<P> The conversion efficiency of energy from respiration into mechanical ( physical ) power depends on the type of food and on the type of physical energy usage ( e.g. , which muscles are used , whether the muscle is used aerobically or anaerobically ) . In general , the efficiency of muscles is rather low : only 18 to 26 % of the energy available from respiration is converted into mechanical energy . This low efficiency is the result of about 40 % efficiency of generating ATP from the respiration of food , losses in converting energy from ATP into mechanical work inside the muscle , and mechanical losses inside the body . The latter two losses are dependent on the type of exercise and the type of muscle fibers being used ( fast - twitch or slow - twitch ) . For an overall efficiency of 20 % , one watt of mechanical power is equivalent to 4.3 kcal ( 18 kJ ) per hour . For example , a manufacturer of rowing equipment shows calories released from ' burning ' food as four times the actual mechanical work , plus 300 kcal ( 1,300 kJ ) per hour , which amounts to about 20 % efficiency at 250 watts of mechanical output . It can take up to 20 hours of little physical output ( e.g. , walking ) to `` burn off '' 4,000 kcal ( 17,000 kJ ) more than a body would otherwise consume . For reference , each kilogram of body fat is roughly equivalent to 32,300 kilojoules or 7,700 kilocalories of food energy ( i.e. , 3,500 kilocalories per pound ) . </P>
<P> The conversion efficiency of energy from respiration into mechanical ( physical ) power depends on the type of food and on the type of physical energy usage ( e.g. , which muscles are used , whether the muscle is used aerobically or anaerobically ) . In general , the efficiency of muscles is rather low : only 18 to 26 % of the energy available from respiration is converted into mechanical energy . This low efficiency is the result of about 40 % efficiency of generating ATP from the respiration of food , losses in converting energy from ATP into mechanical work inside the muscle , and mechanical losses inside the body . The latter two losses are dependent on the type of exercise and the type of muscle fibers being used ( fast - twitch or slow - twitch ) . For an overall efficiency of 20 % , one watt of mechanical power is equivalent to 4.3 kcal ( 18 kJ ) per hour . For example , a manufacturer of rowing equipment shows calories released from ' burning ' food as four times the actual mechanical work , plus 300 kcal ( 1,300 kJ ) per hour , which amounts to about 20 % efficiency at 250 watts of mechanical output . It can take up to 20 hours of little physical output ( e.g. , walking ) to `` burn off '' 4,000 kcal ( 17,000 kJ ) more than a body would otherwise consume . For reference , each kilogram of body fat is roughly equivalent to 32,300 kilojoules or 7,700 kilocalories of food energy ( i.e. , 3,500 kilocalories per pound ) . </P>
where is the energy released from when food is metabolized
where is the energy released from when food is metabolized
into mechanical ( physical ) power
into mechanical ( physical ) power