Motor unit control mechanisms during a sinusoidal force-matching task
Date
2022
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Background: A motor unit is a single alpha motor neuron and all of the muscle fibers that it innervates. The nervous system modulates force through two mechanisms: recruitment (the activation or deactivation of motor units) or rate coding (the modulation of a motor unit’s firing rate). Motor units typically exhibit two types of firing rate behavior, tonic or phasic, depending on task parameters and the motor unit’s recruitment threshold force. Reported motor unit firing rates have ranged from 8 pulses per second (pps), to 120 pps, and in rare cases, up to 200-300pps. High firing rates above those seen during maximal voluntary contractions signify the motor unit is firing in its secondary range, which is typically elicited during ballistic contractions. A gap exists in motor unit literature to describe motor unit discharge behavior during movements that fall in the transition area from primary to secondary range behavior. This range may be important for activities of daily living that require submaximal forces that are modulated with intermediate rates of change. Purpose: The purpose of this thesis was to examine motor unit rate coding mechanisms during submaximal force conditions with oscillating rates of change by examining isometric contractions with varying frequencies (0.3, 0.9, 1.5 Hz) around the 20% MVC force level. The first aim was to determine the effects of task frequency on various rate coding parameters. The second aim was to compare the range of firing rate modulation between motor units displaying tonic or phasic discharge behavior, and it was hypothesized that phasic motor units would utilize a greater range of firing rates than tonic motor units. The third aim was to test the feasibility of observing secondary range behavior in a new 1.5Hz sinusoidal force-matching condition. It was hypothesized that the 1.5Hz condition would result in increased peak rates of force development (RFD) compared to the 0.3 and 0.9 Hz conditions, and that evidence of secondary range behavior would be revealed through a quadratic relationship between peak firing rates and the associated peak RFD from data across all conditions. Results: A total of 18 healthy, young adults participated in this study, providing a total of 418 motor unit observations. Minimum firing rate and mean firing rate increased significantly with increases in task frequency. The first 3 interspike intervals (time between consecutive firings of a motor unit) and number of firings per sinusoidal cycle or phasic burst of action potentials decreased significantly as task frequency increased. In the 0.3Hz condition, there was no significant difference between discharge behavior type and firing rate modulation. In the 0.9Hz condition, firing rate modulation was significantly greater for motor units exhibiting phasic discharge behavior compared to tonic. In the 1.5Hz condition, firing rate modulation was significantly greater for motor units exhibiting tonic discharge behavior compared to phasic. Peak rates of force development increased significantly with increasing task frequency. However, linear and quadratic models revealed that peak rates of force development only explained less than 4% of the variance in peak firing rates. Instead, firing rates indicative of secondary range behavior were seen during rapid force corrections throughout all conditions. Conclusion: These findings help fill the gap in our understanding of motor unit rate coding mechanisms during submaximal tasks with intermediate rates of change, and provide information on phasic discharge behavior, which is seldom reported on in the motor unit literature. In addition, the role of secondary range behavior during rapid motor corrections was revealed.
Description
Keywords
Motor unit control, Sinusoidal force-matching