In 1922 A.V. Hill and H. Lupton theorized that the body needs to replace the oxygen used by working muscles during mild to intense bouts of exercise. More recently, researchers have used the term EPOC to describe the several different events that occur as the body restores itself to homeostasis, or rest.[1] After you exercise either by weight training or cardiovascular training your body continues to demand oxygen at a higher rate then before you began. Four things are known to happen during this post excessive phase when you are utilizing this excess oxygen build up.

First there is a replenishment of energy resources. This includes replenishment of the immediate source of energy (phospagen system) in which creatine phosphate (the first source of energy utilized within the first 2 seconds of muscular activity) and ATP.[1] Moreover, lactate produced during more intense exercise (approximately 60% VO2 max) is converted to pyruvate for use as fuel in either the krebs cycle or glycolosis. At this time the body is also restoring the muscle glycogen that has been used during the exercise bout.

Secondly re-oxygenation of blood and restoration of circulatory hormones occurs. Large amounts of oxygen are used during exercise to break down food substrates for energy. Thus, the body continues to expend energy after exercise to re-oxygenate the blood. In addition, the body restores the levels of circulatory hormones, which increased during exercise, to normal.[1]

Third there is a decrease in body temperature. This is due to the fact that body's core temperature has risen during exercise and now needs to return to normal. To do this the body must expend energy.

Lastly there is a return to normal ventilation and heart rate. Energy expenditure is greatly elevated as the body rapidly returns to a normal breathing rate. Heart rate is normalizing as well during the EPOC.

Weight Management and EPOC

It has been shown by Bruleson et al. 1998 that resistance training produces greater EPOC responses than aerobic exercise. This suggest that high-intensity resistance exercise disturbs the body's homeostasis to a greater degree than aerobic exercise. The result is a larger energy requirement after exercise to restore the body's systems to normal and thus an explanation for the higher EPOC.[1] A few of the causes of this mechanism are due to an increase in circulating catecholamines and anabolic hormones. In conclusion, the majority of fat metabolized during exercise occurs actually later during in fact, EPOC.

Practical Application (Reynolds, J. and Kravitz, L.)
"Intermittent, high-intensity weight training appears to have the greatest effect on EPOC (Melby et al. 1993; Laforgia et al 1997) and individuals who perform high volume (2- Color 5 sets of 10 to 15 reps at 60- 70% of 1-RM) resistance exercise will see minimal weight management benefits from EPOC (Burleson et al. 1997; Osterberg However, it should be emphasized that the overall weight-control benefits of EPOC, for men and women, from participation in resistance exercise occur over a significant time period, since kilocalories are expended at a low rate in the individual postexercise."[2]


References:
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1. Burleson, M.A. et al. 1998. Effect of weight training exercise and treadmill exercise on elevated post-exercise oxygen consumption. Medicine & Science in Sports & Exercise, 30, 518-22.
2. Kravitz L. Resistance Training and EPOC
3. Schuenke MD, Mikat RP, McBride JM. Effect of an acute period of resistance exercise on excess post-exercise oxygen consumption: implications for body mass management. Eur J Appl Physiol. 2002 Mar;86(5):411-7. Epub 2002 Jan 29.