Foot placement control underlies stable locomotion across species

Abstract

Animals navigate their environment stably without inefficient course corrections despite unavoidable errors. In humans and some robots, this stability is achieved by controlling the placement of the foot on the ground such that recent movement errors are corrected. However, it is unknown whether and how animals with diverse nervous systems and body mechanics use such foot placement control: foot trajectories of many-legged animals are considered as stereotypical velocity-driven patterns, as opposed to error-driven. Here, we posit a control structure for stabilizing foot placement in any legged embodiment by unifying velocity-driven and body state-driven contributions, and develop a framework to discover control strategies used across species from natural locomotor variability. Using this framework, we find evidence for body state-dependent foot placement control in flies and mice, previously only shown to exist in humans. We discover that the urgency and centralization of the foot placement control strategy is shaped by the animal's neuromechanical embodiment. More inherently stable many-legged embodiment is associated with a lower control magnitude and timescale. Further, many-legged embodiment is accompanied by decentralized control with modular control functions, timescales, and gains, whereas analogous functions are centralized across both legs in humans. Our approach discovers signatures of stabilizing control across species and reveals how different neuromechanical embodiments achieve a shared functional goal: foot placement control.