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Exp Brain Res. 2016 Apr;234(4):985-95. doi: 10.1007/s00221-015-4527-y. Epub 2015 Dec 21.

Delayed grip relaxation and altered modulation of intracortical inhibition with aging.

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Department of Occupational Science and Technology, University of Wisconsin-Milwaukee, Milwaukee, WI, 53201-0413, USA.
Department of Sport and Exercise Science, University of Auckland, Auckland, St Johns, 1072, New Zealand.
Department of Public Health Sciences, Medical University of South Carolina, Charleston, SC, USA.
Division of Occupational Therapy, Department of Health Professions, Department of Health Sciences and Research, Medical University of South Carolina, 151B Rutledge Ave., Charleston, SC, USA.


Grip relaxation is a voluntary action that requires an increase in short-interval intracortical inhibition (SICI) in healthy young adults, rather than a simple termination of excitatory drive. The way aging affects this voluntary inhibitory action and timing of grip relaxation is currently unknown. The objective of this study was to examine aging-related delays in grip relaxation and SICI modulation for the flexor digitorum superficialis muscle during grip relaxation. The main finding was that young adults increased SICI to relax their grips, whereas older adults did not increase SICI with a prolonged grip relaxation time (p < 0.05 for both SICI modulation and grip relaxation time). A secondary experiment showed that both young and older adults did not change H reflex excitability during grip relaxation. Our data suggest that grip relaxation is mediated by increased cortical inhibitory output in young adults, and aging-related impairment in increasing cortical inhibitory output may hamper timely cessation of muscle activity. Our data also suggest a lesser role of the spinal circuits in grip muscle relaxation. This knowledge may contribute to understanding of aging-related movement deterioration and development of interventions for improving modulation of SICI to improve muscle relaxation and movement coordination.


Aging; Grip relaxation; H reflex; Hand; SICI; TMS

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