The causes of a charley horse include muscle injury, dehydration, insufficient blood supply to the muscle and neck muscle stress, notes Healthline. Low blood potassium due to use of hypertension diuretics, and low levels of blood minerals such as calcium and sodium may also cause a charley horse.
A charley horse, also known as a muscle spasm, is a condition in which muscles contract involuntarily and fail to relax, explains MedlinePlus. Although not specific to any muscle, this condition mostly occurs in the legs. Muscles that are in spasm become tight and severely painful. Irrespective of the severity of the muscle spasm, it is advisable to seek medical care if the pain persists.