Partial weight support differentially affects corticomotor excitability across muscles of the upper limb

• Type of Material: Article
• Authors: Keith D. Runnalls, University of Auckland; Greg Anson, University of Auckland; Steven Wolf, Emory University; Winston D. Byblow, University of Auckland
• Language: English
• Date: 2014-12-11
• Publisher: Wiley Open Access
• Publication Version: Final Published Version
• Copyright Statement: © 2014 The Authors
• License: Creative Commons Attribution 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.14814/phy2.12183
• Title of Journal or Parent Work: Physiological Reports
• ISSN: 2051-817X
• Volume: 2
• Issue: 12
• Start Page: e12183
• End Page: e12183
• Grant/Funding Information: This research was supported by a University of Auckland FRDF grant to WB.; KR was supported by a University of Auckland Doctoral Scholarship.
• Keywords: Gravity compensation; motor cortex; integrated control; intracortical inhibition; transcranial magnetic stimulation
• Research Categories: Biology, Neuroscience; Biology, Physiology

Abstract

Partial weight support may hold promise as a therapeutic adjuvant during rehabilitation after stroke by providing a permissive environment for reducing the expression of abnormal muscle synergies that cause upper limb impairment. We explored the neurophysiological effects of upper limb weight support in 13 healthy young adults by measuring motor-evoked potentials (MEPs) from transcranial magnetic stimulation (TMS) of primary motor cortex and electromyography from anterior deltoid (AD), biceps brachii (BB), extensor carpi radialis (ECR), and first dorsal interosseous (FDI). Five levels of weight support, varying from none to full, were provided to the arm using a commercial device (Saebo Mobile Arm Support). For each level of support, stimulus–response (SR) curves were derived from MEPs across a range of TMS intensities. Weight support affected background EMG activity in each of the four muscles examined (P < 0.0001 for each muscle). Tonic background activity was primarily reduced in the AD. Weight support had a differential effect on the size of MEPs across muscles. After curve fitting, the SR plateau for ECR increased at the lowest support level (P = 0.004). For FDI, the SR plateau increased at the highest support level (P = 0.0003). These results indicate that weight support of the proximal upper limb modulates corticomotor excitability across the forearm and hand. The findings support a model of integrated control of the upper limb and may inform the use of weight support in clinical settings.