MAIN, R. P.*; GRIFFIN, T. M.; FARLEY, C.T.: Biomechanics of quadrupedal walking: How do four-legged animals walk like inverted pendulums?

Walking bipedal and quadrupedal animals conserve mechanical energy by converting kinetic energy into gravitational potential energy and vice-versa. This inverted pendulum-like exchange is well understood for bipeds in which the center of mass (COM) vaults up and over the stiff support leg. However, walking quadrupeds have 2, 3, and 4 legs in contact with the ground at various times during a stride, making it difficult to understand how the pendulum-like energy exchange occurs. In addition, the typical 25% limb phase (percentage of stride time between successive limb contacts) of walking mammals seems likely to cause the pectoral and pelvic girdles to oscillate out of phase, thus minimizing the vertical oscillations of the COM. We sought to understand how the limbs of quadrupeds function to allow the COM to move like an inverted pendulum. We collected ground reaction forces and video of 6 dogs walking at a range of speeds. We integrated the forces to determine the velocity and vertical displacement of the COM, which were used to calculate the kinetic and potential energy fluctuations. Dogs saved up to 70% of the mechanical work required to lift and accelerate the COM via the inverted pendulum mechanism. Two factors were primarily responsible for this inverted pendulum-like movement. First, the forelimbs supported the majority of body weight (63%). Consequently, the COM movements followed the forelimb movements, mimicking a walking biped. Second, dogs walked with a relatively short time between their ipsilateral hindlimb and forelimb ground contacts (limb phase = 15%), perhaps preventing the pectoral and pelvic girdles from moving out of phase with each other. Supported by UC Berkeley Gompertz Fellowship, NIH AR44688 and AR44008.