It was not until recently that lactic acid was viewed upon as a potential source of energy, and, in fact, is not the reason for muscle fatigue. There are several different types of muscle fatigue, the varying differences are dependent on the type of physical activity as well as the muscle fiber type. The first type of fatigue is due to a buildup of K+ in the transverse-tubular system and in the adjacent muscle fibers. This buildup depolarizes the fiber and hinders Na+ channels from recovering. This type of fatigue can be significant since the concentration of K+ during intense activities can reach high levels. Another type of muscle fatigue is known as “metabolic fatigue.” This type of fatigue emerges due to the indirect or direct effects of metabolite buildup and decrease in substrates within muscle fibers. In fast twitch muscle fibers, cellular ATP drops to extremely low levels and can cause a reduction in the release of Ca2+. This fatigue can also occur due to the direct or indirect effects of glycogen depletion. Decreasing the pH of muscle fibers from 7.1 to less than 6.7 does not expedite or cause onset fatigue, instead, it slows its onset. In fact, decreasing intracellular pH to 6.7 negates the inhibitory effects of an increase in extracellular potassium concentration.

According to a paper in the American Journal of Physiology, UC Berkeley researchers concluded from their lab results that muscle cells use carbohydrates anaerobically for energy, which produces lactase as a byproduct, and then uses the lactate alongside oxygen to generate more energy. The first method is called the “glycolytic pathway”, and is the main pathway during normal exercise. During this method, lactate trickles out of the muscle cells to be used elsewhere. However, during intense workouts, the rapidly accumulating lactate is oxidatively removed to create more energy.

https://www.sciencedaily.com/releases/2006/04/060420235214.htm