Muscle prestimulation tunes velocity preflex in simulated perturbed hopping

Abstract

Muscle fibres possess unique visco-elastic properties, which generate a stabilising zero-delay response to unexpected perturbations. This instantaneous response—termed “preflex”—mitigates neuro-transmission delays, which are hazardous during fast locomotion due to the short stance duration. While the elastic contribution to preflexes has been studied extensively, the function of fibre viscosity due to the force–velocity relation remains unknown. In this study, we present a novel approach to isolate and quantify the preflex force produced by the force–velocity relation in musculo-skeletal computer simulations. We used our approach to analyse the muscle response to ground-level perturbations in simulated vertical hopping. Our analysis focused on the preflex-phase—the first 30 ms after impact—where neuronal delays render a controlled response impossible. We found that muscle force at impact and dissipated energy increase with perturbation height, helping reject the perturbations. However, the muscle fibres reject only 15% of step-down perturbation energy with constant stimulation. An open-loop rising stimulation, observed in locomotion experiments, amplified the regulatory effects of the muscle fibre’s force–velocity relation, resulting in 68% perturbation energy rejection. We conclude that open-loop neuronal tuning of muscle activity around impact allows for adequate feed-forward tuning of muscle fibre viscous capacity, facilitating energy adjustment to unexpected ground-level perturbations.

Muscle Preflex Response to Perturbations in locomotion: In-vitro experiments and simulations with realistic boundary conditions

“…The following sections provide details of the experiments and simulations conducted. (Geyer et al 2003;Haeufle et al 2014;Izzi et al 2022). Single-leg hopping is computer-simulated (A) with three perturbation scenarios: 5 cm step-up (P"), no perturbation (P0), and 5 cm step-down (P#).…”

“…To identify realistic boundary conditions for in-vitro muscle fiber experiments, we extracted contractile element kinematics from a single-leg hopper simulation ( (Izzi et al 2022) based on the model by Geyer et al (2003)). The single-leg hopper is driven by a Hill-type muscle-tendon unit (MTU) model.…”

“…Figure 1: Simulations and in-vitro experiments applied in this study. A Hill-type model muscle-tendon unit drives a knee extensor unit of the hopper(Geyer et al 2003;Haeufle et al 2014;Izzi et al 2022). Single-leg hopping is computer-simulated (A) with three perturbation scenarios: 5 cm step-up (P"), no perturbation (P0), and 5 cm step-down (P#).…”

Muscle preflex response to perturbations in locomotion: In vitro experiments and simulations with realistic boundary conditions

“…Boundary conditions for in vitro muscle fiber experiments were derived from a human hopping simulation ( Figure 1A ). It is a simplified model of human hopping which consist of a point mass, a single leg with two segments, and a classical Hill-type muscle model connected as a knee extensor ( Geyer et al, 2003 ; Haeufle et al, 2014 ; Izzi et al, 2023 ) (For details of the model see Supplementary Section S1 ). We performed three simulations with the hopping model: a no-perturbation reference hopping (P0), a step-up perturbation (P ↑ ), and a step-down perturbation (P ↓ ).…”

MIMo: A Multi-Modal Infant Model for Studying Cognitive Development in Humans and AIs

“…When a limb is perturbed from a stable position, the muscles and tendons stretch elastically, altering the force response. It has been shown that these properties support motion stabilization during certain tasks [44], [45]. The degree of compliance can be adapted by the relative contraction levels of the muscle pair.…”