A low-cost method for carrying loads during human walking

Abstract

Humans often perform tasks that require them to carry loads, but the metabolic cost of carrying loads depends on where the loads are positioned on the body. We reasoned that carrying loads at the arms’ center of mass (COM) during walking might be cheap since arm swing is thought to be dominated by passive pendulum dynamics. In contrast, we expected that carrying loads at the leg's COM would be relatively expensive because muscular actuation is necessary to initiate and propagate leg swing. Therefore, we hypothesized that carrying loads at the arm's COM while swinging would be cheaper than carrying loads at the leg's COM. We further hypothesized that carrying loads at the arm's COM while swinging would be more expensive than carrying loads at the waist, where the mass does not swing relative to the body. We measured net metabolic power, arm and leg motion, and the free vertical moment while subjects (n=12) walked on a treadmill (1.25 m/s) with no-load, and with 8-kg added to the arms (swinging vs. not swinging), legs, or waist. We found that carrying loads on the arms or legs altered arm swinging amplitude; however, the free vertical moment remained similar across conditions. Most notably, the cost of carrying loads on the swinging arms was 9% less than carrying at the leg's COM (P<0.001), but similar to the waist (P=0.529). Overall, we find that carrying loads at the arm's COM is just as cheap as carrying loads at the waist.