Motor Planning of Walking in Novel Environments


Before initiating a walking movement, the central nervous system forms a motor plan, a set of motor commands predicted to accomplish task-specific goals. To be effective, motor plans must be continually updated to consider an ever-changing external environment. Despite being an inherent part of walking, how the nervous system adapts motor plans to interact with a dynamic environment is poorly understood. The purpose of this work was to explore how the nervous system adapts motor plans to interact with novel environments during walking. Explicitly, I evaluated how whole-body center-of-mass (COM) walking trajectories are adapted in response to either predictable or unpredictable changes. I first evaluated how people adapt to a novel and predictable environment, a viscous force field applied at the pelvis. I found that participants adapted their motor plans to create a COM trajectory that offset the force field. Additionally, when forces were unexpectedly removed, COM trajectories deviated in the opposite direction of initial error. These findings suggest that participants formed a predictive internal model to control COM trajectories in this predictable environment. Next, I changed the direction of the viscous force field to evaluate a critical assumption of this experimental paradigm, that the COM trajectory is a control objective of the nervous system. Of note, while COM trajectories were consistent across participants, the stepping strategies used were variable. These findings provide support that the COM trajectory is a control objective of the nervous system during walking. Finally, I evaluated how people adapt to walking in an unpredictable environment. I found that participants updated feedforward control strategies in response to their recent experience in the changing environment. With practice the large initial lateral deviations of the COM trajectory in the direction of the force field were significantly reduced. Additionally, COM trajectories immediately returned to baseline when forces were unexpectedly removed. These results suggest that participants adopted an impedance control strategy when walking in this unpredictable environment. Together, this work provides foundational information about motor planning during walking – people use internal models to control their COM trajectories in predictable environments and rely on impedance control in unpredictable environments.

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