Improved motor unit number estimate when motor unit alternation is addressed

Area of Science:

• Neuroscience
• Motor Control
• Biomedical Engineering

Background:

• Motor unit number estimation (MUNE) is crucial for assessing motoneuron survival in aging and neurological conditions like ALS.
• Traditional MUNE methods are limited by motor unit alternation, where varying axon thresholds lead to inconsistent motor unit recruitment, inflating estimates.
• Existing methods to minimize alternation have limitations, potentially still overestimating motor unit counts.

Purpose of the Study:

• To systematically quantify motor unit alternation in the thenar muscles of healthy adults.
• To compare MUNE results with and without accounting for motor unit alternation.
• To assess the impact of addressing alternation on MUNE reliability and precision.

Main Methods:

• Digital subtraction of electromyography (EMG) and force signals was employed to identify and quantify motor unit alternation.
• MUNE was calculated both with and without accounting for identified alternation.
• The study included 35 healthy adult participants, analyzing thenar muscle activity.

Main Results:

• Motor unit alternation was highly prevalent, occurring in 34 of 35 muscles within the first 7-11 EMG steps.
• Alternation led to an underestimation of the number of motor unit recruitment steps compared to the actual number of motor units.
• Accounting for alternation via digital subtraction reduced MUNE by up to 50% and significantly decreased day-to-day and between-participant variability.

Conclusions:

• Motor unit alternation is a significant factor that systematically inflates MUNE, necessitating its quantification.
• Digital subtraction provides a reliable method to correct for alternation, improving MUNE precision.
• Accurate MUNE, accounting for alternation, enhances the ability to detect subtle changes in motoneuron survival relevant to aging and disease.