Abstract
Different neurophysiological strategies are used to perform angle adjustments during motor tasks such as car driving and force-control tasks using a fixed-rigid pedal. However, the difference in motor unit behavior in response to an increasing exerted force between tasks is unknown. This study aimed to investigate the difference in motor unit responsiveness on increasing force between force and position tasks. Twelve healthy participants performed ramp and hold contractions during ankle plantarflexion at 20% and 30% of the maximal voluntary contraction using a rigid pedal (force task) and a free pedal with an inertial load (position task). High-density surface electromyograms were recorded of the medial gastrocnemius muscle and decomposed into individual motor unit firing patterns. Ninety and hundred and nine motor units could be tracked between different target torques in each task. The mean firing rate increased and firing rate variability decreased on 10% maximal voluntary contraction force gain during both force and position tasks. There were no significant differences in these responses between the two tasks. Our results suggest that the motor unit firing rate is similarly regulated between force and position tasks in the medial gastrocnemius muscle with an increase in the exerted force.NEW & NOTEWORTHY Different neurophysiological strategies are used to perform a force control task and angle adjustment task. Our results showed that motor unit firing rate is similarly regulated between the two tasks in the medial gastrocnemius muscle with an increase in the exerted force. Although it is reported that position tasks contribute to early fatigue, it does not seem to be a particular problem for the increase in force.
Highlights
One of the major causes of road accidents is excessive speed, and speed variation among drivers is a factor affecting traffic congestion [1, 2]
Our results showed that motor unit firing rate is regulated between the two tasks in the medial gastrocnemius muscle with an increase in the exerted force
We hypothesized that the change in motor unit (MU) firing rate variability with increasing muscle force would be larger in the position than force task
Summary
One of the major causes of road accidents is excessive speed, and speed variation among drivers is a factor affecting traffic congestion [1, 2]. It is necessary to exert an appropriate force and adjust the ankle angle on a pedal to maintain the required driving speed. This motor task is similar to the position task used in laboratory settings to investigate motor control mechanisms. The MU firing rate increases with an increasing muscle force, but its variability decreases and shows similar behavior to the force fluctuation during isometric contraction [19]. The greater rate of increase in the rating of perceived exertion, the force fluctuations, and firing rate variability are observed during the fatigue condition in the position task, indicating that the augmentation of the descending drive is more likely to occur [8]. We hypothesized that the change in MU firing rate variability with increasing muscle force would be larger in the position than force task
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