Interactions between motor unit firing and fascicle dynamics during shortening and lengthening contractions

The integration of high-density surface electromyography with ultrasound-transparent electrodes (HDsEMG-US) and B-mode ultrasonography enables concurrent assessment of motor unit (MU) firing properties and fascicle length (FL) from the same muscle region. Although the isometric relations between MU discharge, FL, and torque have been characterized, their interactions during anisometric contractions remain poorly understood. This study examined MU discharge behavior and its relation with FL changes during shortening and lengthening contractions of the tibialis anterior muscle. Ten healthy males performed isokinetic dorsiflexion contractions at a constant angular velocity of 2°/s while maintaining a constant submaximal torque of 25% maximum voluntary contraction torque, measured at a long muscle-tendon length (∼30° of plantarflexion), with HDsEMG-US signals and ultrasound images recorded simultaneously. MUs were identified using blind-source separation decomposition and FL was tracked with an optical flow algorithm. Cross-correlation analysis revealed high associations between cumulative spike train (CST), FL, and torque in both contraction phases. Neuromechanical delays were significantly longer during lengthening than shortening (phase effect: F_1,7 = 8.51, P = 0.023; mean difference: -25.04 ms, 95% CI: -45.35 to -4.74). Discharge rate changed at a similar rate with variations in FL during both shortening and lengthening contractions (similar discharge rate-FL slopes, P = 0.44); however, absolute discharge rates were systematically lower during lengthening (difference in intercepts: P < 0.0001). These findings demonstrate the feasibility of HDsEMG-US for characterizing neuromechanical coupling during dynamic contractions and provide new evidence of neural drive regulation during shortening and lengthening muscle actions.NEW & NOTEWORTHY Using ultrasound-transparent high-density surface EMG, we simultaneously measured motor unit discharge and tibialis anterior fascicle length during shortening and lengthening contractions. Discharge rate was linearly related to fascicle length changes during both contraction types. However, when considering the same level of fascicle shortening between contractions, discharge rates were systematically lower during lengthening, and neuromechanical delays were prolonged, consistent with length-dependent adjustments in contractile properties. These findings provide new evidence of neuromechanical coupling during anisometric contractions.