Muscle Synergies: Neural Mechanisms and Changes after Stroke

Abstract

Human movement requires the coordination of multiple muscles at the same time. The characterisation of these patterns of muscle activity is essential to understand the neural strategies underlying the generation of movement. A modular motor control strategy known as muscle synergies (MSs) has been proposed as the building blocks of human movement. MSs are fundamentally invariant patterns of activation across multiple muscles that are linearly combined to produce complex muscle activation patterns. The main objective of my research was to examine the underlying neurophysiological mechanisms associated with MSs, and changes to MSs that occur after stroke. For the experimental component of my thesis, I first looked at the neurophysiological basis of upper extremity MSs. To study the underlying mechanisms associated with MSs, I proposed a novel method to study properties of individual synergies based on the preferred direction of a MSs. I applied this novel method, first, to study the influence of muscle fatigue over the expression of MSs. I concluded that MSs are adapted to fatigue by modulations of their activation coefficients while conserving their structure. Then, I examined the functional connectivity between the cortex and MSs’ muscles, and between pairs of MSs’ muscles based on their structural weights. I found that muscles with higher weight seem to have a stronger functional connectivity, sharing information probably descending from subcortical structures. Both the fatigue and functional connectivity experiments suggest an underlying neural strategy to generate MSs. Then in the second part of my experimental work, I explored the changes of MSs after a stroke, first, by studying the influence of different levels of arm weight support over synergy expression. Finding that WS can upregulate synergy expression after stroke. Finally, I studied the relationship between motor impairment and synergy expression, and found that they are correlated, but also the impact of methodological choices on synergy expression. Together these studies align with the idea of a consistent, integrated mechanism reflecting the emergence of MSs from interneuronalnetworks, modulated by the motor cortex; providing evidence of synergies as a neurally derived strategy to control movement.