Lactic acid isn't the enemy it's been made out to be.
It enables us to perform at high intensities and enormous rates. In fact, lactic acid is the preferred fuel source of the heart and many other tissues.
Contrary to popular belief, it's not responsible for next day soreness, which is a result of muscle damage and tissue inflammation.
You know that immediate burning sensation you get from intense exercise? The real culprit appears to be hydrogen ions, which come, in part, from lactic acid.
Lactic acid is derived from the breakdown of glucose. Because our bodies are constantly breaking down glucose for energy, we're constantly producing lactic acid. During moderate exercise, when lactate is taken up from lactic acid to be oxidized for fuel, the hydrogen ion that results from this separation is removed by the body.
During very intense exercise though, eventually we can't take oxygen in fast enough to meet our increased energy requirements. In order to keep performing, the body speeds up anaerobic metabolism.
For our purposes, this means that lactic acid is produced at a faster rate than it's removed. It becomes more concentrated in our blood. While the lactate makes itself useful in fuel production, the accumulated hydrogen ions lower the PH of muscle. The resulting acidity interferes with muscle performance and metabolism, and seems to be the culprit behind that immediate burning sensation.
With training, however, our tissues seem to adapt to using lactic acid more efficiently, preventing it from reaching high concentrations. In fact, some sports drinks contain the lactate part of lactic acid with the idea that it will help fuel your workout.