Merging of Healthy Motor Modules Predicts Reduced Locomotor Performance and Muscle Coordination Complexity Post-Stroke

Abstract

Evidence suggests that the nervous system controls motor tasks using a low-dimensional modular organization of muscle activation. However, it is not clear if such an organization applies to coordination of human walking, nor how nervous system injury may alter the organization of motor modules and their biomechanical outputs. We first tested the hypothesis that muscle activation patterns during walking are produced through the variable activation of a small set of motor modules. In 20 healthy control subjects, EMG signals from eight leg muscles were measured across a range of walking speeds. Four motor modules identified through nonnegative matrix factorization were sufficient to account for variability of muscle activation from step to step and across speeds. Next, consistent with the clinical notion of abnormal limb flexion-extension synergies post-stroke, we tested the hypothesis that subjects with post-stroke hemiparesis would have altered motor modules, leading to impaired walking performance. In post-stroke subjects (n = 55), a less complex coordination pattern was shown. Fewer modules were needed to account for muscle activation during walking at preferred speed compared with controls. Fewer modules resulted from merging of the modules observed in healthy controls, suggesting reduced independence of neural control signals. The number of modules was correlated to preferred walking speed, speed modulation, step length asymmetry, and propulsive asymmetry. Our results suggest a common modular organization of muscle coordination underlying walking in both healthy and post-stroke subjects. Identification of motor modules may lead to new insight into impaired locomotor coordination and the underlying neural systems.

Analysis of muscle synergies and activation–deactivation patterns in subjects with anterior cruciate ligament deficiency during walking

“…The differences in the tendency of SVs and NCs were measured by means of the Pearson's correlation coefficient (Clark et al, 2010). If this value was close to 1, it meant that the shape of the compared sets of data was similar.…”

“…The number of modules was fixed to 5 and the cross correlation of the NCs and SVs between modules was analyzed within each group as in (Clark et al, 2010). Table 1 shows that the correlation among modules within each group was overall low, which means that the SVs and the NCs of the modules were independent from each other.…”

“…It is believed that the number of synergies used by a human being when walking is between 4 and 6 (Allen and Neptune, 2012;Clark et al, 2010;De Groote et al, 2014;Ivanenko et al, 2005Ivanenko et al, , 2004Oliveira et al, 2014). The Variance Accounted For (VAF) between the reconstructed and the original signals is evaluated to select the proper number of modules to be used when factorizing the signals.…”

“…The Variance Accounted For (VAF) between the reconstructed and the original signals is evaluated to select the proper number of modules to be used when factorizing the signals. Most authors consider that a VAF>0.9 is the threshold to accept the reconstruction (Clark et al, 2010;Oliveira et al, 2014). It is reported that there are similarities in the muscle synergies when performing the same movement across subjects.…”

“…It is reported that there are similarities in the muscle synergies when performing the same movement across subjects. Several authors reported muscle synergies when walking (Clark et al, 2010;Dominici et al, 2011;Ivanenko et al, 2004;Neptune et al, 2009;Oliveira et al, 2014), walking with perturbations (Ivanenko et al, 2005) or performing other tasks (Rugy et al, 2013). Clark et al (2010) applied the muscle synergy analysis in post-stroke injured subjects.…”