Lower complexity of motor primitives ensures robust control of high-speed human locomotion

Alessandro Santuz,Antonis Ekizos,Yoko Kunimasa,Kota Kijima,Masaki Ishikawa,Adamantios Arampatzis

doi:10.1016/j.heliyon.2020.e05377

Alessandro Santuz, Antonis Ekizos + Show 4 more

Open Access

https://doi.org/10.1016/j.heliyon.2020.e05377

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Abstract

Walking and running are mechanically and energetically different locomotion modes. For selecting one or another, speed is a parameter of paramount importance. Yet, both are likely controlled by similar low-dimensional neuronal networks that reflect in patterned muscle activations called muscle synergies. Here, we challenged human locomotion by having our participants walk and run at a very broad spectrum of submaximal and maximal speeds. The synergistic activations of lower limb locomotor muscles were obtained through decomposition of electromyographic data via non-negative matrix factorization. We analyzed the duration and complexity (via fractal analysis) over time of motor primitives, the temporal components of muscle synergies. We found that the motor control of high-speed locomotion was so challenging that the neuromotor system was forced to produce wider and less complex muscle activation patterns. The motor modules, or time-independent coefficients, were redistributed as locomotion speed changed. These outcomes show that humans cope with the challenges of high-speed locomotion by adapting the neuromotor dynamics through a set of strategies that allow for efficient creation and control of locomotion.

Highlights

Humans can locomote at a very broad range of speeds even though walking and running, the two most common gait modes, are profoundly different from both a mechanic and energetic point of view [1, 2, 3]
Our analysis of the modular organization of muscle activations in adult males showed that increasing the locomotion speed and transitioning from walking to running forced the motor system to produce locally less complex and relatively longer basic activation patterns
While not generalizable to the female population due to the male sample, these findings provide evidence that the neuromotor control of locomotion via muscle synergies was spatially and temporally modulated to withstand the constraints imposed by high locomotion speeds

Summary

Introduction

Humans can locomote at a very broad range of speeds even though walking and running, the two most common gait modes, are profoundly different from both a mechanic and energetic point of view [1, 2, 3]. The energy cost function of walking has a peculiar U-shape with a minimum close to each individual's preferred speed, which lies around 1.4 m/s in the average human [5]. Humans often decide to switch from walking to running at lower speeds [5], on average around 2.0 m/s. Despite the profound mechanical and energetic differences, walking and running seem to be sharing similar neural control [4, 8, 9]

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