Electromyogram and force fluctuation during different linearly varying isometric motor tasks

Area of Science:

• Neuroscience
• Human Physiology
• Biomechanics

Background:

• Motor control systems rely on precise motor unit activation and de-activation strategies.
• Understanding deviations from mirror-like behavior is crucial for diagnosing motor control errors.
• Isometric contractions are standard tasks for studying muscle force regulation.

Purpose of the Study:

• To investigate if motor unit activation and de-activation strategies deviate from mirror-like behavior during isometric tasks.
• To determine if this deviation depends on maximum force or rate of force change.
• To assess the degree of error in the motor control system under varying conditions.

Main Methods:

• Surface electromyography (EMG) and force output were recorded from the first dorsal interosseous muscle in 12 male subjects.
• Subjects performed trapezoid isometric contractions at 50% and 100% maximal voluntary contraction (MVC) with varying rates of force change (6.7% and 13.3% MVC/s).
• Ten force steps at various % MVC were also recorded to analyze motor unit recruitment and de-recruitment.

Main Results:

• The relationships between EMG amplitude/frequency and % MVC were not overlapped when comparing sustained steps, up-going ramps, and down-going ramps.
• Force output error relative to target (% ERR) differed between trapezoid contractions and sustained steps below 20% MVC.
• Motor unit activation and de-activation strategies were found to be non-mirroring.

Conclusions:

• Motor unit activation and de-activation strategies are not symmetrical due to influences of maximum effort and rate of force change on recruitment/de-recruitment thresholds.
• The findings suggest potential mechanical or central nervous system hysteresis affecting force decrement control.
• This study highlights the complexity of motor control and force regulation during dynamic isometric contractions.