All cells, whether part of the human body, plant life or even bacteria, must use cellular respiration to produce the energy they need to live. Cells use enzymes to create usable energy out of glucose. Cells can either break down glucose through aerobic respiration, which uses oxygen, or anaerobic respiration, which does not. While aerobic respiration is more efficient, human muscle cells can switch to anaerobic respiration when they lack a sufficient oxygen supply.
Anaerobic respiration occurs most often in humans during vigorous exercise. When muscle cells use up their oxygen, they have to switch to an anaerobic pathway to process energy. According to Dr. Paul Decelle’s online biology course, anaerobic respiration in humans occurs most often in fast-twitch muscle cells, which have a lower capacity for storing oxygen than slow-twitch muscles. The fast-twitch muscle cells run out of oxygen quickly and have to switch to a lactic acid fermentation process to create energy. Humans use their fast-twitch muscle cells during vigorous exercise, particularly running.
According to the Royal Society of Chemistry, glycolysis, the most basic form of metabolism found in all living things, is also the first step in both the aerobic and anaerobic respiration processes. Glycolysis uses enzymes to break down glucose into other types of molecules, releasing energy in the process.
In aerobic respiration, oxygen helps enzymes metabolize energy through the glycolysis process, so according to the Royal Society of Chemistry, in anaerobic respiration, the cells have to use fermentation to keep the metabolism going. In anaerobic respiration, fermentation produces the enzymes that are necessary for cell operation, but it also creates a waste byproduct. In plants and yeast, this byproduct is ethanol, but in humans, the byproduct is called lactic acid.
The cells cannot use the ethanol and lactic acid byproducts and therefore have to get rid of them. In humans, this means lactic acid is excreted through the muscle fibers and processed by the liver. The lactic acid can build up in muscle fibers, however, causing side effects of anaerobic respiration.
Anaerobic respiration is a less-efficient form of cellular respiration than aerobic respiration, and according to the Royal Society of Chemistry, fermentation only produces about 10 percent of the energy that the cells can create with oxygen. Therefore, anaerobic respiration only gives human muscles little bursts of energy, leading to muscle fatigue and cramping. Lactic acid buildup stiffens up the muscle fibers and contributes to muscle cramping.