A cramp is a sudden, involuntary muscle contraction or overshortening; while generally temporary and non-damaging, they can cause significant pain and a paralysis-like immobility of the affected muscle. Onset is usually sudden and it resolves on its own over a period of several seconds, minutes, or hours. Cramps may occur in a skeletal muscle or smooth muscle. Skeletal muscle cramps may be caused by muscle fatigue or a lack of electrolytes such as sodium
Muscle contraction begins with the brain setting off action potentials, which are waves in the electrical charges that extend along neurons. The waves travel to a group of cells in a muscle, letting calcium ions out from the cells’ sarcoplasmic reticula (SR), which are storage areas for calcium. The released calcium lets myofibrils contract under the power of energy-carrying adenosine triphosphate (ATP) molecules. Meanwhile, the calcium is quickly pumped back into the SR by fast calcium pumps.
A lack of potassium or sodium would prevent the sodium-potassium gradient from being strong enough to power the calcium pumps; the calcium ions would remain in the myofibrils, forcing the muscle to stay contracted and causing a cramp. The cramp eventually eases as slow calcium pumps, powered by ATP instead of the sodium gradient, push the calcium back into storage.
Nocturnal leg cramps are involuntary muscle contractions that occur in the calves, soles of the feet, or other muscles in the body during the night or (less commonly) while resting. The duration of nocturnal leg cramps is variable, with cramps lasting anywhere from a few seconds to several minutes. Muscle soreness may remain after the cramp itself ends. These cramps are more common in older people. They happen quite frequently in teenagers and in some people while exercising at night. Besides being painful, a nocturnal leg cramp can cause much distress and anxiety.