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

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.

Intermuscular coherence analysis in older adults reveals that gait‐related arm swing drives lower limb muscles via subcortical and cortical pathways

“…When interpreting the results, it is important to keep in mind that intermuscular coherence during gait can be dependent on certain circumstances, such as age (dos Santos et al 2020) or the frequency ratio between arm and leg swing (Kerkman et al 2020). The latter is found to be dependent on walking speed, where a 1:1 ratio is found during a normal walking speed and a 2:1 ratio during very slow walking (<0.8 m/s) (Wagenaar & Van Emmerik, 2000).…”

“…2020) or the frequency ratio between arm and leg swing (Kerkman et al . 2020). The latter is found to be dependent on walking speed, where a 1:1 ratio is found during a normal walking speed and a 2:1 ratio during very slow walking (<0.8 m/s) (Wagenaar & Van Emmerik, 2000).…”

Neuromuscular Control before and after Independent Walking Onset in Children with Cerebral Palsy

“…The reconstruction accuracy of the extracted synergies was determined by the variability accounted for (VAF), which is the ratio of the sum of squared errors to the total sum of squares computed with respect to the mean [ 7 , 37 , 38 ]. Next to VAF we also determined the synergies’ contribution to the matrix (or Frobenius) norm [ 39 , 40 , 41 ], which revealed comparable results. For the sake of legibility, we here present the conventionally used VAF.…”

Alterations in Muscle Networks in the Upper Extremity of Chronic Stroke Survivors

“…Muscle synergies group together muscles based on the co-variation of EMG amplitude during force generation whereas muscle networks group together muscles based on their common neural input in distinct spectral regions, seen as the co-modulation of EMG frequencies, as assessed through the decomposition of the coherence. Previous studies have shown that muscle synergies and coherence muscle networks are complementary to each other and both necessary to understand neuromuscular motor control [30] [31]. Muscle networks have been studied in healthy individuals during postural tasks [30] [32] as well as gait analysis [31], but not yet in individuals with impaired motor control function such as stroke.…”

“…Previous studies have shown that muscle synergies and coherence muscle networks are complementary to each other and both necessary to understand neuromuscular motor control [30] [31]. Muscle networks have been studied in healthy individuals during postural tasks [30] [32] as well as gait analysis [31], but not yet in individuals with impaired motor control function such as stroke. Thus, how stroke alters muscle networks remains unknown even though the scientific findings can support a better understanding of the changes in intermuscular coordination as the severity of post-stroke impairment increases.…”