Charley horse

"A charley horse" is an American term for a very painful involuntary cramp in the legs (usually located in the calf muscle) and/or foot, lasting anywhere from a few seconds to a couple days. The phrase formerly referred more commonly to bruising of the quadriceps muscle of the anterior or lateral thigh, or contusion of the femur, that commonly results in a haematoma and sometimes several weeks of pain and disability. In this latter sense, such an injury is known as dead leg.

Dead legs and charley horses are two different types of injuries: A charley horse involves the muscles contracting without warning, and can last from a few seconds to a couple days. A dead leg often occurs in contact sports, such as football, when an athlete suffers a knee or other blunt trauma to the lateral quadriceps causing a haematoma or temporary paresis and antalgic gait as a result of pain.

Colloquially, taking a hit in the thigh area (thigh contusion) can also be referred to as a charley horse or even simply as a charley.

Etymology
The first known use of the term was in 1886 in a West Virginia newspaper. The originator of the phrase is credited to two baseball players, Jack Glasscock and Joe Quest.

Cause
Charley horses have many possible causes directly resulting from high or low pH or substrate concentrations in the blood, including hormonal imbalances, dehydration, low levels of magnesium, potassium, or calcium (evidence has been mixed),  side effects of medication, or, more seriously, diseases such as amyotrophic lateral sclerosis and neuropathy. Charley horses seem to be most common in individuals who engage in strenuous physical activities, such as those who work in construction or play sports. They are also a common complaint during pregnancy.

Treatment
Relief is given by massaging or stretching the leg or foot in the opposite direction of the cramp. Relief also comes from standing up, which serves to counter the muscle-tightening signal.