All cells of the human body employ biochemical reactions known as cellular respiration to produce the energy they need to function and stay alive. The sugar glucose serves as the primary fuel for human cellular respiration. Cells can break down glucose to generate energy using oxygen-dependent aerobic respiration or anaerobic respiration, which does not require oxygen. While aerobic respiration generates energy more efficiently, human muscle cells can utilize anaerobic respiration when they lack sufficient oxygen or require a quick burst of energy.
Role in Exercise
Anaerobic respiration in humans occurs primarily in muscle cells during high-intensity exercise. This might occur if you're pushing your limits during an aerobic activity, like spinning or a cardio workout, and the oxygen supply to your muscles is insufficient to maintain aerobic-only respiration. Anaerobic respiration also occurs with activities that require short, intense bursts of muscle power, such as sprinting or power lifting.
All muscles contain two types of muscle fibers called fast-twitch and slow-twitch fibers. The proportions vary in different muscles. Slow-twitch fibers are geared toward sustained activity and normally rely primarily on aerobic respiration, although they can employ anaerobic respiration if necessary. Fast-twitch muscle fibers are functionally geared toward anaerobic respiration because it generates energy much faster -- up to 100 times faster -- than aerobic respiration. However, since anaerobic respiration is less efficient than aerobic respiration, fast-twitch muscle fibers fatigue relatively quickly.
Glycolysis is the first biochemical process in both aerobic and anaerobic respiration. This multistep process employs several enzymes to breakdown glucose. Each molecule of glucose broken down ultimately yields 2 molecules of pyruvate and 2 molecules of adenosine triphosphate (ATP). ATP stores the energy needed to power cellular functions. With aerobic respiration, the pyruvate generated from glycolysis goes through an additional series of biochemical reactions to generate more ATP. This does not occur with anaerobic respiration.
Lactic Acid Fermentation
With anaerobic respiration in humans, the pyruvate molecules generated during glycolysis are converted into lactate. This process, called lactic acid fermentation, does not generate more energy. However, it does replenish some of the cofactors needed to keep the process of glycolysis going during anaerobic respiration.
Lactate produced during fermentation is of no further use to cells in terms of energy generation. Therefore, it is transported out of the cells and carried in the blood to the liver. There it is converted back to pyruvate, which can then be used to produce more glucose for future use to generate more energy. This biochemical form of recycling is called the Cori cycle.
Lactic acid buildup was previously thought to be the primary cause for muscle fatigue during exercise and delayed soreness afterward. However, more recent data refutes the notion that lactic acid is responsible for delayed muscle soreness. Its possible role in muscle fatigue remains an area of active research.
Reviewed and revised by: Tina M. St. John, M.D.