Meeting Abstract

6.1  Friday, Jan. 4  The basic mechanics of pronking, bounding or frog-hopping â€" the costs of pitching accounts for much of the diversity of fast quadrupedal gaits. USHERWOOD, JR; The Royal Veterinary College

Quadrupeds show a fascinating range of gaits, both between species and across speeds. Accounting for the selection of these gaits, and understanding them within the context of mechanics, body form and locomotory requirements remains challenging. Current extreme reductionist models provide a range of insights, but fail to account for many aspects of gait selection. Here, I build on the principles of collisional mechanics developed for quadrupedal locomotion pioneered by Ruina, Bertram and Srinivasan, and develop a numerical ‘pseudo-impulsive’ approach to account for the energetic requirements of pronking, bounding and frog-hopping, including the consequences of pitching. This allows two complications to the point-mass model to be considered: points of force application on the ground being distributed (because of a finite back length); and the forces are allowed to apply torques about the centre of mass (because of a finite pitch moment of inertia). In effect, this model treats a quadruped as a stiff table. This approach successfully accounts for why horses gallop with only a gathered aerial phase (and frogs extended). However, if the body geometry does not vary with speed, no account is made for a transition from pronking to pitching gaits (or trotting to galloping) with increasing speed. Indeed, the energetic costs of non-pitching gaits (pronking, trotting and pacing) are predicted to be independent of speed, while pitching gaits (bounding, frog-hopping, galloping etc.) are predicted to increase with speed. So, while the model provides novel and, in retrospect, intuitive insight into the footfall timing and direction of forces during pitching gaits, it also predicts a gallop to trot transition with increasing speed. Likely limitations of the model assumptions will be considered.