Energy Metabolism

Energy metabolism consists of a series of chemical reactions that break down foodstuffs and thereby produce energy. The body traps about 20 percent of the energy that is produced and releases the remaining 80 percent as heat. This is why the body heats up during exercise. 

Energy production in the human body revolves around the rebuilding of ATP molecules after they have been broken down for energy. ATP is the molecule that stores energy in a form that the body can use. This rebuilding of ATP molecules is accomplished in a number of ways, all of which correlate to the four main purposes for which energy is utilized during athletic performance - power, speed, strength, and endurance - and to the four basic types of physical activity - strength - power, sustained power, anaerobic power - endurance, and aerobic endurance.

The National Research Council of Canada states that the human body maintains a basic minimum rate of heat production at about 250 Btu/hr during sleep, the heat equivalent of about 75 watts, and about 400 Btu/hr (120 watts) when awake but sedentary. As bodily activity increases, the rate of oxidation of food, with its attendant release of energy, must increase. The level of heat production for light work will be about 650 Btu/hr (190 watts), the extreme value for heavy work, about 2400 Btu/hr (700 watts).