Your body produces lactic acid, or lactate, in the absence of enough oxygen to convert glucose to energy. As your level of intensity increases, so do your levels of lactic acid. The muscle fatigue and pain that accompany lactic acid have given it a bad rap among athletes in general, and runners in particular, who experience pain in their knee joints following intense exercise.
Your body produces lactate when oxygen levels are low, in order to allow for the breakdown of glucose for energy. Your body normally receives fuel through a process known as glycolysis, which turns glucose into a substance known as pyruvate, and then, when sufficient oxygen is available, delivers it through aerobic pathways as energy. However, when oxygen levels are depleted, as often occurs during periods of strenuous exercise or infectious disease, your body converts pyruvate to lactate to facilitate glucose breakdown.
Lactic acidosis is a condition that occurs when too much lactic acid builds up in your blood at a rate that is faster than it is used to. This condition contributes symptoms of nausea and weakness, possibly hampering athletic performance. It turns out that it is not actually lactic acid that causes the muscle weakness but other metabolites that are not yet well understood. These metabolites contribute to fatigue, a burning sensation and post-exercise soreness.
Although lactic acid is burned quickly and leaves the body within an hour or so of exercise, many athletes begin to experience delayed onset muscle soreness, or DOMS. This condition, often resulting in sore muscles and knees, has often been attributed to lactic acid buildup. However, it is not that simple reports an article published in 2004 in "American Journal of Physiology Regulatory Integrative and Comparative Physiology." This article reported that lactic acid delays fatigue and soreness, but it occurs as a result of other biological processes.
While the former logic of lactic acid's causal relationship with muscle fatigue has been proven faulty, the message remains the same for runners looking to protect their knees. Lactic threshold is determined by blood testing and has been used to determine the optimal training regime to maximize speed while minimizing muscle fatigue. While the relationship may not be direct, lactic threshold appears to still play a role in determining optimal training. An article published in March 2011 in the "Journal of Strength & Conditioning Research" reported that middle-distance runners improved significantly when training at their lactic threshold as opposed to training at a slower pace.