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Front. Neurol. | doi: 10.3389/fneur.2018.00795

Motor Overflow In Spastic Hemiplegia After Stroke: A Phenomenon Of Disinhibition?

  • 1University of Texas Health Science Center at Houston, United States

The phenomenon of exaggerated motor overflow is well documented in stroke survivors with spasticity. However, the mechanism underlying the abnormal motor overflow remains unclear. In this study, we aimed to investigate the possible mechanisms behind abnormal motor overflow and its possible relations with post-stroke spasticity. 11 stroke patients (63.6 yrs ± 6.4 yrs; 4 women) and 11 healthy subjects (31.18 yrs ± 6.18 yrs; 2 women) were recruited. All of them were asked to perform unilateral isometric elbow flexion at submaximal levels (10%, 30%, and 60% of maximum voluntary contraction). Electromyogram (EMG) was measured from the contracting biceps (iBiceps) muscle and resting contralateral biceps (cBiceps), ipsilateral flexor digitorum superficialis (iFDS), and contralateral FDS (cFDS) muscles. Motor overflow was quantified as the normalized EMG of the resting muscles. The severity of stroke was quantified through reflex torque (spasticity) and weakness. To investigate the possible cortical and subcortical origins of the abnormal motor overflow, EMG-EMG coherence was calculated between the contracting muscle and each of the resting muscles. During elbow flexion on the impaired side, stroke subjects exhibited significant higher motor overflow to the iFDS muscle compared with healthy subjects (ipsilateral motor overflow). Stroke subjects exhibited significantly higher motor overflow to the contralateral spastic muscles (cBiceps and cFDS) during elbow flexion on the non-impaired side (contralateral motor overflow), compared with healthy subjects. Moreover, there was significantly high EMG-EMG coherence in the alpha band (6-12Hz) between the contracting muscle and all other resting muscles during elbow flexion on the non-impaired side. Our results of diffuse ipsilateral and contralateral motor overflow with EMG-EMG coherence in the alpha band suggest subcortical origins of motor overflow. Furthermore, correlation between contralateral motor overflow to contralateral spastic elbow and finger flexors and their spasticity was consistently at moderate to high levels. Post-stroke spasticity is attributed to reticulospinal hyperexcitability secondary to the unmasking of ponto-medullary reticular formation. A high correlation between spasticity and motor overflow supports a potential role of reticulospinal activation in motor overflow. Collectively, these results suggest that diffuse motor overflow to the impaired side is likely related to activation of hyperexcitable reticulospinal pathways after stroke.

Keywords: motor overflow, Stroke, Spasticity, EMG-EMG coherence, Reticulospinal tract

Received: 12 Mar 2018; Accepted: 04 Sep 2018.

Edited by:

Yves VANDERMEEREN, Université Catholique de Louvain, Belgium

Reviewed by:

Bernhard Sehm, Max-Planck-Institut für Kognitions- und Neurowissenschaften, Germany
Michelle L. Harris-Love, George Mason University, United States
Thierry Deltombe, CHU Dinant Godinne UCL Namur, Belgium  

Copyright: © 2018 CHEN, Li, Magat, Zhou and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Sheng Li, University of Texas Health Science Center at Houston, Houston, United States, Sheng.Li@uth.tmc.edu