Abstract
This study presents a model-based sensitivity analysis of the strength of voluntary muscle contraction with respect to different patterns of motor unit loss. A motor unit pool model was implemented including simulation of a motor neuron pool, muscle force, and surface electromyogram (EMG) signals. Three different patterns of motor unit loss were simulated, including (1) motor unit loss restricted to the largest ones, (2) motor unit loss restricted to the smallest ones, and (3) motor unit loss without size restriction. The model outputs including muscle force amplitude, variability, and the resultant EMG-force relation were quantified under two different motor neuron firing strategies. It was found that motor unit loss restricted to the largest ones had the most dominant impact on muscle strength and significantly changed the EMG-force relation, while loss restricted to the smallest motor units had a pronounced effect on force variability. These findings provide valuable insight toward our understanding of the neurophysiological mechanisms underlying experimental observations of muscle strength, force control, and EMG-force relation in both normal and pathological conditions.
Highlights
Voluntary muscle activation is mainly controlled by motor unit recruitment and rate modulation
For the loss of 60% motor units restricted to the smallest ones, the maximum muscle force was only reduced from 5685 au to 4624 au when the motor units followed the “onion skin” firing strategy and reduced from 6133 au to 5090 au, when the motor units followed the reverse “onion skin” firing strategy
This study implemented a model to estimate the effect of different patterns of motor unit loss on muscle force strength, variability, and the EMG-force relation
Summary
Voluntary muscle activation is mainly controlled by motor unit recruitment and rate modulation. Previous studies have revealed various changes in motor unit properties of poststroke patients, such as loss of functional motor units [1,2,3,4,5,6], impaired motor unit control properties (reduced motor unit peak firing rates and compressed ranges of motor unit recruitment) [7,8,9,10], and altered motor unit morphological features [11,12,13] These factors significantly impair muscle force generation and the EMG-force relation. The slope of the EMG-force relation of the paretic first dorsal interosseous and biceps brachii muscles was reported to be significantly greater in hemiparetic stroke survivors compared with the contralateral or neurologically intact muscles [7, 14]
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