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.

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.

When a muscle goes into spasm it feels very tight and is sometimes described as a knot. The pain can be severe.

Your health care provider can diagnose muscle spasms by the presence of tight or hard muscles that are very tender to the touch. There are no imaging studies or blood tests that can diagnose this condition. If the spasm is caused by nerve irritation, such as in the back, an MRI may be helpful to find the cause of the irritation.