Understanding corticomotor mechanisms for activation of non-target muscles during unilateral isometric contractions of leg muscles after stroke

Abstract

Purpose

Muscle activation often occurs in muscles ipsilateral to a voluntarily activated muscle and to a greater extent after stroke. In this study, we measured muscle activation in non-target, ipsilateral leg muscles and used transcranial magnetic stimulation (TMS) to provide insight into whether corticomotor pathways contribute to involuntary activation.

Materials and methods

Individuals with stroke performed unilateral isometric ankle dorsiflexion, ankle plantarflexion, knee extension, and knee flexion. To quantify involuntary muscle activation in non-target muscles, muscle activation was measured during contractions from the ipsilateral tibialis anterior (TA), medial gastrocnemius (MG), rectus femoris (RF), and biceps femoris (BF) and normalized to resting muscle activity. To provide insight into mechanisms of involuntary non-target muscle activation, TMS was applied to the contralateral hemisphere, and motor evoked potentials (MEPs) were recorded.

Results

We found significant muscle activation in nearly every non-target muscle during isometric unilateral contractions. MEPs were frequently observed in non-target muscles, but greater non-target MEP amplitude was not associated with greater non-target muscle activation.

Conclusions

Our results suggest that non-target muscle activation occurs frequently in individuals with chronic stroke. The lack of association between non-target TMS responses and non-target muscle activation suggests that non-target muscle activation may have a subcortical or spinal origin. Non-target muscle activation has important clinical implications because it may impair torque production, out-of-synergy movement, and muscle activation timing.

Keywords:

• Stroke
• electromyography
• motor activity
• lower extremity
• transcranial magnetic stimulation

Author contributions

Brice Cleland: conceptualization, formal analysis, funding acquisition, investigation, methodology, validation, visualization, writing-original draft, writing – review & editing. Matt Giffhorn: methodology, investigation, writing -review & editing. Arun Jayaraman: methodology, project administration, resources, supervision, funding acquisition, writing – review & editing. Sangeetha Madhavan: funding acquisition, methodology, project administration, resources, supervision, writing – review & editing.

Disclosure statement

No potential conflict of interest was reported by the author(s)

Data availability statement

The data that support the findings of this study are openly available at the following link: https://drive.google.com/drive/folders/1im7TBb4ehRGUd_zDjnHERKjRzDLy9SeX?usp=sharing.

Additional information

Funding

This work was funded by Eunice Kennedy Shriver National Institute of Child Health and Human Development. This project was partially funded by Samsung Electronics and the Max Nader Center for Rehabilitation Technologies and Outcomes Research (AJ, MG). This work was also partially supported by the National Institutes of Health (SM, BC) [F32HD102214 & R01HD075777]. Study sponsors had no role in study design, data collection, data analysis, data interpretation, writing the manuscript, or the decision to submit this manuscript for publication.