I know, you have a lot of educational baggage when it comes to lactate and lactic acid. Yet as the chemical equation to the left shows, lactate and not lactic acid is produced in the lactate dehydrogenase reaction, and more importantly, lactate production consumes a proton (H+). Thus, lactate production retards, not contributes to a metabolic acidosis. Yes, this is the opposite to historical and what were once conventional explanations. However, that just shows you how polluted science can be when apathy and bias infect the process of the scientific method. You will be amazed how many more examples of such "bad" science exist across the physiological sciences, and especially within exercise physiology. Now, the other very important aspect of lactate production relates to the second product of this reaction, NAD+. The LDH reaction, and therefore lactate production, rapidly regenerates NAD+ and helps maintain cytosolic redox ([NAD+] / [NADH]). As you now know, NAD+ is an important substrate for the glyceraldehyde-3-phosphate dehydrogenase reaction of phase II of glycolysis. Remember, this is also the phase of glycolysis that regenerates the ATP. Thus, biochemically speaking, lactate production in contracting skeletal muscle support the sustained ATP demand of intense muscle contraction by providing a cytosolic re-supply of NAD+, which in turn allows for glycolytic ATP regeneration and a much larger non-mitochondrial ATP turnover capacity. Consequently, if it were not for lactate production in skeletal muscle during intense exercise, we could not exercise for as long, acidosis would be more severe, and as a result our tolerance of more intense exercise would be severely compromised. In this context, muscle lactate is not a "bad" molecule. Lactate is needed to allow skeletal muscle to have the tolerance of sustained intense contractions. The "fatigue" and acidosis have other origins, as I explain in other sections of this electronic textbook.
Recommended sequence of topics:
What is lactate?
Lactate efflux from muscle