Does normalization of voluntary EMG amplitude to M_MAX account for the influence of electrode location and adiposity?

Voluntary surface electromyography (sEMG) amplitude is known to be influenced by both electrode position and subcutaneous adipose tissue thickness, and these factors likely compromise both between- and within-individual comparisons. Normalization of voluntary sEMG amplitude to evoked maximum M-wave parameters (M_MAX peak-to-peak [P-P] and Area) may remove the influence of electrode position and subcutaneous tissue thickness. The purpose of this study was to: (a) assess the influence of electrode position on voluntary, evoked (M_MAX P-P and Area), and normalized sEMG measurements across the surface of the vastus lateralis (VL; experiment 1: n = 10); and (b) investigate if M_MAX normalization removes the confounding influence of subcutaneous tissue thickness [muscle-electrode distance (MED) from ultrasound imaging] on sEMG amplitude (experiment 2; n = 41). Healthy young men performed maximum voluntary contractions (MVCs) and evoked twitch contractions during both experiments. Experiment 1: voluntary sEMG during MVCs was influenced by electrode location (P ≤ 0.046, ES≥1.49 "large"), but when normalized to M_MAX P-P showed no differences between VL sites (P = 0.929) which was not the case when normalized to M_MAX Area (P < 0.004). Experiment 2: voluntary sEMG amplitude was related to MED, which explained 31%-38% of the variance. Normalization of voluntary sEMG amplitude to M_MAX P-P or M_MAX Area reduced but did not consistently remove the influence of MED which still explained up to 16% (M_MAX P-P) and 23% (M_MAX Area) of the variance. In conclusion, M_MAX P-P was the better normalization parameter for removing the influence of electrode location and substantially reduced but did not consistently remove the influence of subcutaneous adiposity.