Neural excitation of muscle in rate of force development and function

Date
2018
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University of Delaware
Abstract
Movement velocity, and its isometric surrogate rate of force development (RFD), are associated with functional mobility. High levels of neural excitation are necessary for rapid movement to occur. Neural excitation, measured via surface electromyography (EMG), can be quantified using a variety of measures. Early feline experiments revealed a bilinear input:output relationship of the alpha motor neuron that had a primary firing rate range and a secondary firing rate range in response to injected current. Both ranges have been characterized as linear and the secondary range has a greater slope and firing rates that are characteristic of most-rapid contractions or movements. More recent literature showed a bilinear relationship between movement velocity and motor unit firing rate in humans. Older adults and people with Parkinson’s disease (PwPD) are two populations that experience decreased rates of force development and neural excitation. It may be the case that not all functional assessments used in these populations require high rates of neural excitation, especially those performed at preferred rather than fastest rates. PURPOSE: The purpose of this research was threefold: Aim 1) to compare candidate surface EMG measures of neural excitation, Aim 2) to determine the nature of the relationship between EMG measures and peak RFD, with a specific interest in potential bilinearity, and Aim 3) to describe and compare the rate of neural excitation during common dynamic assessments performed at increasing movement velocities in young adults, healthy older adults, and people with Parkinson’s disease. METHODS: EMG, isometric force and limb accelerations were recorded during isometric and dynamic movement performed at increasing speeds. The purpose of Aim 1 was to describe the relationship between peak rate of EMG rise and surface electromyogram outcome measures with the strongest measures then used in a comparison of linear and nonlinear models of the relationship between RFD and neural excitation. In the cross-sectional study involving dynamic movements (Aim 2 and 3), analysis of variance was used to test a hypothesized group-by-speed interaction on peak rate of EMG rise (RER) in two lower extremity and two upper extremity movements. The peak RER of the four movement conditions being performed as fast as possible was compared. RESULTS: Aim 1.1: Root mean square of the initial 75ms (RMS75) had the strongest relationship while peak RER had the strongest relationship for measures not reliant on EMG onset. Aim 1.2: For RMS75, a linear relationship was the best fit for 14/21 participants. A bilinear relationship was the best fit for 7/21 participants. Neither Log-log nor exponential was best fit for any participants. For Peak RER, a linear relationship was the best fit for 6/21 participants. A bilinear relationship was the best fit for 13/21 participants. Log-log was not the best fit for any participants. An exponential relationship was the best fit for 2/21 participants. Aim 2: Only recumbent cycling revealed a group by speed interaction on peak RER and this finding was consistent in vastus lateralis, soleus, and tibialis anterior muscles. Elbow extension and 4-meter walk revealed group-by-speed interactions for peak accelerations. Comparing NE during the AFAP condition revealed a group by condition interaction with all groups having differences in peak RER across the movements. In the upper extremity, no group had any significance in RER differences between weighted arm curl and elbow extension. In the lower extremity, YA and PD had significant differences in RER between walking and low-resistance, high velocity bicycling. CONCLUSION: Neural excitation is a key determinant of rapid isometric contractions and rapid movement. These findings present evidence to suggest that the known bilinearity of motor unit firing rates is observable in surface EMG measures. Results also provide guidance on the specific movements and rate conditions that are suitable for understanding the role of neural excitation in function and mobility. Assessing movement at a rapid rather than preferred pace will elicit the high levels of neural excitation often associated with functional independence.
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