Many people commonly, but wrongly, view lactic acid as a waste product that causes fatigue and muscle soreness. However, while lactic acid production often accompanies fatigue, it does not directly cause tired muscles, nor is it responsible for the muscle soreness you may feel a day or two after your workout. In fact, rather than being a waste product, lactic acid is an important energy source for your body both during and after your workout.
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Lactic Acid Production
During very intense exercise, your circulatory system cannot keep up with your muscles' demand for oxygen. To maintain a steady supply of energy, muscles shift from aerobic metabolism, which requires oxygen, to anaerobic metabolism, which does not. Muscles can break down carbohydrates anaerobically to provide energy, resulting in a compound called pyruvate. When oxygen is available, pyruvate can be further broken down aerobically to provide more energy. But when sufficient oxygen is not available, pyruvate is converted into lactic acid.
Lactic Acid and Fatigue
Lactic acid is rapidly broken down into a compound called lactate, resulting in the release of hydrogen ions. Your body can clear lactate by metabolizing it for energy, but when lactate production exceeds the clearance rate, it accumulates in your muscles and bloodstream. While rising levels of lactate are associated with tired muscles, lactate does not actually cause fatigue. Rather, it is the increased acidity in your tissues, due to the buildup of hydrogen ions, that contributes to the sensation of fatigue.
Delayed Onset Muscle Soreness
Lactic acid may cause a temporary burning sensation in your muscles while you're working out. However, contrary to popular belief, it is not responsible for delayed onset muscle soreness, or DOMS, which is muscle soreness that typically develops a day or two after exercise. Because lactate is cleared from your muscles rapidly after exercise, it plays no role in DOMS. Rather, the condition results from microscopic damage to your muscles, usually from exercises that you are not accustomed to doing.
Clearing Lactic Acid
During and after your workout, your muscles and heart can metabolize lactate for energy. Your liver clears lactate from the bloodstream by converting it into glucose, or blood sugar. The liver can also convert lactate into amino acids, the building blocks of proteins. Some lactate is lost in your sweat as well. All of these processes contribute to the rapid clearance of lactate from your bloodstream after exercise. Nearly all of the lactate you produce during a workout is cleared with 30 to 60 minutes, even after very intense exercise. Performing an active cool-down after your workout will help clear lactate more quickly than simply resting.