Robinovitch, S. N. | bioRxiv Science

bioRxiv2023

Neuromuscular fatigue reduces responsiveness when controlling leg external forces

In legged movement, our legs push against the ground, generating external force vectors that enable agile movements. Neuromuscular fatigue can reduce agility by causing physiological changes, such as slowing muscle reaction time, altering proprioception, and delaying neuromuscular control. Fatigue may deteriorate the nervous systems control of leg external forces, contributing to reductions in agility. In this study, we investigated the effect of fatigue on the performance of the nervous system in controlling the vertical component of leg external force ground reaction forces. We hypothesized that increased leg fatigue would lead to declines in both the responsiveness (speed) and accuracy of leg force control. To test this hypothesis, we used an apparatus that allowed participants to exert controlled vertical forces with one leg against a force plate while immobilizing the rest of their bodies. Participants adjusted their leg external force to match step targets displayed on a screen. We induced fatigue by having participants maintain submaximal leg forces, and we measured leg force control performance between fatigue trials. Results showed a significant 26% reduction in mean maximum force production, leading to a substantial decline in leg force control responsiveness, as evidenced by a 23% increase in rise time and a 25% narrowing of bandwidth. However, fatigue did not significantly reduce leg force control accuracy. Understanding the effects of fatigue on leg force control can inform the development of strategies and technologies to sustain agile performance, even in the presence of fatigue. New and NoteworthyWe developed a method to probe the influence of neuromuscular fatigue on the control of leg external forces. Our findings demonstrate that while fatigue significantly diminishes responsiveness (speed), it does not compromise the accuracy of control. These insights enhance our understanding of legged agility and could guide the development of strategies for optimizing leg force control performance. This study paves the way for future research aimed at identifying and employing effective strategies to maintain agility in the face of fatigue.

neuroscience↗

bioRxiv2021

Characterizing the performance of human leg external force control

Our legs act as our primary contact with the surrounding environment, generating external forces that enable agile motion. To be agile, the nervous system has to control both the magnitude of the force that the feet apply to the ground and the point of application of this force. The purpose of this study was to characterize the performance of the healthy human neuromechanical system in controlling the force-magnitude and position of an externally applied force. To accomplish this, we built an apparatus that immobilized participants but allowed them to exert variable but controlled external forces with a single leg onto a ground embedded force plate. We provided real-time visual feedback of either the leg force-magnitude or position that participants were exerting against the force platform and instructed participants to best match their real-time signal to prescribed target step functions. We tested target step functions of a range of sizes and quantified the responsiveness and accuracy of the control. For the control of force-magnitude and for intermediate step sizes of 0.45 bodyweights, we found a bandwidth of 1.8{+/-}0.5 Hz, a steady-state error of 2.6{+/-}0.9%, and a steady-state variability of 2.7{+/-}0.9%. We found similar control performance in terms of responsiveness and accuracy across step sizes and between force-magnitude and position control. Increases in responsiveness correlated with reductions in other measures of control performance, such as a greater magnitude of overshooting. We modelled the observed control performance and found that a second-order model was a good predictor of external leg force control. We discuss how benchmarking force control performance in young healthy humans aids in understanding differences in agility between humans, between humans and other animals, and between humans and engineered systems.

physiology↗