Sex-related differences in motor unit yield, subcutaneous tissue thickness, and maximal force

Area of Science:

• Neurophysiology
• Biomechanics
• Human Physiology

Background:

• Sex-based disparities in neurophysiology are documented, with males typically showing greater maximal strength and motor unit yield.
• Subcutaneous tissue thickness differences between sexes may impact electromyography (EMG) signal quality and motor unit yield.
• Previous research often attributes lower motor unit yield in females to increased subcutaneous fat.

Purpose of the Study:

• To investigate sex-related differences in motor unit number estimation (MUNE) and subcutaneous tissue thickness.
• To explore the relationship between MUNE, subcutaneous tissue thickness, and maximal voluntary contraction (MVC) force.
• To clarify the influence of sex and tissue depth on EMG-based motor unit analysis.

Main Methods:

• Surface electromyography (sEMG) and ultrasound were used to measure motor unit yield and subcutaneous tissue thickness in the tibialis anterior muscle.
• Participants (16 females, 15 males) performed isometric dorsiflexion at various submaximal and maximal intensities.
• Analysis focused on comparing MUNE, subcutaneous thickness, and MVC force between sexes and examining their correlations.

Main Results:

• No significant sex differences were found in the number of identified motor units across all contraction intensities.
• Males exhibited significantly higher maximal voluntary contraction force compared to females.
• Females demonstrated significantly greater subcutaneous tissue thickness than males.
• No significant correlations were observed between subcutaneous tissue thickness and motor unit yield, nor between MVC force and motor unit yield.

Conclusions:

• Contrary to some assumptions, this study did not find sex differences in motor unit yield despite observed differences in maximal strength and subcutaneous tissue thickness.
• Subcutaneous tissue thickness and maximal voluntary contraction force do not appear to be limiting factors for motor unit yield in this cohort.
• Further research is needed to identify the factors contributing to documented sex-based neurophysiological differences beyond peripheral measures.