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Why does strenuous physical work require much more energy than resting?

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USDA MyPlate dietary guidelines — human nutrition MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.

Strenuous physical work requires significantly more energy than resting because muscle contraction is an energy-intensive process that relies on the continuous breakdown and replenishment of adenosine triphosphate (ATP).12

At rest, the body expends energy primarily to support basal metabolic processes, such as maintaining body temperature, breathing, and pumping blood. However, when muscles perform physical work, they require a rapid and substantial supply of ATP to fuel the mechanical contraction of muscle fibers. As the intensity of physical activity increases, the demand for ATP rises linearly with the work rate.3415

To meet this heightened demand, muscle cells utilize glycogen and fats through metabolic pathways to synthesize more ATP. While the body can produce ATP anaerobically, aerobic metabolism is much more efficient for sustained activity, requiring increased oxygen delivery to the mitochondria via the respiratory and circulatory systems. Consequently, strenuous work can increase an individual's oxygen consumption and total energy expenditure by 10-fold or more compared to resting levels.4153