Limitations of the surface electromyography technique for estimating motor unit synchronization

Area of Science:

• Neuroscience
• Human Physiology
• Biomedical Engineering

Background:

• Motor unit synchronization plays a crucial role in force production and fine motor control.
• Accurate estimation of motor unit synchronization is essential for understanding neuromuscular function.

Purpose of the Study:

• To evaluate the validity of a simplified surface electromyography (EMG) technique for estimating motor unit synchronization.
• To investigate the influence of motor unit discharge rate and muscle activity level on surface-EMG-based synchronization estimates.

Main Methods:

• Surface EMG signals from the first dorsal interosseus muscle were recorded in human volunteers.
• Intramuscular recordings identified single motor units to create spike-triggered averages of the surface EMG.
• Computer simulations were used to model surface-EMG signals with controlled motor unit activity and no inherent synchrony.

Main Results:

• The surface-EMG estimate of motor unit synchronization was significantly higher at high reference motor unit discharge rates compared to low rates.
• A significant correlation was found between the calculated synchronization ratio and the overall level of EMG activity.
• Computer simulations revealed that signal rectification artifacts and reduced signal-to-noise ratio influenced the surface-EMG index, particularly with increased motor unit activity.

Conclusions:

• The simplified surface-EMG method is not a reliable index for motor unit synchronization.
• Methodological artifacts, including rectification and signal-to-noise ratio fluctuations, confound the estimation of motor unit synchronization.
• The technique's sensitivity decreases with increased muscle activity, limiting its utility in dynamic conditions.