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

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: r_s=-0.508 to r_s=-0.415; DXA: r_s=-0.531 to r_s=-0.337), leg FM% (15%-70% MVF: r_s=-0.579 to r_s=-0.582), thigh FM% (15%-70% MVF: r_s=-0.614 to r_s=-0.620), and MED (15%-70% MVF: r_s=-0.581 to r_s=-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.