Muscle cramps in athletes: causes and their link to exercise load and pharmacology

A sudden, painful spasm of the calf muscle in the final kilometers of a marathon or during a heavy set is familiar to almost everyone who trains seriously. For a long time cramps were explained solely by a 'lack of salts,' but modern data show a more complex picture. Our editors examined what actually triggers cramps during exercise, how they differ from nighttime spasms, and which medications and supplements can raise their risk.

What exercise-associated cramps are

Sports medicine uses the term 'exercise-associated muscle cramps' (EAMC). These are painful, involuntary, sudden contractions of a skeletal muscle that occur during training or shortly afterward. The muscles most often affected are two-joint muscles: the calf, the hamstrings, the quadriceps, and less often the muscles of the foot and hand.

A characteristic feature of such cramps is their connection to a specific muscle that has worked for a long time in a shortened position. The spasm usually lasts from a few seconds to a few minutes and stops after passive stretching. After an episode the muscle may remain tender, and in the following days sore, as after an intense workout.

Cramps often occur in endurance sports: marathon running, triathlon, cycling, football, tennis. Yet they are not rare in strength disciplines either — especially during the 'cutting' phase, when a calorie deficit, fluid restriction, and the use of a number of drugs combine. It is precisely this combination that makes the topic important for our editors.

It is important to distinguish EAMC from nocturnal leg cramps, which occur at rest, often in older people, and from spasms that are a symptom of neurological or metabolic diseases. The mechanisms of these conditions partly overlap, but the approaches to prevention differ.

Two main theories: electrolytes versus neuromuscular fatigue

The classic 'electrolyte-dehydration' theory arose from observations of workers who labored in the heat and lost sweat en masse. The logic is simple: sweat carries out sodium and fluid, the volume of the intercellular space changes, and nerve endings become more excitable. This theory is still popular among athletes and drink manufacturers.

However, prospective studies of marathon runners and triathletes, in particular the work of Schwellnus's group, found no substantial difference in plasma sodium levels or the degree of dehydration between those who had cramps and those who did not. Instead, athletes with cramps more often ran faster than their usual pace or had a history of cramps.

This gave rise to the 'neuromuscular' theory. According to it, fatigue disrupts the balance between excitation from the muscle spindles and inhibition from the Golgi tendon organs. The alpha motor neurons of the spinal cord receive an excess of excitatory signals and get 'stuck' in a state of continuous discharge. This is why passive stretching, which activates the Golgi organs, relieves a spasm so quickly.

The current consensus is that the two mechanisms are not mutually exclusive. Neuromuscular fatigue is most likely the main cause of most cramps during exercise, while significant losses of fluid and sodium can lower the threshold for their occurrence in certain people — for example, in 'salty sweaters' who lose a lot of sodium in their sweat.

Training-related risk factors

The signs most consistently linked with cramps in research are those pointing to overload: intensity higher than usual, duration longer than the athlete is adapted to, insufficient sleep and recovery before the start. Cramps appear more often in the second half of a competition, when fatigue accumulates.

A significant predictor is previous episodes of cramps. If an athlete has already had spasms, the likelihood of their recurrence under similar conditions is higher. This may reflect both an individual predisposition of the nervous system and the repetition of the same mistakes in preparation.

• a sharp increase in training volume or intensity;
• starting at a pace higher than one's actual fitness;
• training in the heat without acclimatization;
• a prolonged deficit of calories and carbohydrates during 'cutting';
• sleep deprivation and psychological stress before competitions;
• shortened, poorly stretched muscles and working in a shortened range of motion.

Nutrition deserves a separate mention. Low-carbohydrate diets reduce glycogen stores, accelerating fatigue, while a sharp restriction of salt combined with profuse sweating can lower the cramp threshold in susceptible people. Bodybuilders before going on stage often simultaneously restrict water, salt, and calories — a classic scenario for generalized spasms.

Finally, specialization matters: cyclists more often cramp in the quadriceps, runners in the calf, swimmers in the foot muscles. These patterns fit the neuromuscular theory well: it is precisely the muscles that tire the most that suffer.

Pharmacology and supplements that affect cramps

A number of medications list muscle cramps among their known side effects. For athletes this is especially relevant, because some of these substances are used both for treatment and for non-medical purposes. Below is a summary from drug package inserts and review literature.

Diuretics deserve special attention. In sports they are used for rapid weight loss or to mask other substances, and that is exactly why they are on the WADA Prohibited List. A sharp drop in potassium and magnesium can manifest not only as cramps but also as dangerous heart rhythm disturbances — so our editors regard the unsupervised use of diuretics as a serious risk.

Clenbuterol and other beta-2 agonists cause a redistribution of potassium into cells. Users are often described as having tremor, tachycardia, and painful spasms. These symptoms are not a 'sign of the drug working' but a signal of an electrolyte disturbance that requires medical evaluation.

There is a persistent myth about creatine that it 'causes cramps.' Controlled observations of college football players (Greenwood et al., 2003) found no increase in the frequency of cramps or injuries, and the ISSN position statement (2017) does not classify cramps as a proven side effect of creatine. Magnesium is also often considered a universal remedy, but a Cochrane review (2020) did not confirm its effectiveness for typical cramps in adults not related to pregnancy.

When a cramp is a sign of another problem

Isolated cramps after an exhausting workout are usually not a cause for alarm. However, there are situations where a spasm is only the 'tip of the iceberg.' Frequent cramps at rest, during light activity, or in several muscle groups at once require examination.

Possible causes include thyroid dysfunction, kidney disease, diabetes mellitus, calcium or magnesium deficiency, peripheral nerve damage, and also rare metabolic myopathies. A doctor may order tests for electrolytes, creatine kinase, TSH, glucose, and kidney function.

A dangerous sign is the combination of cramps with severe muscle pain, swelling, and dark urine — this may be how rhabdomyolysis manifests. Generalized spasms against a background of confusion, nausea, and headache during a long event may indicate hyponatremia associated with excessive water intake. Both conditions require emergency care.

You should also see a doctor if cramps appeared after starting a new medication — for example, a statin or a diuretic prescribed for treatment. You must not stop or change such therapy on your own: that decision is made by a doctor.

Editorial conclusions

Cramps during exercise are mostly the result of neuromuscular fatigue, not just a 'lack of salts.' The most important risk factors are intensity above one's fitness level, insufficient recovery, and previous episodes.

Dehydration and electrolyte loss can lower the cramp threshold in some athletes, and diuretics, beta-2 agonists, insulin, statins, and stimulants are capable of provoking spasms through disruption of the electrolyte balance or a direct effect on the muscles.

According to available data, creatine does not increase the frequency of cramps, and magnesium should not be regarded as a universal solution. Frequent or atypical cramps, especially with dark urine or confusion, are a reason to seek medical help immediately.

We also recommend reading our materials on the prevention and diagnosis of cramps, on rhabdomyolysis in athletes, and on the role of sodium and potassium in hydration.

References

1. Schwellnus MP. Cause of exercise associated muscle cramps (EAMC) — altered neuromuscular control, dehydration or electrolyte depletion? Br J Sports Med. 2009;43(6):401–408.
2. Schwellnus MP, Derman EW, Noakes TD. Aetiology of skeletal muscle "cramps" during exercise: a novel hypothesis. J Sports Sci. 1997;15(3):277–285.
3. Garrison SR, Korownyk CS, Kolber MR, et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020;(9):CD009402.
4. Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.
5. Stroes ES, Thompson PD, Corsini A, et al. Statin-associated muscle symptoms: impact on statin therapy. European Atherosclerosis Society Consensus Panel Statement. Eur Heart J. 2015;36(17):1012–1022.
6. Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–390.
7. World Anti-Doping Agency. The World Anti-Doping Code: International Standard. Prohibited List. Montreal: WADA; чинна редакція.