying the inter-muscle and inter-individual variability in cramp susceptibility (e.g. genetics?), as well why cramps are painful?"