Changes in Motor Unit Activity of Co-activated Muscles During Dynamic Force Field Adaptation

Muscle co-contraction plays a critical role in motor adaptation by minimizing movement errors and enhancing joint stability in novel dynamic environments. However, the underlying changes in motor unit (MU) activity within co-activated muscles during adaptation remain largely unexplored. To investigate this, we employed advanced electromyography sensor arrays and signal processing to examine MU activation in the triceps brachii (agonist) and biceps brachii (antagonist) during a reaching task under force-field perturbation. Our results revealed a gradual reduction in movement errors and an increase in velocity with adaptation, accompanied by a decrease in muscle co-contraction from early to late adaptation phases. This reduction was primarily driven by increased triceps activity, while biceps activity remained unchanged throughout the adaptation process. At the MU level, recruitment, amplitude, and firing rate increased in both muscles during adaptation compared to baseline (without force-field perturbation). However, from early to late adaptation phases, triceps MU amplitude continued to increase, while its firing rate stabilized, suggesting a shift in force generation strategy. In contrast, biceps MU activity remained stable throughout the adaptation. These findings indicate that the reduction in co-contraction during motor adaptation is likely mediated by a shift in motor unit control strategy within the agonist muscle. The increased reliance on MU amplitude modulation rather than firing rate in later adaptation may represent a mechanism for optimizing force production while maintaining movement accuracy and joint stability in dynamic environments.