Carrying an additional 1 kg of weight while walking at 1.25 m/s increases the rate of metabolic energy expenditure by approximately 7.6 W in healthy adults, and this increase is closely associated with a proportional rise in mechanical work performed on the center of mass, indicating that the metabolic cost of load carriage is primarily driven by the need to redirect greater mass during gait.

When you carry extra weight while walking, your body has to move more mass with each step. This makes your center of mass harder to speed up and slow down, so your ankle and knee muscles must push harder to redirect your body forward. These muscles use more energy to produce this force because they are inefficient — much of the energy becomes heat instead of motion. The extra energy used matches exactly how much extra work the joints do, showing that moving the heavier mass is the main reason you burn more calories.