Modulations in motor unit discharge are related to changes in fascicle length during isometric contractions

The integration of electromyography (EMG) and ultrasound imaging has provided important information about the mechanisms
of muscle activation and contraction. Unfortunately, conventional bipolar EMG does not allow an accurate assessment of the
interplay between the neural drive received by muscles, changes in fascicle length and torque. We aimed to assess the relationship
between modulations in tibialis anterior muscle (TA) motor unit (MU) discharge, fascicle length, and dorsiflexion torque using
ultrasound-transparent high-density EMG electrodes. EMG and ultrasound images were recorded simultaneously from TA using a
32-electrode silicon matrix while performing isometric dorsiflexion contractions at two ankle joint positions (0 or 30 plantar flexion)
and torques (20% or 40% of maximum). EMG signals were decomposed into MUs and changes in fascicle length were
assessed with a fascicle-tracking algorithm. MU firings were converted into a cumulative spike train (CST) that was cross-correlated
with torque (CST-torque) and fascicle length (CST-length). High cross-correlations were found for CST-length (0.60, range:
0.31–0.85) and CST-torque (0.71, range: 0.31–0.88). Cross-correlation delays revealed that the delay between CST-fascicle length
(75 ms) was smaller than CST-torque (150 ms, P < 0.001). These delays affected MU recruitment and de-recruitment thresholds
since the fascicle length at which MUs were recruited and de-recruited was similar but MU recruitment-de-recruitment torque
varied. This study demonstrates that changes in TA fascicle length are related to modulations in MU firing and dorsiflexion
torque. These relationships allow assessment of the interplay between neural drive, muscle contraction and torque, enabling the
time required to convert neural activity into movement to be quantified.