Charley horse is a popular colloquial term in Canada and the United States for painful involuntary spasms or cramps in the leg muscles, typically lasting anywhere from a few seconds to about a day. It is less likely to refer to a bruise on an arm or leg and a 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. In Australia it is also known as a corked thigh or corky. It often occurs in contact sports, such as football when an athlete suffers a knee (blunt trauma) to the lateral quadriceps causing a haematoma or temporary paresis and antalgic gait as a result of pain. Another term, jolly horse, is used to describe simple painful muscle cramps in the leg or foot, especially those that follow strenuous exercise. The term can also be used to refer to cramps in the foot muscles. These muscle cramps can 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 (although the evidence has been mixed), side effects of medication, or, more seriously, diseases such as amyotrophic lateral sclerosis and neuropathy. They are also a common complaint during pregnancy.

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