Mechanisms for the functional differentiation of the propulsive and braking roles of the forelimbs and hindlimbs during quadrupedal walking in primates and felines.

During quadrupedal walking in most animals, the forelimbs play a net braking role while the hindlimbs are net propulsive. However, the mechanism by which this differentiation occurs remains unclear. Here we test two models to explain this pattern using primates and felines: (1) the Horizontal Strut Effect (in which limbs are modeled as independent struts), and (2) the Linked Strut Model (in which limbs are modeled as linked struts with a center of mass in between). Video-recordings were used to determine point of contact, mid-stance, and limb protraction/retraction duration. Single limb forces were used to calculate contact time, impulses, and the proportion of the stride at which the braking to propulsive transition (BP) occurred for each limb. We found no association of the occurrence of the BP and mid-stance, little influence of protraction and retraction duration on the braking-propulsive function of a limb, and a causative relationship between vertical force distribution between limbs and the patterns of horizontal forces. These findings reject the Horizontal Strut Effect, and provide some support for the Linked Strut Model, although predictions were not perfectly matched. We suggest that the position of the center of mass relative to limb contact points is a very important but not the only factor driving functional differentiation of the braking/propulsive roles of the limbs in quadrupeds. It was also found that primates have greater differences in horizontal impulse between their limbs compared to felines, a pattern that may reflect a fundamental arboreal adaptation in primates.