Muscle pre-stimulation tunes viscous-like perturbation rejection in legged hopping

TL;DR

This study investigates how muscle fibre viscosity influences immediate stabilizing responses during fast hopping, revealing that viscous properties contribute to perturbation rejection and can inform robotic stabilizer design.

Contribution

The paper isolates and quantifies the role of muscle fibre viscosity in preflex responses, highlighting its importance beyond elastic effects and its potential application in robotics.

Findings

Viscous-like fibre engagement varies with force-velocity conditions.

Stable hopping occurs even with reduced viscous response in saturated force-velocity cases.

A simple damper could mimic muscle preflex stabilization in robots.

Abstract

Muscle fibres possess unique visco-elastic properties, capable of generating a stabilising zero-delay response to unexpected perturbations. This instantaneous response -- termed "preflex" -- is crucial in the presence of neuro-transmission delays, which are particularly hazardous during fast locomotion due to the short stance duration. While the elastic contribution to preflexes has been studied extensively, research on the role of fibre viscosity due to the force-velocity relation remains unexplored. Moreover, muscle models predict conditions with saturated force-velocity relations resulting in reduced viscous-like fibre engagement. The goal of our study is to isolate and quantify the preflex force produced by the force-velocity relation. We use our approach to analyse two perturbed vertical hopping conditions, differing in their viscosity engagement at touch-down. Both cases showed stable hopping patterns despite the reduced viscous-like response to ground perturbations in the saturated case. Moreover, the force-velocity relation was not the predominant factor driving energy adjustment to disturbance intensity. From a robotics perspective, our results suggest that already a simple, constant damper mounted in parallel to an actuator could provide stabilising preflexes at and shortly after impact.