You probably have heard of the term "oxygen debt" and wonder what it means. To cite one of the many definitions that can be found on the Internet, oxygen debt is "the amount of oxygen required to repay the oxygen deficit by the removal of lactic acid and other metabolic products that accumulate when the supply of oxygen was below the needs of the individual during intense activity." - (Feltham Press Ltd: Exercise Physiology Glossary online)
To further explain what oxygen debt means, we briefly discuss here the processes involved in muscle metabolism. Even with the maximal rate and degree of intensity of respiration, and the maximal rate of the flow of blood through the skeletal muscles, there is an inadequate provision of oxygen to supply - by the burning of foods - the energy required for intense muscular work. The energy obtained by the muscles is derived from other reactions. One such reaction is the breakdown of glycogen to lactic acid. In this reaction, however, some of the lactic acid, in turn, is burned to carbon dioxide and water; free oxygen is required for this particular reaction.
However, inasmuch as the oxidative reaction gives off no energy needed directly for muscle contraction, it need not be fully carried out during the period of muscular activity. While such process of oxidizing takes place during the actual muscle activity, much of it is inevitably put off by the limitations of the oxygen provision until both the exercise and the lactic acid production have ceased. It is during the period of muscle activity, while the lactic acid is accumulating, that the body is said to be in oxygen debt.
When the body is in oxygen debt, it means that it has borrowed against the time when the exercise will come to an end, and, with a continuation of the rapid respiratory rate, the oxygen debt can be requited, and the lactic acid disposed of. Most of the lactic acid produced is reconverted into glycogen; for this reaction, a supply of energy is required, which is provided by the oxidation of a small part of the lactic acid.
Let's take a look at this example. During a period of exercise lasting a couple minutes, the anaerobic energy-releasing processes may bring forth such a quantity of lactic acid that ten liters of oxygen are needed for its combustion or reconversion to glycogen. If such volume of oxygen would be taken into the blood in the lungs and sent to the muscles during the period of exercise, the lactic acid could be dealt with as quickly as it is brought forth. In such a case, there would be no oxygen debt.
In reality, though, only about five extra liters of oxygen might be conveyed to the active muscles during such period of exercise. Here, therefore, the amount of oxygen debt incurred is five liters. This oxygen debt is repaid by continuation of the rapid breathing (at a gradually decreasing rate) for possibly ten minutes after the exercise is brought to a stop.
The body has limited borrowing capacity. In extreme cases, a total oxygen debt of about twenty liters may be accumulated. Usually, though, before this limit is reached, further muscle contractions are prevented by fatigue. Prolonged exercise of a less severe nature is often associated with the term "second wind." In this condition, the increased breathing and blood flow convey oxygen to the muscles quickly enough so that the rate of disposition of lactic acid tends to stay even with the rate of its production.
1. "Oxygen Debt", on BrianMAC Sports Coach (online) - http://www.brianmac.co.uk/oxdebit.htm
2. "Oxygen Debt" - http://www.personalpowertraining.net/Articles/oxygen_debt.htm