During anaerobic exercise (exercise that does not require oxygen to produce the necessary energy, ATP, to carry out the activity, such as weight lifting), lactic acid (lactate) is formed when pyruvate (an intermediate substance in glucose metabolism) combines with hydrogen, in the presence of an enzyme, lactate dehydrogenase. Once lactic acid is formed, it quickly diffuses from the muscle into the blood, where it is buffered and carried away. If this didn't occur, you'd tire out very, very quickly. Eventually, though, as you continue to work out, the clearance of lactic acid can't keep up with its formation, and eventually builds up in the muscle and blood, causing fatigue. The acid build-up inactivates certain enzymes that are involved in energy transfer. It also decreases the muscles' ability to contract. These processes lead to fatigue. The lactic acid that has remained in the muscle can be reversed back into an energy source (pyruvate) during recovery or when the pace of exercise is slowed. How quickly this recovery can occur will vary from individual to individual. This process doesn't create more energy than you began with. Some of the lactate has dissipated from the muscle. Although some of the dissipated lactate may be converted into glucose during the Cori Cycle. Hope this info helps.
