Shaking limbs don't mean you should stop engaging in your favorite exercise routine. Running, especially hard running, can wring you out, causing rapid, deep breathing, a burning sensation in your legs, a pounding heart, and even nausea and dizziness -- particularly if you've been running outside in the sweltering heat. One thing that occasionally happens after a good run that you may not anticipate is post-workout muscle tremors. While generally not a dire sign, muscle tremors are uncomfortable and worth preventing if at all possible.
As trivial as it sounds, the usual reason your muscles twitch after a run is simple -- they're tired. At the level of individual motor units -- groups of up to 2,000 or so individual muscle cells innervated by a single motor neuron -- what this means is that fatigue causes some units to "drop out" altogether while others maintain full function. According to physiology professor Loren Martin, the result is that the motor units still "firing" tend to do so an an increasingly synchronized manner, leading to muscle behavior manifesting itself in what we see as twitching or tremors. A short rest period is generally all that's need to return affected muscles to normal functioning.
Electrolytes and Metabolites
Sometimes, post-run muscle tremors are driven not by fatigue but by chemical changes that occur within the muscle, often but not always secondary to unusually stressful environmental conditions such as extreme heat and humidity. Dr. Andrew Turrisi notes that imbalances in sodium, potassium, calcium or magnesium -- each a common result of dehydration -- as well as lactic acid accumulation brought on by running can all lead to post-workout muscle tremors. Dr. Turrisi says that massaging the affected muscles mobilizes metabolic waste products, increases blood circulation and re-establishes a balance among the various chemicals in your bloodstream and muscles.
According to the National Institutes of Health, exercise can precipitate muscle twitching and tremors in the presence of factors predisposing people to such issues that are wholly unrelated to exercise itself. These factors include autoimmune disorders; the overuse or abuse of drugs such as caffeine, amphetamines or other stimulants; the side effects of prescription drugs such as diuretics, corticosteroids or estrogens; nutritional deficiencies and stress. Muscles not participating in exercise may be affected, such as those controlling the eyelids, because the tiniest muscles in the body are the most strongly affected by unusual blood levels of lactic acid and other offending substances.
Prevention and Treatment
While at times there's not much you can to to prevent muscle tremors after running -- and the tremors, while annoying, are normally no cause for alarm -- a number of strategies minimize their potential, and are good for your health anyway. Muscle tremors usually cease on their own, but they can progress into outright spasms; if this happens, stretch and massage the area. If the problem becomes chronic, you may need some combination of chiropractic adjustments, physiotherapy, ultrasound and acupuncture.
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