Hypokalemia causes muscle cramps because potassium affects the way neuromuscular cells discharge and regenerate energy, and low potassium makes the cells unable to repolarize and fire repeatedly, which can result in a variety of muscle and nerve dysfunctions, including muscle cramps, explains MedicineNet. Hyponatremia causes muscle cramps and other symptoms because it creates an imbalance in the levels of sodium and water that prevents the cells from functioning properly, MedlinePlus reports.
When the amount of sodium outside a cell drops, the cell compensates by allowing in more water, thereby restoring the correct balance of sodium to water. When this happens, cells become swollen with excess water, causing many of the symptoms of hyponatremia, states MedlinePlus.
A possible explanation for how hyponatremia causes muscle cramps in athletes is that, after losing a lot of salt and water through sweat, dehydration and lack of salt make the spaces between the muscle cells contract, increasing pressure on the nerve terminals and causing pain, states the BBC. However, there is not enough evidence to prove this. For example, researchers at North Dakota State University induced muscle cramps in volunteers by using electrical stimulation to test whether dehydrated subjects, who would have low sodium levels, would cramp more easily, but they did not.