The Existence of Shared Muscle Synergies Underlying Perturbed and Unperturbed Gait Depends on Walking Speed

Lotte Hagedoorn,Edwin Van Asseldonk,Matjaž Zadravec,Zlatko Matjačić,Andrej Olenšek

doi:10.3390/app12042135

Abstract

Muscle synergy theory assumes that the central nervous system generates a wide range of complex motor outputs by recruiting muscle synergies with different strengths and timings. The current understanding is that a common set of muscle synergies underlies unperturbed as well as perturbed walking at self-selected speeds. However, it is not known whether this is the case for substantially slower walking. The aim of this study was to investigate whether a shared set of muscle synergies underlies balance recovery responses following inward- and outward-directed perturbations in the mediolateral direction at various perturbation onsets and walking speeds. Twelve healthy subjects walked at three walking speeds (0.4, 0.6, and 0.8 m/s) on a treadmill while perturbations were applied to the pelvis using the balance assessment robot. A set of sixteen EMG signals, i.e., eight muscles per leg, was measured and decomposed into muscle synergies and weighting curves using non-negative matrix factorization. The muscles included were left and right tibialis anterior, soleus, gastrocnemius medialis, gastrocnemius lateralis, rectus femoris, hamstring, gluteus medius, and gluteus maximus. In general, four muscle synergies were needed to adequately reconstruct the data. Muscle synergies were similar for unperturbed and perturbed walking at a high walking speed (0.8 m/s). However, the number of similar muscle synergies between perturbed and unperturbed walking was significantly lower for low walking speeds (0.4 and 0.6 m/s). These results indicate that shared muscle synergies underlying perturbed and unperturbed walking are less present during slow walking compared to fast walking.

Highlights

Several studies support the conception that a small set of muscle synergies underlies walking [1,2,3]
Electromyography (EMG) data of a set of muscles can be decomposed into a smaller set of synergies by a dimensionality reduction algorithm such as the frequently used non-negative matrix factorization (NNMF)
The weight acceptance muscle synergy primarily consists of the activity of the gluteus maximus, which extends and abducts the hip, and is mostly active during the initial contact of the stance phase

Summary

Introduction

Several studies support the conception that a small set of muscle synergies underlies walking [1,2,3]. Four muscle synergies have been shown to sufficiently explain the activity of lower limb muscles during gait at a self-selected speed in healthy subjects. The characteristics of these muscle synergies are described by Clark et al [2] as follows. The weight acceptance muscle synergy primarily consists of the activity of the gluteus maximus, which extends and abducts the hip, and is mostly active during the initial contact of the stance phase. The propulsion muscle synergy mainly involves the calf muscles soleus and medial gastrocnemius and is mainly active during the late stance. The late swing muscle synergy primarily involves hamstrings during terminal swing and initial contact. Chvatal et al [3] investigated whether these similar synergies were used when

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