Simulation Study to Investigate the Accuracy of in Vivo Motor-Unit Twitch Force Measurements

Abstract

Motor-unit behaviour is central to understanding muscle contractile function. Commonly, in vivo experimental measurements of motor-unit twitches are taken at points biomechanically linked to the muscle. Here, we investigate whether these measurements accurately represent muscle twitch force behaviour by using a 3D, continuum-mechanical masticatory system model. Selected motor-units within the masseters were individually stimulated, and twitch force measured at the occlusal surface (typical for experiments) and at the muscle origin (representing true twitch force). Occlusal measurements underpredicted twitch force across all motor-units. This was due to force (re)distribution caused by motor-unit location and muscle architecture. Additionally, we investigated the assumption of twitch force summation by comparing twitch forces from co-contracting motor-units with linearly summed counterparts. Shear stress comparisons within the muscle revealed a lower lateral force dispersion for the co-contracting case, partly explaining the underpredicted magnitudes of the linearly summed forces. These findings have implications for twitch force characterisation and motor-unit based muscle modelling.