Mechanical work in terrestrial locomotion: two basic mechanisms for minimizing energy expenditure

TLDR

Walking in animals involves alternating gravitational‑potential and kinetic energy transfer like a pendulum, whereas running, hopping, and trotting rely on elastic body bounce to conserve energy. The authors measured the work required to lift and accelerate the center of mass during each step across bipeds, quadrupeds, and hoppers. They found that at intermediate walking speeds up to 70 % of stride energy changes arise from kinetic–gravitational transfer, galloping animals combine this with elastic bounce, and across species the power per unit weight to maintain speed, lift the center of mass, and their sum are largely speed‑independent or linear.

Abstract

The work done during each step to lift and to reaccelerate (in the forward direction) and center of mass has been measured during locomotion in bipeds (rhea and turkey), quadrupeds (dogs, stump-tailed macaques, and ram), and hoppers (kangaroo and springhare). Walking, in all animals (as in man), involves an alternate transfer between gravitational-potential energy and kinetic energy within each stride (as takes place in a pendulum). This transfer is greatest at intermediate walking speeds and can account for up to 70% of the total energy changes taking place within a stride, leaving only 30% to be supplied by muscles. No kinetic-gravitational energy transfer takes place during running, hopping, and trotting, but energy is conserved by another mechanism: an elastic "bounce" of the body. Galloping animals utilize a combination of these two energy-conserving mechanisms. During running, trotting, hopping, and galloping, 1) the power per unit weight required to maintain the forward speed of the center of mass is almost the same in all the species studied; 2) the power per unit weight required to lift the center of mass is almost independent of speed; and 3) the sum of these two powers is almost a linear function of speed.