Vary Slow Motion: Effect of Task Forces on Movement Variability and Implications for a Novel Skill Augmentation Mechanism

This article presents the results of a human subject experiment aimed at answering the question, "can increased muscle force variability in low force levels explain increased variability or intermittency of slow movements?" To address this research question, we conducted an experiment with eight subjects, which involved the completion of slow elbow flexion movements at two target speed levels and under five resistive torque fields implemented via an elbow exoskeleton. The results of this experiment demonstrated that increasing levels of resistive torques decreased movement speed variability only until a certain torque level. This observation indicates that a motor-unit pool-based muscle force generation variability, which is known to increase at low force levels, can indeed underlie increased variability in slow movements. Our results imply that resistive torques may be used to significantly decrease movement speed variability, opening up new possibilities for novel assistive devices for motor skill augmentation.