Anaerobic glycolysis is the process used by living things, including humans, to convert glucose into energy in the absence of oxygen. In humans, it is an essential process for creating energy used for the intense use of muscles, typical of sprinting or weightlifting.
In the presence of oxygen, a complicated chemical reaction takes place that converts oxygen and glucose into ATP and carbon dioxide. During this process, which takes place in the mitochondria, oxygen is combined with molecules that have been created in other parts of the cell to create a very large number of ATP molecules instead of the few created during the previous steps. This creates a steady supply of ATP, but is relatively slow.
In Anaerobic glycolysis, glucose is converted as usual until the oxygen adding step, after which the mitochondria can created even larger numbers of ATP molecules much more quickly. However, instead of creating carbon dioxide as a by-product, it creates lactate.
Although lactate can be processed by the liver back into glucose, this reaction also requires oxygen. In the absence of oxygen (such as is the case with a person running very quickly who cannot take in enough oxygen to meet the demand, or a person with respiratory problems) the lactate builds up as lactic acid. As such, the process is self-limiting. In a well trained individual, the body can continue to process glucose for a period of up to two minutes. However, the process is typically exhausted within thirty seconds.