The Real Role of Lactic Acid in Exercise and Recovery

Learn why lactic acid is a natural and necessary response to exercise and why post-workout cool-downs might not be so great.

'Zugspitze' - Mountain Sprint

A long-standing theory about exercise cool-down is that it plays a role in preventing delayed onset muscle soreness (DOMS) by flushing lactic acid from your system. Not only is this notion not entirely accurate, neither is the conception that lactic acid build-up from intense exercise is what causes soreness in the first place. A warm-up, on the other hand (according to this study), is likely to reduce DOMS.

Lactic acid forms whenever the body breaks down carbohydrates for energy. It forms faster as the body's oxygen becomes more limited; muscle cells take metabolized glucose and break it down for energy.  The lactic acid then splits into the lactate ion (lactate) and hydrogen ion, which provide the acid that interferes with the same metabolic pathways that permit the initial breakdown of glucose. This effect takes its toll on muscle and nerve signaling, slows energy reactions and impairs muscle contractions.

The burning sensation you feel during high-intensity exercise is a result of the hydrogen ion buildup. While it may seem counter-intuitive that the acid in lactic acid reduces the body's work capacity, it actually is a defense mechanism, an alert, preventing permanent damage to your muscles. Muscle soreness, after all, is caused by the damaging and micro-tearing of muscle tissue and post-exercise tissue inflammation.

As far as the lactate goes, Professor of Exercise Physiology Dr. Thomas Fahey asserts that it's an important fuel for the body during rest and exercise, actually the source of energy preferred by the heart and muscles during high exertion activities. "Lactate is used rapidly for fuel during exercise and recovery and doesn't remain in the muscles like motor oil," Fahey says.

In a state of rest and during light (aerobic) exercise, the body mainly uses fat for fuel. But during maximal exercise, as the body is required to use carbohydrates at higher rates, more lactate is produced, and some carbohydrates are converted into lactate. The lactate is then used as muscle fuel or is transported by the bloodstream to other areas in the body that require energy.  If more lactic acid is made than can be used or removed, you get that buildup. As you slow down your exercise intensity or pace, the rate of lactate used for energy eventually catches up to the rate of lactate production. Fahey recommends both high-intensity exercise and extended moderate exercise to improve your capacity to remove the acid and use the lactate as fuel.

RELATED: The Truth About Aerobic and Anaerobic Training

During long endurance races like marathons or triathalons, the  balance between lactic acid used and produced actually stabilizes. Even though lactic acid is produced at a high rate, a tremendous increase occurs in the rate of muscle uptake.

So, what does this all mean for the recovery?

One study found that following intense rowing machine exercise, subjects who actively recovered (cooled-down) displayed less muscle glycogen re-synthesis than those who passively recovered. Another found that when cyclists stopped exercise abruptly, lactate levels stabilized faster than for those who tapered off their exercise, leaving muscle lactate levels significantly lower in the passive recovery group. During a cool-down, it seems that less lactic acid is transported throughout and removed from the system than during passive recovery. These results suggest that while long cool-downs are certainly not harmful, your muscles might be better served by a passive recovery.

However, if you have a cardiovascular condition or stop abruptly—which can cause blood to pool and lead to dizziness or lightheadedness—skipping the cool-down is probably not the best idea.

At the very least, a cool-down probably won't prevent muscle soreness. An active warm-up, along with foam rolling, is the way to go.

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