Gait transitions in load-pulling quadrupeds: insights from sled dogs and a minimal spring-loaded inverted pendulum model

Quadrupedal animals employ diverse galloping strategies to optimize speed, stability and energy efficiency. However, the biomechanical mechanisms that enable adaptive gait transitions during high-speed locomotion under load remain poorly understood. In this study, we present new empirical and modelling insights into the biomechanics of load-pulling quadrupeds, using sprint sled dogs as a model system. High-speed video and force recordings reveal that sled dogs often switch between rotary and transverse galloping gaits within just a few strides and with minimal changes in speed and stride duration, suggestive of locomotor multi-stability during high-speed load pulling. To investigate the mechanical basis of these transitions, a physics-based quadrupedal spring-loaded inverted pendulum (SLIP) model with hybrid dynamics and prescribed footfall sequences was used to reproduce the asymmetric galloping patterns observed in racing sled dogs. Through trajectory optimization, we replicate experimentally observed gait sequences and identify swing-leg stiffness modulation as a key control mechanism for inducing transitions. This work provides a much-needed biomechanical perspective on high-speed animal draft and establishes a modelling framework for studying locomotion in pulling quadrupeds, with implications for both biological understanding and the design of adaptive legged systems.