Coordinating fore and hind limb locomotion: when two strategies are better than one

Constraining step length and speed in humans can result in pronounced changes in gait, but such situations seldom occur naturally. In contrast, many quadrupeds have pairs of legs with differing lengths. Since these pairs translate at the same absolute speed and stride lengths, they present a naturally-constrained step length / speed relationship. Using a model optimizing cost of transport and mimicking the morphology of Belgian Malinois dogs, we show that the shorter pair of legs is always more compliant than the longer pair, and that some speeds require both pairs to use different bipedal gaits to minimize cost. This suggests that quadrupeds, if selecting the most economical gait, will on occasion simultaneously use different control strategies for the front and hind limb pairs.