# Greater muscle electrode distance and fat mass affect motor units identification from high-density surface EMG in the vastus lateralis muscle

## TL;DR

This study shows that higher fat mass and greater muscle-electrode distance reduce the number of motor units identified from high-density surface EMG in the vastus lateralis muscle.

## Contribution

The study identifies a threshold for muscle-electrode distance and quantifies the impact of fat mass on motor unit identification from HDsEMG.

## Key findings

- Higher fat mass and muscle-electrode distance are significantly correlated with fewer identified motor units.
- A muscle-electrode distance threshold of ~0.7 cm was found to influence motor unit identification.
- Females had greater muscle-electrode distance and fat mass, leading to fewer identified motor units compared to males.

## Abstract

Decomposing high-density surface electromyography (HDsEMG) signals enables non-invasive identification of motor units (MU); however, anatomical factors contribute to variability in the identified MU number across individuals. This study examined the influence of fat mass (FM%), muscle-electrode distance (MED), and muscle size on MU identification in the vastus lateralis. Thirty-three young (19–30 years) and twenty-eight older adults (66–82 years), including both sexes, performed isometric knee extensions at 15%, 35%, 50%, and 70% of maximal voluntary force (MVF) while HDsEMG signals were recorded. Whole-body and regional FM% were assessed using bioelectrical impedance analysis (BIA) and dual-energy X-ray absorptiometry (DXA), while MED and muscle size were measured with ultrasonography. Females exhibited greater MED and FM%, resulting in fewer identified MU than males. Significant negative correlations were found between the identified MU number and whole-body FM% (15%-70% MVF: BIA: rs=-0.508 to rs=-0.415; DXA: rs=-0.531 to rs=-0.337), leg FM% (15%-70% MVF: rs=-0.579 to rs=-0.582), thigh FM% (15%-70% MVF: rs=-0.614 to rs=-0.620), and MED (15%-70% MVF: rs=-0.581 to rs=-0.670). Notably, segmented regression analysis revealed a threshold at ~ 0.7 cm MED, below which a greater number of MU could be identified. Our findings highlight the negative impact of high FM% and MED on MU identification from HDsEMG decomposition.