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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.
Brinker MR, O’Connor DP, Almekinders LC, et al. Physiology of Injury to Musculoskeletal Structures: 1. Muscle and Tendon Injury. In: DeLee JC, Drez D Jr, Miller MD, eds. DeLee and Drez’s Orthopaedic Sports Medicine. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2009:chap 1, section A.
Geiderman JM, Katz D. General principles of orthopedic injuries. In: Marx JA, Hockberger RS, Walls RM, et al, eds. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier; 2009:chap 46.
Ronthal M. Arm and neck pain. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Neurology in Clinical Practice. 5th ed. Philadelphia, Pa: Butterworth-Heinemann; 2008:chap 32.
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