There are numerous causes of leg muscle spasms. According to the U.S. National Library of Medicine and the National Institutes of Health, muscle spasms or cramps are sudden, involuntary contractions in one or more of a person's muscles that last for several seconds to a few minutes and are one of the most common muscle problems. Leg muscle spasms may be caused by soft tissue damage, medical conditions or metabolic abnormalities.
Muscle Strains
Lower extremity muscle strain can cause leg muscle spasms. According to the National Institutes of Health, muscle injuries sustained while running and jumping may cause upper leg muscle spasms, and one of the most common leg injuries that may lead to muscle spasm is muscle strain or a tearing of the muscle fibers.
The American Academy of Orthoaedic Surgeons, or AAOS, states that muscle strains are graded on a scale of 1 to 3 based on their severity. A grade 1 muscle strain is mild and typically heals quickly, whereas a grade 3 strain is a severe muscle tear or complete rupture of the muscle and can take several months to heal.
The AAOS notes that lower extremity muscle strains often happen when a leg muscle is stretched farther than its limit, which tears the muscle fibers, and that muscle strains typically occur at the musculotendinous junction where the muscle joins its tendon. Muscle spasms, which manifest after the injury, are the body's way to protect the affected area from further injury.
Muscle Fatigue
Muscle fatigue following prolonged exercise can cause leg muscle spasms. According to a February 14, 2008, article on "The New York Times" website, one of the most common causes of cramping is an imbalance between nerve signals that stimulate a muscle and those that inhibit a muscle, and this imbalance occurs when a muscle becomes fatigued, such as during prolonged exercise.
The AAOS states that an athlete is more likely to experience muscle spasms or cramps, including leg muscle spasms, in the preseason when her body is not yet conditioned to the demands of her sport and she is susceptible to fatigue, and that muscle spasms typically develop near the end of intense or prolonged exercise of four to six hours duration.
Other people who are particularly susceptible to muscle fatigue-induced spasms include young children; people ages 65 years and older; people who are sick, overweight or take certain medications; and generally sedentary people who overexert themselves during work or exercise.
Electrolyte Imbalance
Electrolyte imbalances can cause leg muscle spasms. According to the Chemo Care website, electrolytes are chemicals in a person's bloodstream that regulate important functions in the body, and that, when dissolved in water, electrolytes become positively or negatively charged ions. Examples of electrolytes include calcium, magnesium, potassium and sodium.
The Chemo Care website states that a person's nerve reactions and muscle function are governed by the proper cellular exchange of electrolytes, and that electrolyte imbalance—especially altered potassium, magnesium, sodium or calcium levels—can cause a variety of symptoms, including leg muscle weakness, twitching, convulsions and spasm. If the electrolyte imbalance is minor, it can usually be corrected by diet changes. But if the imbalance is severe, a person may require intravenous fluids to replace lost electrolytes.
