Mapping the Neural Control of Force and Impedance of Wrist Movements Using Robotics and fMRI.

While robots are becoming increasingly valuable tools in neurorehabilitation, our limited understanding of the brain's response during human-robot interaction tasks restricts advancements in training programs to restore neural pathways after injury. Co-contraction is characteristic of several neuromuscular disorders, such as stroke and cerebral palsy, and it is often targeted by assessments or rehabilitation programs. Despite its importance, the neural mechanisms underlying cocontraction remain poorly understood. To address this gap, this study investigates the neural substrates of muscle co-contraction via functional magnetic resonance imaging (fMRI) during a dynamic motor task with an MR-compatible wrist robot. To establish suitable fMRI experimental conditions, we first conducted a behavioral study assessing muscle activity during a wrist-pointing task with four participants. Participants reached toward a target while experiencing four main perturbation conditions (no force, divergent force, constant force up, and constant force down), designed to elicit distinct force and impedance responses. Following this behavioral validation, five additional participants performed the wrist-pointing task during fMRI. Our results suggest localization of force and impedance control within the cortico-thalamic-cerebellar network. These findings provide new insights into the neural mechanisms of co-contraction, supporting the development of neurorehabilitation paradigms.