While exercise recovery needn’t take an epoch, it does involve EPOC (Excess Post Exercise Oxygen Consumption).
Using the classic example of a short, intense exercise such a sprint, the body uses anaerobic means to produce enough energy to get from the starting block to the finish line. The difference between the oxygen the body required and what it actually was able to take in during that sprint is often referred to as oxygen deficit, or oxygen debt. It describes the lack of oxygen the body has had to make due with while under increased energy demands. It is during these periods, when the output of energy exceeds the ability of the body to provide needed oxygen, that there is an oxygen ‘debt’ and the participant experiences more rapid breathing and stronger heart muscle contraction. The cardiorespiratory system is trying to ‘come up with the down payment’ to this debt by more labored functioning.
Payback Time, with Interest
After the exercise is over, oxygen consumption doesn’t return to resting levels immediately. Oxygen consumption during the exercise recovery is still beyond what is needed to maintain the body’s resting metabolic rate, even though energy demands have already dropped back to resting levels.
Usually, the amount of EPOC is usually greater than the amount of the oxygen deficit. So even though the initial amount of oxygen to be replaced could be thought of as the principal, the additional amount of oxygen consumed during EPOC could be thought of as the interest the body has accrued for enduring the exercise in order to adapt and return to a resting state.
After a bout of exercise, some of the reasons the body demands additional oxygen include:
- To replenish fuel stores. Not only does EPOC involve the use of more oxygen, it involves the use of more fuel. During exercise, the body breaks down its fats stores and free fatty acids (FFA) are released into the blood. During the recovery process, the opposite occurs as there is a direct oxidation of free fatty acids as fuel and a re-conversion of FFAs back into fat storage, both requiring more energy.
- To aid removal of elevated amounts of carbon dioxide from increased respiratory and heart rates
- To fuel the increase in metabolism associated with an elevated body temperature.
- The increase in oxygen consumption triggered by the release of adrenaline and noradrenaline
- To replenish the phosphagen system. During recovery, Adenosine-5′-triphosphate (often referred to as the body’s “molecular unit of currency” for intracellular energy transfer) is synthesized. Some of this ATP donates its phosphate groups to creatine until both ATP and creatine levels are restored to their resting state levels.
- To oxidize lactic acid. A great deal of lactic acid is produced during exercise and finds its way through the blood stream to the heart, the kidneys and the liver. Oxygen is necessary in order to convert lactic acid into the more readily usable pyruvic acid.
How long does EPOC last?
In geologic terms, an epoch can take millions of years. The EPOC effect doesn’t take nearly that long. In general, it can take anywhere from a quarter hour to three quarters of a hour for the body to return to its resting state. EPOC’s effects are greatest soon after the exercise is completed, followed by a gradual decay.
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4. Mosby’s Medical Dictionary, 8th edition. 2009, Elsevier.