ABSTRACT
Previous studies assessing relationships between muscle mechanics and neural activity have concurrently assessed changes in fascicle length (FL) and neural activation with electromyography (EMG) with low spatial sampling from different muscle regions. We used a new ultrasound-transparent high-density EMG electrode (HDEMG-US) to assess changes in FL and motor unit (MU) firing, simultaneously, on the same region of interest. EMG signals and ultrasound images were recorded simultaneously from the tibialis anterior muscle, using a silicon matrix of 32 electrodes, while performing sustained and torque-varying isometric ankle dorsiflexion contractions, at diverse joint positions (0° and 30° plantar flexion) and torques (20% and 40% of maximum (MVC)). 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 60% (range: 31-85%) and 71% (range: 31-88%) of the variance in FL and torque, respectively. Cross-correlation lags revealed that the delay between CST-FL (~75ms) was considerably smaller than CST-torque (~150ms, p<0.001). These delays affected the interpretation of MU recruitment/de-recruitment thresholds, with FL showing consistent lengths for both recruitment and de-recruitment. This study is the first to demonstrate the feasibility of recording single-MU activity with HDEMG-US whilst simultaneously evaluating changes in FL, which provides new opportunities for more complex examinations of the interplay between fascicle dynamics and motor unit discharge rates under different contraction conditions.
KEY POINTS
- We used ultrasound-transparent high-density surface EMG (HDEMG-US) electrodes to examine motor unit firing properties and how this relates to changes in fascicle length, a novel method to better understand the interplay between neural activity and muscle mechanics
- For the first time, we showed that it is possible to identify multiple tibialis anterior motor units with HDEMG-US electrodes, revealing close relationships between fluctuations in discharge rate, fascicle length and dorsiflexion torque
- Delays between neural drive and muscle contraction as well as muscle shortening are reduced compared to the external torque, making this methodology more suitable for understanding motor unit recruitment strategies
Competing Interest Statement
The authors have declared no competing interest.