The grazing gait, and implications of toppling table geometry for primate footfall sequences

Abstract

Many medium and large herbivores locomote forwards very slowly and intermittently when grazing. While the footfall order during grazing is the same as for walking, the relative fore-hind timing-phasing-is quite different. Extended periods of static stability are clearly required during grazing; however, stability requirements are insufficient to account for the timing. Aspects of relatively rapid rolling and pitching-toppling due to the resistance of the back to bending and twisting-can be included in a simplifying geometric model to explain the observation that, in grazing livestock, a step forward with a forefoot is consistently and immediately followed by a step forward from the hind; but not vice versa. The same principles and geometry, but applied to the footfall pattern of walking primates, show that toppling would occur at a different point in the gait cycle. This provides a potential account for the distinctive diagonal-sequence footfall pattern of primates, as it prevents the instant of toppling from being at forefoot placement. Careful and controlled hand positioning would thus be facilitated, presumably beneficial to walking on top of branches, despite a slight energetic cost compared with the usual lateral sequence pattern of horses.

Keywords: gait; quadruped; static stability; walk.

Figure 1.

Intervals between hind and fore (H-F), and fore and hind (F-H) foot placements during grazing. A step forwards with a forefoot is directly followed by a step forwards with the hindfoot; the opposite is not true.

Figure 2.

Changes in height due to stiff-limbed vaulting over stance (black low, red high) for a single hip or shoulder (a) or all hips and shoulders for instants of hind–fore and fore–hind foot transitions for ‘normal’ horse-like (b) or typical ‘primate’ walking (c) foot sequencing. The height of three corners of the table-top-back are defined by the height of the hips and shoulders connected to legs in stance; the fourth corner (blue circle outline) is defined by the plane of the back. At the instant of one of the foot placements, there is a discontinuity in back orientation, requiring rapid pitching and rolling (red arrows). This instant occurs at forefoot placement in ‘normal’ walking, but hindfoot placement with the ‘primate’ sequence. The primate sequence may promote careful, controlled forefoot placement suitable for walking along branches. Horse and loris [10] show geometry at the instant of forefoot placement.