Meeting Abstract

86.1  Tuesday, Jan. 6  Compass-gait mechanics constrains walking speed in bipeds USHERWOOD, JR; The Royal Veterinary College jusherwood@rvc.ac.uk

The constraints to maximum walking speed and the underlying cause of the walk-run transition remains controversial. However, the motions of the body and legs can be reduced to a few mechanical principles, which, if valid, impose simple physics-based limits to walking speed. Bipedal walking may be viewed as a vaulting gait, with the centre of mass passing over a stiff stance leg (an inverted pendulum), while the swing leg swings forward (as a pendulum). At its simplest, this forms a compass gait walker, which has a maximum walking speed constrained by simple mechanics: walk too fast, or with too high a step length, and gravity fails to keep the stance foot attached to the floor. But how useful is such an extremely reductionist model? Here, we report forceplate-derived measurements on a range of bird species. Ducks represent relatively unspecialized, non-planar, crouch-limbed walkers; turkeys, guinea fowl, pheasants and emu may be viewed as more competent cursors. These measurements are compared with the theoretical predictions derived from compass gait mechanics. Ducks walked as inverted pendulums with near-passive swing-legs up to relative velocities around 0.5, remarkably consistent with the theoretical model. In contrast, top walking speeds (around 0.7) in guinea fowl – as for humans – cannot be achieved with passive swing legs: more specialist cursors, while still constrained by compass gait mechanics, extend their envelope of walking speeds by using relatively high step frequencies. Therefore, the capacity to drive the swing leg forward by competent walkers may be an important and previously little considered specialization, allowing near-passive vaulting of the centre of mass at walking speeds 4/3 that possible with a passive (duck-like) swing leg.