BERTRAM, John E.A.; Florida State Univ., Tallahassee: The mechanics behind galloping in larger mammals: not what you would expect!

Interpretation of the details of morphological form and functional strategies requires an understanding of the critical determinants influencing the performance of the system. In locomotion, substantial insight into the mechanics of a system can sometimes be gained by evaluating the fundamental mechanisms that form the basis of the functional behavior, as these often determine minimum requirements. For example, in recent decades much progress in understanding gait mechanics has come from recognizing the fundamental role of pendular energy exchange in walking and strain energy storage in running. Although numerous studies model the neural control of the asymmetric quadrupedal running gaits collectively known as �galloping�, to date there exists no compelling mechanical explanation of the advantages provided by this gait. Kinetic and kinematic assessments are routinely ambiguous and, based on currently available mechanisms, have not led to an obvious conclusion as to why these gaits are so universal in fast moving quadrupeds. This paper describes an alternative, simple model of galloping that is based on the minimization of energy loss that results from collisional contacts with the substrate. Collisions, discontinuities in center of mass motion, are a major, but largely under-appreciated, source of energetic cost in locomotion. Using the geometry of the center of mass motion mediated by the limb contact, the advantage of sequencing the limbs in the manner employed by large quadrupedal runners is quantified. These predictions are then compared to the canter (slow gallop) of the domestic horse, for which some detailed data are available. The simple, collision-based model is able to predict a remarkable amount of detail of the dynamics of a galloping horse.