Corticospinal excitability during motor preparation of upper extremity reaches reflects flexor muscle synergies: A novel principal component-based motor evoked potential analyses

Article type: Research Article

Authors: Augenstein, Thomas E.^{a; b} | Oh, Seonga^a | Norris, Trevor A.^a | Mekler, Joshua^d | Sethi, Amit^e | Krishnan, Chandramouli^{a; b; c; *}

Affiliations: [a] Department of Physical Medicine and Rehabilitation, NeuRRo Lab, Michigan Medicine, Ann Arbor, MI, USA | [b] Department of Robotics, University of Michigan, Ann Arbor, MI, USA | [c] Physical Medicine and Rehabilitation, Michigan Medicine, Ann Arbor, MI, USA | [d] Neurology, Michigan Medicine, Ann Arbor, MI, USA | [e] Department of Occupational Therapy, University of Pittsburgh, Pittsburgh, PA, USA

Correspondence: [*] Corresponding author: Chandramouli Krishnan, Director, Department of Physical Medicine and Rehabilitation, Neuromuscular & Rehabilitation Robotics Laboratory (NeuRRo Lab), University of Michigan, 325 E Eisenhower Parkway (Suite 3013), Ann Arbor, MI –48108, USA. Phone: +1 319 321 0117; Fax: +1 734 615 1770; E-mail: [email protected].

Abstract: Background: Previous research has shown that noninvasive brain stimulation can be used to study how the central nervous system (CNS) prepares the execution of a motor task. However, these previous studies have been limited to a single muscle or single degree of freedom movements (e.g., wrist flexion). It is currently unclear if the findings of these studies generalize to multi-joint movements involving multiple muscles, which may be influenced by kinematic redundancy and muscle synergies. Objective: The objective of this study was to characterize corticospinal excitability during motor preparation in the cortex prior to functional upper extremity reaches. Methods: 20 participants without neurological impairments volunteered for this study. During the experiment, the participants reached for a cup in response to a visual “Go Cue”. Prior to movement onset, we used transcranial magnetic stimulation (TMS) to stimulate the motor cortex and measured the changes in motor evoked potentials (MEPs) in several upper extremity muscles. We varied each participant’s initial arm posture and used a novel synergy-based MEP analysis to examine the effect of muscle coordination on MEPs. Additionally, we varied the timing of the stimulation between the Go Cue and movement onset to examine the time course of motor preparation. Results: We found that synergies with strong proximal muscle (shoulder and elbow) components emerged as the stimulation was delivered closer to movement onset, regardless of arm posture, but MEPs in the distal (wrist and finger) muscles were not facilitated. We also found that synergies varied with arm posture in a manner that reflected the muscle coordination of the reach. Conclusions: We believe that these findings provide useful insight into the way the CNS plans motor skills.

Keywords: Motor control, muscle synergy, movement planning, cortical excitability, stroke

DOI: 10.3233/RNN-231367

Journal: Restorative Neurology and Neuroscience, vol. 42, no. 2, pp. 121-138, 2024