# Foot placement control underlies stable locomotion across species

## TL;DR

Animals like flies, mice, and humans use foot placement control to maintain stable movement despite errors, with variations based on their body structure and nervous system.

## Contribution

The paper introduces a unified feedforward-feedback control structure for stable locomotion across species, supported by empirical data.

## Key findings

- Flies, mice, and humans use error-driven foot placement control to stabilize locomotion.
- Multilegged animals show less urgent and slower foot placement corrections compared to humans.
- Control signatures in foot placement vary with neuromechanical embodiment across species.

## Abstract

Legged animals move from place to place without deviating from their desired movements despite inherent errors. Humans and some robots achieve such stability by placing the swinging foot in a manner that corrects errors. However, we do not understand how such stability is achieved by animals with diverse brains and bodies. Here, we find that other legged animals also move their swinging leg in response to inherent errors. We further find that the inherent mechanical stability of the animal’s body shapes aspects of its control strategy. Our approach to investigate strategies for stable locomotion across species can help advance comparative neuroscience and legged robotics.

Animals navigate their environment stably without inefficient course corrections despite unavoidable errors. In humans, this stability is achieved by modulating the placement of the foot on each step such that recent errors are corrected. However, it is unknown whether animals with diverse nervous systems and body mechanics use such foot placement control; foot trajectories of many-legged animals are considered to be stereotypical velocity-driven patterns, as opposed to error-driven. Here, we put forth a unified “feedforward-feedback” control structure for stable locomotion that combines velocity-driven and body state error-driven foot placement. We provide empirical support for this control structure across flies, mice, and humans by mining their natural locomotor variability, finding that a competing control structure with purely velocity-driven foot placement is not supported by the data. This work finds shared behavioral signatures of foot placement control in flies, mice, and humans. We find that key characteristics of these signatures, such as their urgency and centralization, vary with neuromechanical embodiment across species. For example, more inherently stable multilegged animals exhibit less urgent control with a lower control magnitude and a slower correction timescale compared to humans. Furthermore, many-legged animals display modular, direction-, and leg-specific control signatures, whereas humans exhibit common signatures across both legs. Overall, our findings provide insight into stable locomotion across species, revealing how species with diverse neuromechanics achieve a shared functional goal: foot placement control.