Low-frequency fluctuations of plantar flexion torque in force and position control tasks studied experimentally and by a neuromusculoskeletal model

There are many feedback loops potentially active while a subject performs a plantar flexion. One of these loops encompasses the proprioceptive feedback from muscle spindle Ia afferents, motor neurons (MNs), and the muscle. The aim of this study was to investigate the role of the Ia feedback loop on the low-frequency fluctuations of the plantar flexion torque during the performance of a force task (FT) and a position task (PT). Experiments were conducted on healthy human subjects and provided reference data for computer simulations that were carried out on a biologically-based large-scale neuromusculoskeletal model. Spectral analysis was performed on both simulated and experimental torque (equivalent to 20% MV C). In addition, coherence analysis was performed on several simulated neuronal and biomechanical variables. The results suggest that Ia afferents are largely involved in the low-frequency fluctuations of plantar flexion torque.