Why Do Your Muscles Get Tired When You Exercise?

Lessen muscle fatigue by learning why it happens.
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At some point while exercising, your muscles will send up the white flag of defeat and succumb to fatigue. You may be on the last leg of the Boston Marathon or sprinting through the last five minutes on the treadmill, but it happens: muscle burnout. Tired muscles are an exercise reality, but knowing why it happens gives you an edge. Follow up with your doctor if you're concerned about prolonged muscle fatigue.

Lactic Acid Theory

    Since 1929, the sports medicine community has argued over lactic acid's role in the muscle fatigue conundrum and whether it is as insidious as once thought. You may walk at a moderate pace and effortlessly go the distance before your muscles say stop; yet sprinting quickly turns your muscles to jelly. Why is that? Lactic acid rapidly clears from your blood during normal exercise, unlike during high-intensity sprints when it builds up faster than it clears. Fitness pros toe the lactic acid line, weighing in on the pros and cons, but Dr. George A. Brooks of the University of California, Berkeley believes lactic acid is actually a beneficial fuel source that helps performance. Add spurts of high-intensity exercise to give your muscles a more efficient workout, according to Brooks.

Keeping Your Blood Flowing

    Tight clothing -- pants, socks, even a cinched waistband -- also contribute to muscle fatigue, as it constricts blood flow. When your blood flow is reduced, oxygen and other vital nutrients can't reach your muscles and lactic acid can't escape. The blood flow dam doesn't cause tired muscles, but it helps speed it along. Your muscles need a constant circulation of oxygenated, nutrient-rich blood for optimal performance. When you stop the supply with tight clothing, armbands or ill-fitting footwear, expect muscle fatigue.

Chalk it up to Calcium

    Like the stench of sweaty, unwashed socks in your gym bag, muscle fatigue can also stick around after your workout. Yet again, lactic acid isn't to blame. This time, it may be calcium making your muscles feel like jelly, says Columbia University Medical Center's Dr. Andrew Marks. When you exercise, microscopic tears form in your muscles. Calcium, which normally stays in the muscle, slips through these tears and into the bloodstream. Your muscles end up weak and tired. If you exercise, tears and fatigue are unavoidable. Pacing yourself and resting in between workout gives your muscles time to recover, and lets the tears heal.

Starting at the Source

    Minor tears, leaky calcium and lactic acid; it must start somewhere. It does -- with adenosine triphosphate, ATP, a cellular enzyme that helps cell metabolism. Your muscles need energy to flex, and that comes from the high-octane ATP, which moonlights as a muscle fuel. Like a car's gas tank, your muscles can only hold so much of the enzyme. Low-energy exercises keep your muscles from burning ATP faster than it can be replaced, whereas high-intensity exercises burn ATP faster than your muscles can rebuild the supply. When your ATP tank hits zero, your muscle turns to its small storage of glucose for energy. At that point, your muscles are crying for rest.

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