Exercise Physiology - Energy

Energy

Humans have a high capacity to expend energy for many hours doing sustained exercise. For example, one individual cycling at a speed of 26.4 km/h (16.4 mph) across 8,204 km (5,098 mi) on 50 consecutive days may expended a total of 1,145 MJ (273,850 kcal) with an average power output of 182.5 W.

Skeletal muscle burns 90 mg (0.5 mmol) of glucose each minute in continuous activity (such as when repetitively extending the human knee), generating ≈24 W of mechanical energy, and since muscle energy conversion is only 22-26% efficient, ≈76 W of heat energy. Resting skeletal muscle has a basal metabolic rate (resting energy consumption) of 0.63 W/kg making a 160 fold difference between the energy consumption of inactive and active muscles. For short muscular exertion, energy expenditure can be far greater: an adult human male when jumping up from a squat mechanically generates 314 W/kg, and such rapid movement can generate twice this power in nonhuman animals such as bonobos, and in some small lizards.

This energy expenditure is very large compared to the resting metabolism basal metabolic rate of the adult human body. This varies somewhat with size, gender and age but is typically between 45 W and 85 W. Total energy expenditure (TEE) due to muscular expended energy is much higher and depends upon the average level of physical work and exercise done during a day. Thus exercise, particularly if sustained for very long periods, dominates the energy metabolism of the body.