Lactic acid buildup occurs during anaerobic exercises, which leads to the burn felt in muscles. However, this buildup involves a much more complicated metabolic process. The buildup continues to be the focus in exercise physiology courses, a buildup that contributes to acidosis in the muscles. Universities and colleges teach students that anaerobic exercises can change the pH in the muscles, leading to metabolic acidosis, but the acidosis isn't from lactic acid. Increases in hydrogen proton and pyruvate production are bigger contributors to metabolic acidosis. Yet most colleges and universities don't teach about all of the contributors of acidic pH change in muscles.
Anaerobic Exercise
The drop in oxygen use during intense exercise leads to a buildup of acidic metabolic compounds in muscles. Free hydrogen protons, pyruvate and lactic acids build up in the working muscle cells. Without oxygen being present to help form energy, the acidic compounds build up until the exercise has ceased. Oxygen then becomes available as part of the recovery process, allowing the pyruvate and hydrogen particles to become lactic acid.
Cause of Acidosis
Protons from hydrogen cause a shift in pH levels---the measurement of acidity of liquids--in the muscles. The accumulation free hydrogen in your working muscles causes your muscles to experience an acidic environment, as stated by Len Kravitz in "Lactate: Not Guilty as Charged."
Lactic Acid
The buildup of lactic acid produces the burn felt by muscles during strenuous exercise. You may notice that with this buildup of lactic acid, your ability to perform exercise diminishes, eventually forcing you to stop. For decades, lactic acid was thought to be the cause of metabolic acidosis when in fact it is produced as a means of neutralizing the increased number of hydrogen particles in the muscles, according to Kravitz.
Cori Cycle
When exercise intensity is so great that oxygen consumption is limited, pyruvic acid---acid metabolite of stored sugar in muscle used for energy--changes into lactic acid in the liver. Lactic acid levels begin to rise, causing the body to recycle this acid to remove its buildup in the muscles. Lactic acid buildup causes muscular pain and burning during exercise. The liver takes the excess lactic acid and converts it back to pyruvic acid for more energy to be made for muscles. This is called the Cori Cycle, and it works best when anaerobic exercise has stopped.
Hyperventilation
After you have performed intense, short bursts of exercise, your breathing rate quickens and deepens. This rapid, deep breathing is your body's mechanism for increasing oxygen availability for the muscles. Oxygen is unloaded more efficiently and transports the excess pyruvate and hydrogen protons to the liver to raise pH levels by making lactic acid, as stated by Illinois Wesleyan University in "Respiratory Regulation During Exercise."
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