What Causes Muscle Cramping in Endurance Athletes?

Lots of athletes will know the feeling. It’s a bit like there’s a small knot forming beneath the skin, in your hamstrings or calves. The tension grows, and the muscle starts to contract involuntarily. The cramp usually begins as repeated twinges or flutter-like waves of contractions. It only builds from there, and shifts from discomfort to sharp, intense pain. You have to stop to stretch it out, which relieves the cramp momentarily, before it comes back, over-and-over again.

Cramp can be debilitating for some athletes, effectively ending races in a painful stop-start jerk to the finish. For lucky athletes, it never seems to happen at all. Exercise-induced cramps are, amazingly, still a bit of a mystery.

In this blog, we’ll discuss what we think might be going on with exercise-induced muscle cramping, and suggest some strategies that athletes might use to combat them.

 What causes exercise-induced muscle cramping?

Like I said, the precise cause of exercise-induced muscle cramping is not clear (1). Two main theories exist, which we call the ‘dehydration and electrolyte theory’ and ‘altered neuromuscular control theory’ (2). I dive deep into muscle cramping in our course of long-distance triathlon and heat stress – LDT103 – so check that out if this stuff interests you!

Dehydration and electrolyte theory

During exercise, we often become dehydrated, as we lose fluid in sweat (3). If we lose more fluid in sweat than we put back through drinking, there is a net loss of fluid from the body. As we become dehydrated, we also lose electrolytes like sodium, as sweat contains sodium (3). Even though we lose sodium from the body in sweat, when we become dehydrated during exercise, the sodium concentration of our blood may actually rise, as the concentration of sodium in sweat is lower than the concentration of sodium in our blood (4).

Dehydration and electrolyte theory proposes that these losses – of water and sodium during exercise, due to sweating – contribute to exercise-induced muscle cramping. Specifically, dehydration and electrolyte theory suggests that as we become dehydrated, the interstitial fluid compartment around muscle – fluid in the space between muscle cells and the bloodstream – tightens, which causes misfiring of nerves and disrupts the function of the pumps on the muscle membrane that move electrolytes like sodium and potassium around and allow muscles to contract. This, so the theory goes, wreaks havoc, and the ensuing chaotic environment around the muscle leads to the involuntary contractions we know as cramp.

It seems plausible that dehydration and electrolyte depletion might be involved in cramp – after all, a cramp is an involuntary muscle contraction, and we get our muscles to contract by sending signals to them through the movement of electrolytes like sodium and potassium across membranes. Also, if it happens, cramp tends to occur towards the end of demanding endurance events, when we are likely the most dehydrated and therefore electrolyte-depleted.

However, as plausible as it sounds, there isn’t much evidence to support dehydration and electrolyte theory. Most studies have found that crampers do not lose more sodium in sweat, or become more dehydrated, than non-crampers (5–8). These studies have assessed things like hydration status at the end of endurance events, and compared those who cramped with those who didn’t cramp. If salt losses and/or dehydration were at the root of the problem, one would expect that athletes who cramp would be losing more salt in sweat and urine, and would be more dehydrated, than non-crampers.

However, a recent experimental study found that consumption of an electrolyte-containing drink in already well-hydrated, cramp-prone volunteers made it harder to electrically-induce cramp through artificial stimulation in a lab, but didn’t prevent it completely (9). It’s hard to know what to make of those results, other than it may suggest that sodium has small role in cramping, but it certainly doesn’t provide compelling evidence that it is the major factor. And I would say, you wouldn’t have to worry about sodium during competition, beyond that which is already in your standard sports drinks.

Another theory within the electrolyte balance domain suggests that excessive electrolyte consumption could cause cramping. High levels of sodium and potassium, known as hypernatremia and hyperkalemia, may interfere with normal muscle function and contractions. For sodium, this can be particularly problematic when too much is consumed without adequate water intake – a situation that can easily occur with highly-concentrated electrolyte supplements. In the case of potassium, excessively high levels can disrupt the balance across muscle cell membranes, leading to abnormal muscle contractions and cramping.

While this remains speculative, it makes intuitive sense, especially given the belief that "salty sweaters" tend to cramp more often. Perhaps this cramping is due to an imbalance caused by too much sodium or potassium in the body, not too little. The high salt content in sweat might be the body’s attempt to remove excess electrolytes as efficiently as possible. After all, it is well established that the salt content in sweat is largely influenced by total sodium intake in the diet (see Figure 1 from Malhotra et al. (10)).

Therefore, if you are someone who often suffers with cramps during racing, I don’t think you hang your hat on better hydration as your pathway out of cramping episodes. Sorry!

Figure 1: Relationship between daily sodium intake, and sodium losses in sweat and urine.

 Altered neuromuscular control theory

An alternative theory behind cramp relates instead to neuromuscular fatigue, and disruption of some of the receptors around muscle that cause reflex contractions (2). Specifically, altered neuromuscular control theory suggests muscle fatigue increases the excitability of a receptor called the muscle spindle, and decreases the excitability of the Golgi tendon organ.

The muscle spindle is a sensory receptor located within skeletal muscles that plays a critical role in detecting changes in muscle length and rate of stretch. The muscle spindle is best known for the ‘stretch reflex’ – whereby stretch of a muscle activates the muscle spindle, leading to a reflex contraction that resists further stretch and protects the muscle against injury. The Golgi tendon organ is a different sensory receptor, located at the junction between muscle fibres and tendons. The Golgi tendon organ detects tension within a muscle. When a muscle contracts hard, it generates lots of tension that eventually activates the Golgi tendon organ – this in turn triggers a reflex that causes the muscle to relax, and therefore protects against injury caused by excessive tension.

Therefore, if muscle fatigue were to increase activity of the muscle spindle, and reduce activity of the Golgi tendon organ, it might be prone to painful contractions, which is what we see with cramp.

One factor supporting this theory is that athletes who cramp often experience it in the same body location repeatedly, such as the adductor, calf, or hamstring. Additionally, when cramping occurs, stretching usually alleviates the discomfort (7). This suggests that maintaining good muscle quality and more supple muscles may reduce the risk of cramps in athletes. It’s one reason why incorporating regular stretching and massage into your daily training routine may be important.

Again, whilst this theory makes sense, direct evidence to support it is scant. A relationship between fatigue and cramping seems obvious, and supports some involvement of muscle fatigue in the aetiology of cramping. There is evidence to support altered muscle spindle activity with fatigue in animal muscles (11), but animals are not triathletes! Interestingly, there is evidence that triathletes who race ‘above’ their ability – go out too hard – are more likely to cramp, which, again, indirectly supports, but does not prove, a role for muscle fatigue in cramping (8).

We don’t know the cause of exercise-associated muscle cramp because it is hard to study

So, the exercise-induced cramping literature is a little...frustrating. The lack of clear-cut evidence to identify its cause makes identifying strategies to deal with cramp – for those of us who are more prone to it – difficult.

In defence of the scientists who study cramp, I’ll say here that exercise-induced cramping is really difficult to study. Not everyone cramps, and people that do cramp, don’t cramp predictably. That makes it hard to design protocols to test the effects of a particular intervention on cramping, or to identify the mechanisms behind it. To do that, you’d need a protocol in which people come into your lab and perform exercise that eventually results in cramp. Were you to recruit volunteers and ask them to run on your treadmill until they started cramping, you’d find that most of them had to stop running for some other reason – glycogen depletion, perhaps – without cramp occurring. Therefore, you’d waste a lot of time, money, and effort, without gaining any insight into exercise-induced cramp.

As mentioned, there are electrical stimulation methods that can be used to trigger cramp (9). Researchers have used these techniques to study cramp; however, are these electrically-stimulated cramps the same as the exercise-induced cramps that sometimes occur at the end of a long race? Probably not. Therefore, the application of the results of these studies to cramping in marathon runners or long-distance triathletes is dubious at best.

Strategies for dealing with exercise-induced muscle cramp

Many athletes and coaches report that pickle juice – yes, pickle juice – is a useful tonic to the cramping muscle. The evidence for this is again scant, although this doesn’t mean it doesn’t work – an absence of evidence is not evidence of absence. An experimental study did find that pickle juice ingestion inhibited electrically-induced muscle cramps (12), although, as I said, the application of electrically-stimulated cramping data to athletic settings is tricky. If you want to give pickle juice a go, try ingesting ~1 mL per kilogram of body mass as a ‘shot’ if you feel cramp coming on.

Whilst pickle juice may be a useful ‘cure’ for cramping, effective preparation followed by effective pacing may be a preventative strategy. Observational studies have found that ‘unexpectedly’ fast starts in endurance races are predictors of cramping, rather than dehydration or blood sodium concentrations (8, 13). These results suggest that adopting a realistic pacing strategy – racing to your ability – may be an effective cramp prevention strategy. Also, as already mentioned, studies also found that a history of exercised-induced cramping also predicted cramping, which suggests that some athletes are unfortunately more susceptible to cramp than others. However, the research suggests that this susceptibility has little to do with sweat sodium losses (14).

I have often carried a sachet of soy sauce – one of those small, fish-shaped packets you get with takeaway sushi – to use if I feel cramp coming on. I can’t say that this strategy has a strong evidence base behind it, but my hunch is that it has been helpful. In the absence of a clear mechanism, I am happy to carry it in my race suit, even if it’s just for my peace-of-mind. Just make sure you take it close to aid station .

Summary

Exercise-induced muscle cramps are the bane of some athletes’ racing lives. Frustratingly, we do not have clarity on the precise cause of exercise-induced cramping. It has been suggested that cramps are caused by dehydration and/or electrolyte depletion through sweating, or by altered neuromuscular control caused by muscle fatigue. There is some suggestion that pickle juice is a useful strategy to mitigate cramping, as well as some data to suggest that adopting a realistic pacing strategy reduces the likelihood of muscle cramps.

If you are an athlete who suffers with muscle cramps, I recommend giving the pickle juice a go – why not, you can carry a ~1 mL per kg shot on you in your racing gear. I myself have used a sachet of soy sauce in the past, and found it to be effective, but I can’t say that that strategy is based on hard data.

Most importantly, I also recommend having an honest look at your pacing, and therefore preparation for races in which you have cramped – to make sure that it wasn’t caused by going out too fast. In my earlier Ironman career, I experienced more cramping. However, as I became fitter and better understood the demands of the event through more specific training, I stopped cramping during races. In fact, from 2015 to 2024, I can’t recall having a single cramp while racing.

Keep moving forward and Endure on!

 References

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