Muscle Synergies and Coherence Networks Reflect Different Modes of Coordination During Walking.

Jennifer N Kerkman,Annike Bekius,Tjeerd W Boonstra,Nadia Dominici,Andreas Daffertshofer

doi:10.3389/fphys.2020.00751

Jennifer N Kerkman, Annike Bekius + Show 3 more

Open Access

https://doi.org/10.3389/fphys.2020.00751

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Abstract

When walking speed is increased, the frequency ratio between the arm and leg swing switches spontaneously from 2:1 to 1:1. We examined whether these switches are accompanied by changes in functional connectivity between multiple muscles. Subjects walked on a treadmill with their arms swinging along their body while kinematics and surface electromyography (EMG) of 26 bilateral muscles across the body were recorded. Walking speed was varied from very slow to normal. We decomposed EMG envelopes and intermuscular coherence spectra using non-negative matrix factorization (NMF), and the resulting modes were combined into multiplex networks and analyzed for their community structure. We found five relevant muscle synergies that significantly differed in activation patterns between 1:1 and 2:1 arm-leg coordination and the transition period between them. The corresponding multiplex network contained a single module indicating pronounced muscle co-activation patterns across the whole body during a gait cycle. NMF of the coherence spectra distinguished three EMG frequency bands: 4–8, 8–22, and 22–60 Hz. The community structure of the multiplex network revealed four modules, which clustered functional and anatomical linked muscles across modes of coordination. Intermuscular coherence at 4–22 Hz between upper and lower body and within the legs was particularly pronounced for 1:1 arm-leg coordination and was diminished when switching between modes of coordination. These findings suggest that the stability of arm-leg coordination is associated with modulations in long-distant neuromuscular connectivity.

Highlights

Human locomotion requires a well-organized activation of multiple muscles to coordinate movements of upper and lower limbs
Muscle Synergies and Coherence Networks from loaf to normal walking, one can observe a switch in frequency locking from a 2:1 to a 1:1 ratio between the arm and leg swing (Craik et al, 1976; Schöner et al, 1990; Van Emmerik and Wagenaar, 1992, 1996): At very low speeds, the arm swing is phase locked to the step cycle, while at fast speeds it locks to the stride cycle
The arm swing can have little to no influence on leg movement after spinal cord injury (Tester et al, 2012). These findings suggest that a partial interruption of the spinal cord may suffice to limit the interaction between spinal motor neurons such that switches in interlimb coordination no longer emerge

Summary

Introduction

Human locomotion requires a well-organized activation of multiple muscles to coordinate movements of upper and lower limbs. Muscle Synergies and Coherence Networks from loaf (very slow) to normal walking, one can observe a switch in frequency locking from a 2:1 to a 1:1 ratio between the arm and leg swing (Craik et al, 1976; Schöner et al, 1990; Van Emmerik and Wagenaar, 1992, 1996): At very low speeds, the arm swing is phase locked to the step cycle, while at fast speeds it locks to the stride cycle This switch is accompanied by a change in the phase relationship between the arms from in-phase to antiphase phase locking (Wagenaar and van Emmerik, 2000), and in the immediate vicinity of the transition the variability of frequency (phase) locking drastically increases. In the vicinity of a phase transition, one may expect the dynamics’ dimensionality to be drastically reduced and muscle activity patterns to stay on lowdimensional manifolds

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