Concurrent assessment of motor unit firing properties and fascicle length changes with high-density surface electromyography ultrasound-transparent electrodes

Previous studies assessing relationships between muscle mechanics and neural activity have concurrently assessed changes in fascicle length (FL) and neural activation with electromyography (EMG) approaches with low spatial sampling from different muscle regions. This project aimed to assess changes in FL and motor unit (MU) firing, simultaneously, on the same region of interest, with high-density EMG electrodes transparent to ultrasound (HDEMG-US). EMG signals and ultrasound images were recorded simultaneously from the tibialis anterior of 10 participants, using a silicon matrix of 32 electrodes, while performing sustained-isometric ankle-dorsiflexion contractions, at diverse joint positions (0{degrees} and 30{degrees} plantar flexion) and torques (20% and 40% of maximum). EMG signals were decomposed into individual MUs and changes in FL were assessed with a fascicle-tracking algorithm. MU firing data was converted into a cumulative spike train (CST) that was cross-correlated with dorsiflexion torque (CST-torque) and FL (CST-FL). On average, 7 (3) MUs were identified across contractions. Cross-correlations showed that CST could explain on average 60% (range: 31-85%) and 71% (range: 0.31-0.88) of the variance in FL and torque, respectively. Cross-correlation lags revealed that the delay between CST-FL (~75ms) was smaller than CST-torque (~150ms, p<0.001). Finally, we could observe that both delays increased by ~20ms at 30{degrees} of plantar flexion (p<0.05). This study is the first to demonstrate the feasibility of recording single MU activity with HDEMG-US whilst simultaneously evaluating changes in FL. The findings show a close relationship between dorsiflexion torque, FL and CST, also allowing quantification of the delay between each of these signals.