Aerobic and anaerobic metabolism refers to chemical reactions that undergo in our body to release energy in the form of ATP.
When muscles are exercised at a slow pace and not over exerted there is sufficient oxygen present for metabolism to take place. Aerobic respiration takes place during this phase with glucose undergoing the following net reaction with oxygen: C6H12O6 + 6O2(g) --> 6CO2(g) + 6H2O (l) + heat. The heat is energy released in the formation of 29 - 30 ATP molecules for each molecule of glucose. Aerobic metabolism is not very tiring as the substances produced during the reaction are not toxic to muscles.
During intense muscular exertion the amount of oxygen available for respiration is limited. To obtain energy during this phase a set of chemical reactions referred to as anaerobic respiration is used. Anaerobic respiration is very inefficient with the production of only 2 ATP molecules for each molecule of glucose. The reaction is also accompanied by the production of lactic acid that is toxic to muscles and has to be disposed off quickly. Due to this anaerobic respiration can provide energy for only short periods of time.