Answer: Lactic Acid

There’s a long standing fitness myth that after a hard workout, your muscles are sore because of the buildup of lactic acid, or lactate, but by the time your muscles are aching a day or two after a heavy workout, the lactic acid has long since been cleaned up and purged from your muscles. At that point, you’re experiencing inflammation as your muscles repair themselves, and what physiologists refer to as DOMS (Delayed-Onset Muscle Soreness), a post-workout ache that is still not fully understood.

When you’re actually in the process of exercising strenuously (and shortly thereafter), you can thank a temporary buildup of lactic acid in the muscle tissue for that biological reminder to slow down. While our body prefers to generate energy using aerobic mechanisms (which require an adequate supply of oxygen), during periods of intense exercise we burn up the energy reserves in our muscle tissue faster than our oxygen intake can handle. As a result, our muscles briefly switch to an anaerobic energy process called glycolysis, wherein glucose is broken down into a compound called pyruvate. That pyruvate is then, in turn, temporarily converted into lactate for continued energy production by our energy-starved muscles.

The quick buildup of lactate increases the acidity of the muscle tissue and disrupts other metabolic processes. When you’re lifting heavy weights or running at peak capacity, the unpleasant sensation of your muscles burning due to overexertion is a result of high levels of lactic acid. Typically, the onset of that feeling gets us to slow down enough for the body to regain adequate levels of oxygen and begin clearing away the lactic acid, thus reducing the chance that we injure ourselves—a built-in safety mechanism, if you will, against pressing ourselves to the point of injury.