The Biggest Myth About Muscle "Burn" and Soreness
Most of us have, at one time or another, "felt the burn" during exercise, the point when our strained muscles cry out in agony and plead with us to pause for a rest. The reason for this unpleasant sensation is a buildup of lactic acid, conventional wisdom says. But though supplement makers, health magazines, and personal trainers have parroted this factoid for decades, it's actually incorrect!
Making this myth even more of a head-scratcher is the fact that studies in the scientific literature have been debunking it since the 1970s. In one study, when scientists injected lactic acid directly into muscles, they found no signs that it actually boosted fatigue.
As it turns out, lactic acid, and its far more common conjugate base, lactate, are actually quite useful substances to the body. Produced as a byproduct of the metabolic processes that power muscles, lactate gets rapidly recycled to produce even more fuel for exercising muscles, and the balance is sent to the liver to be converted into glucose, which can also be used to make more energy.
Moreover, lactic acid is not involved with delayed onset muscle soreness, or DOMS, the soreness that can hobble exercisers for days after a strenuous bout of physical activity. Numerous studies have dismantled this hypothesis, but the most convincing was published in 1983. Researchers had subjects run on a treadmill for 45 minutes on a level incline as well as a slight decline. They then assessed participants' soreness and lactic acid levels at set intervals for the following 72 hours.
"Lactic acid concentration was significantly increased during running on the level, but subjects experienced no significant postexercise muscular soreness. Lactic acid was never elevated in downhill runners, but subjects experienced significant delayed-onset soreness," they reported.
The leading theory for DOMS is that it results from microtears in muscles and accompanying low-level inflammation.
On the other hand, there is no single, satisfactory explanation for fleeting muscle "burn" and fatigue, just a variety of hypotheses with limited evidential backing. Misadjusted ion levels in the nerves could factor in, as could insufficient calcium levels. Reactive oxygen species might also spell trouble. In certain situations there might be one explanation, other times another. The conundrum is a testament to the complexity of animal physiology.