Dog galloping on rough terrain exhibits similar limb co-ordination patterns and gait variability to that on flat terrain.

Understanding how animals regulate their gait during locomotion can give biological insight and inspire controllers for robots. Why animals use the gallop at the highest speeds remains incompletely explained. Hypothesized reasons for galloping include that it enables recruitment of spinal musculoskeletal structures, that it minimizes energy losses as predicted by collisional theory, or that it provides extended flight phases with more time for leg placement and hence enhances or provides necessary maneuverability [Alexander 1988 Am. Zool. 28 237-45; Ruina, Bertram and Srinivasan 2005 J. Theor. Biol. 237 170-92; Usherwood 2019 J. Exp. Zool. Part A 333 9-19; Hildebrand1989 Bioscience 39 766-75]. The latter-most hypothesis has implications in robotics, where controllers based on the concept of multistability have gained some traction. Here we examine this hypothesis by studying the dynamics of dog gait on flat and rough terrain. This hypothesis predicts that injection of noise into timing and location of ground contacts during the galloping gait by rough terrain will result in an isotropically more noisy gallop gait, centered around the gallop used on flat terrain. We find that dog gait in terms of leg swing timing on rough terrain is not consistently more variable about the mean gait, and constrain the upper limits of this variability to values that are unlikely to be biologically relevant. However the location of the mean gait indeed only shifts by a small amount. Therefore, we find limited support for this hypothesis. This suggests that achieving a target gallop gait with tight regulation is still the desired behavior, and that large amounts of variability in gait are not a desired feature of the gallop. For robotics, our results suggest that the emergent animal-environment dynamics on rough terrain do not exhibit uniformly wider basins of attraction. Future robotics work could test whether controllers that do or do not allow shifts in mean gait and gait variability produce more economical and/or stable gallops.