Definition of Charley horse :

A charley horse is the common name for a muscle spasm, especially in the leg. Muscle spasms can occur in any muscle in the body. When a muscle is in spasm, it contracts without your control and does not relax.

Causes, incidence, and risk factors:

Muscle spasms often occur when a muscle is overused or injured. Working out when you haven't had enough fluids (you're dehydrated) or when you have low levels of minerals such as potassium or calcium can also make you more likely to have muscle spasms.

Some spasms occur because the nerve that connects to a muscle is irritated. The classic example is a herniated disk irritating spinal nerves as they exit the back, causing pain and spasm.

Spasms in the calf often occur while kicking during swimming, and can also occur at night while you're in bed. Upper leg spasms are more common with running or jumping activities. Spasm in the neck (cervical spine) can be a sign of stress.