Understanding mechanisms of generalization following locomotor adaptation

Our nervous system has the remarkable ability to adapt our gait to accommodate changes in our body or surroundings. However, our adapted walking patterns often generalize only partially (or not at all) between different contexts. Here, we sought to understand how the nervous system generalizes adapted gait patterns from one context to another. Through a series of split-belt treadmill walking experiments, we evaluated different mechanistic hypotheses (i.e., the mechanism specific, credit assignment, and slow switching hypotheses) to explain the partial generalization of adapted gait patterns from split-belt treadmill to overground walking. In support of the credit assignment hypothesis, our experiments revealed the central finding that adaptation involves 1) the recalibration of forward models of our body that generalize to overground walking, and 2) models of the treadmill that do not generalize. Methods Participants walked on a split-belt treadmill with two separate belts (Motek, Amsterdam, NL), with the speed of each belt was independently controlled using a custom-written D-Flow program. Participants also walked overground on a 6-meter walkway. Step length asymmetry and other kinematic measures were computed from motion capture data collected with a Vicon motion capture system (Oxford, UK), which uses infrared-emitting markers, or with a Zeno Walkway (ProtoKinetics, Havertown, PA), which uses force plates. We also obtained perceptual measures using a method of adjustment perceptual task to measure the belt speed difference perceived as "equal speeds" after adaptation. All methods are described in the manuscript.