The most common electrolytes are sodium, potassium, calcium, magnesium and chloride. These electrolytes play important roles in muscle contraction, nerve impulse transmission and fluid balance. Electrolyte deficiencies most commonly occur during exercise, as sweat removes large amounts of salt (sodium chloride) and potassium from the horse's body. If these nutrients are not replaced, the muscles become tired and, in the worst case, can even be damaged.