Characterizing sEMG Feature Errors in Lower Limb Muscles Due to Electrode Location and Skin–Electrode Interface Changes

Surface electromyography (sEMG) is popular for monitoring muscle activation. However, it remains underutilized in clinical settings, such as stroke rehabilitation. This is largely due to intersession errors from variabilities in electrode location, the skin-electrode interface, and other intersession effects. These errors obscure the ability to attribute sEMG signal changes to physiological patient changes. In this study, we quantified different sources of error by applying high-density sEMG (HDsEMG) arrays on the gastrocnemius medialis (GM), tibialis anterior (TA), semitendinosus (ST), and tensor fascia latae (TFL) of 12 healthy participants performing isometric and dynamic exercises common in stroke assessments. Between exercise sets (sessions), we shifted and reapplied HDsEMG arrays to analyze three error conditions: 1) the same session errors (SSEs) arising from changes in the recording electrode and electrode location; 2) the same location errors (SLEs) from changes in recording electrodes at the same location across sessions; and 3) the same electrode errors (SEEs) from comparing the same recording electrode at a different location across sessions. For SSE, frequency-domain features (mean, median, and peak frequency) showed <30% error at <10 cm between electrodes. Time-domain features [integrated EMG (iEMG) and max envelope amplitude] exhibited greater errors, with the tensor fasciae latae exhibiting >50% error at 10 cm between electrodes. Contrastingly, we saw <19% error across frequency- and time-domain features for SLE and SEE after correcting for differences in electrode location using SSE trends. These results show that differences in electrode location were most responsible for sEMG feature errors. Thus, accounting for electrode location could improve intersession sEMG feature comparisons, enhancing the viability of sEMG-informed physiological assessments.