The sunlight smiles down as you enjoy a run, but the relaxed feeling is ruined as the agony of a sudden cramp spasms across your side. This phenomenon is not uncommon – a survey found that around 95% of athletes have cramped during exercise. A frequent suggestion to prevent cramps is consuming water, salt and electrolyte tablets as evidence suggests that some cases are caused by disturbances of water and salt balance. But why is this suggestion and how does it help?
First, let’s clarify a common misconception about the causation of cramps due to lactic acid. Lactic acid is often associated with a burning fatigue in the legs or arms, and is occasionally blamed for cramps as well. However, as explored in a previous post, lactate is an important fuel source during anaerobic respiration and is constantly cleared from muscles and converted into energy in the form of glucose or pyruvate, and therefore is not the source of cramps.
The most widely accepted theory on the origin of exercise-associated muscle cramps is due to an electrolyte imbalance. During movement, the body exerts fluid and crucial electrolytes, most popularly sodium but also chloride, potassium, calcium and several others. The average liter of a person’s sweat contains around 150mg of potassium and up to 2,000mg of sodium. These losses are significant because of electrolytes’ necessary role in muscle contractions.
At the center of muscle contraction is the sodium-potassium pump, which transports 3 positive sodium ions out of the cells and 2 positive potassium ions into the cell. The unbalanced exchange of positive ions results in the cell being negatively charged compared to the outside, known as a resting membrane potential. This charged state is necessary for nerve impulses and muscle contractions. After being depleted of sodium ions, voltage gated sodium channels allow sodium ions to rush back in and create the electrical signal of the nerve impulse. The sodium ions are necessary to initiate the electrical signal while the potassium ions are exchanged with the sodium ions, allowing the nerve cell to return to its resting state.
The possible electrolyte deprivation from sweat is concerning for several reasons. First, salt is the main electrolyte that regulates water outside your cells. The sudden, drastic reduction of sodium in your blood possible from excessive sweating causes the fluid outside your cells to become less concentrated with sodium than the fluid inside your cells. In an attempt to neutralize this imbalance, water rushes from outside the cell into the cell through osmosis, causing your nerve and muscle cells to swell. This physical swelling can exert mechanical stress on the cell membrane, possibly hindering the operation of the ion channels located in the plasma membrane. The reduced concentration of potassium outside the cell for the sodium-potassium pump results in the pump struggling to maintain balanced potassium levels. Due to the combined effects of cellular swelling and the imbalance of potassium, the sodium channels may become hyperexcitable and open spontaneously. This results in uncontrolled firing of nerve impulses and excessive instruction for the muscle to contract, resulting in the sustained contraction of the infamous cramp. So perhaps the next time you go on a run, you should bring a container of chips – purely to ward off the cramps, of course!
References:
Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Ion Channels and the Electrical Properties of Membranes. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26910/
Blow, Andy. Weblog post. TrainingPeaks, TrainingPeaks, LLC, www.trainingpeaks.com/blog/everything-you-need-to-know-about-sweat/#:~:text=We%20hear%20a%20lot%20about,your%20capacity%20to%20keep%20training. Accessed 20 July 2025.
Goodsell, David. “Molecule of the Month: Sodium-Potassium Pump.” PDB-101, RCSB PDB, Oct. 2009, pdb101.rcsb.org/motm/118#:~:text=The%20lion’s%20share%2C%20however%2C%20goes,power%20the%20sodium-potassium%20pump. Accessed 20 July 2025.
Knight, Chris. “8th Annual Cramp Survey Results: Common causes of and cures for cramp.” Precision Fuel & Hydration, www.precisionhydration.com/performance-advice/hydration/precision-fuel-and-hydration-8th-annual-cramp-survey-results/. Accessed 20 July 2025.
Mayo Clinic Staff. “Hyponatremia.” Mayo Clinic, Mayo Foundation for Medical Education and Research (MFMER), Oct. 2024, www.mayoclinic.org/diseases-conditions/hyponatremia/symptoms-causes/syc-20373711. Accessed 20 July 2025.