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

Article


Martinez-Valdes, Eduardo, Negro, Francesco, Botter, Alberto, Pincheira, Patricio A., Cerone, Giacinto Luigi, Falla, Deborah, Lichtwark, Glen A. and Cresswell, Andrew G.. 2022. "Modulations in motor unit discharge are related to changes in fascicle length during isometric contractions." Journal of Applied Physiology. 133 (5), pp. 1136-1148. https://doi.org/10.1152/japplphysiol.00758.2021
Article Title

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

ERA Journal ID3161
Article CategoryArticle
AuthorsMartinez-Valdes, Eduardo, Negro, Francesco, Botter, Alberto, Pincheira, Patricio A., Cerone, Giacinto Luigi, Falla, Deborah, Lichtwark, Glen A. and Cresswell, Andrew G.
Journal TitleJournal of Applied Physiology
Journal Citation133 (5), pp. 1136-1148
Number of Pages13
Year2022
PublisherAmerican Physiological Society
Place of PublicationUnited States
ISSN1522-1601
8750-7587
Digital Object Identifier (DOI)https://doi.org/10.1152/japplphysiol.00758.2021
Web Address (URL)https://journals.physiology.org/doi/full/10.1152/japplphysiol.00758.2021
Abstract

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.

NEW & NOTEWORTHY By employing ultrasound-transparent high-density EMG electrodes, we show that modulations in tibialis anterior muscle motor unit discharge rate were related to both changes in fascicle length and resultant torque. These relationships permitted the quantification of the relative delays between fluctuations in neural drive, muscle contraction, and transfer of torque via the tendon during sustained isometric dorsiflexion contractions, providing information on the conversion of neural activity into muscle force during a contraction.

KeywordsEMG; motor unit; muscle contraction; neuromechanics; ultrasound
Contains Sensitive ContentDoes not contain sensitive content
ANZSRC Field of Research 2020420703. Motor control
420702. Exercise physiology
Byline AffiliationsUniversity of Birmingham, United Kingdom
University of Brescia, Italy
Polytechnic University of Turin, Italy
University of Queensland
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