Sudden disturbances (perturbations) to the hand and wrist are commonplace in daily activities and workplaces when interacting with tools and the environment. it is important to understand how perturbations influence forearm musculature and task performance when identifying injury mechanisms. The purpose of this work was to evaluate changes in forearm muscle activity and co-contraction caused by wrist perturbations during a dynamic wrist tracking task. Surface electromyography was recorded from eight muscles of the upper-limb. Participants performed trials consisting of 17 repetitions of ±40° of wrist flexion/extension using a robotic device. During trials, participants received radial or ulnar perturbations that were delivered during flexion or extension, and with known or unknown timing. Co-contraction ratios for all muscle pairs showed significantly greater extensor activity across all experimental conditions. Of all antagonistic muscle pairs, the flexor carpi radialis (FCR)-extensor carpi radialis (ECR) muscle pair had the greatest change in co-contraction, producing 1602% greater co-contraction during flexion trials than during extensions trials. Expected perturbations produced greater anticipatory (immediately prior to the perturbation) muscle activity than unexpected, resulting in a 30% decrease in wrist displacement. While improving performance, this increase in anticipatory muscle activity may leave muscles susceptible to early-onset fatigue, which could lead to chronic overuse injuries in the workplace.www.nature.com/scientificreports www.nature.com/scientificreports/ co-contraction, the CNS has the ability to adapt and selectively alter joint impedance based on the magnitude and direction of instability 8 . Developing an internal model to learn optimal joint impedance is not always required to manage instability; Crevecoeur et al. (2019) explored a model-free strategy, reporting that encountering a single unexpected perturbation led to an immediate increase in co-contraction followed by a gradual decrease during subsequent, perturbation-free trials 9 . When a wrist perturbation is expected but timing is unknown, anticipatory co-contraction is significantly greater than when the timing is known 10 . These findings could be a reflection of the methodology, since participants knew that a perturbation trial would occur, but were unaware of the timing. At the wrist joint, this co-contraction is illustrated by the stabilizing nature of the wrist extensors, which remain active when performing static tasks, even when they are not the primary muscles involved [11][12][13] . This leads to an optimization problem for the CNS as control of impedance is computationally costly 8 , while co-contraction is metabolically costly 6 . Therefore, to optimize control, the CNS is required to provide adequate impedance relative to the magnitude and direction of instability while utilizing minimal metabolic fuel 8,9 . Research utilizing unexpected perturbations at the distal upper extremity has largely been focused around sta...