Motor unit synchronization during fatigue: described with a novel sEMG method based on large motor unit samples

Area of Science:

• * Neuroscience
• * Human Physiology
• * Biomedical Engineering

Background:

• * Motor unit (MU) synchronization changes with fatigue are inconsistently reported, potentially due to methodological limitations and muscle activation variability.
• * Existing methods for assessing MU synchronization and its relationship with force tremor have limitations, particularly with surface electromyography (EMG).

Purpose of the Study:

• * To develop and validate a novel surface EMG descriptor for quantifying MU synchronization across large MU populations.
• * To investigate the effects of fatigue on MU synchronization at different muscle sites.
• * To examine the relationship between MU synchronization and force tremor during sustained isometric contractions.

Main Methods:

• * Twenty-four participants performed isometric elbow flexion at 25% maximal voluntary contraction until exhaustion.
• * Monopolar EMG signals were recorded from the biceps brachii using a high-density electrode grid (130 electrodes).
• * MU synchronization was assessed using sub-band skewness of EMG signals; force tremor was quantified by the coefficient of variation.

Main Results:

• * The novel synchronization descriptor showed dependency on the electrode recording site.
• * Both MU synchronization and force tremor increased significantly with fatigue.
• * A general association was observed between increased MU synchronization and force tremor, though their fluctuations were not directly correlated.

Conclusions:

• * The novel surface EMG descriptor effectively quantifies MU synchronization in large MU populations during fatiguing contractions.
• * Fatigue induces increased MU synchronization and force tremor in the biceps brachii.
• * This method provides valuable insights into MU synchronization during fatigue, complementing or exceeding the capabilities of intramuscular EMG.