Investigating cerebellar control in slow gait adaptations: Insights from predictive simulations of split-belt walking
Authors: Fleischmann Sophie , Shanbhag Julian , Miehling Joerg , Wartzack Sandro , Ong Carmichael , Eskofier Bjoern M , Koelewijn Anne D 
Published: December 2024

During split-belt treadmill walking, neurotypical humans exhibit slow adaptations, characterized by a gradual decrease in step length asymmetry, whereas individuals with cerebellar damage do not show these motor adaptations. We used a neuromusculoskeletal model to better understand individual aspects of the underlying neural control. Specifically, we extended a spinal reflex model by adding a supraspinal layer, representing the cerebellum and its main function of error-driven motor adaptation. The cerebellum, based on the mismatch between an internal prediction and the actual motor outcome, can modulate spinal motor commands within the simulation. Using this model, we investigated the effect of an isolated adaptation of gait timing parameters, in our case the beginning of the liftoff phase. We created 80 s predictive simulations of the model walking on a split-belt treadmill with a 2:1 belt-speed ratio, and evaluated the results by comparing spatiotemporal parameters and kinematics with literature. The simulations exhibited adaptation patterns similar to those observed in human experiments. Specifically, the step length symmetry decreased from an initial asymmetric level toward the baseline, driven primarily by adaptations in the fast step length, while the individual joint kinematics remained similar. The adaptations affected the spatial and temporal domains, represented by a change in the center of oscillation difference and limb phasing. Our findings suggest that reflex gains do not necessarily need to be adapted to achieve changes in step length asymmetry and that, unlike what had been inferred from experiments, the same neural mechanism might account for adaptations in the spatial and temporal domains at different rates. Our simulations demonstrated distinct adaptation patterns corresponding to slow and fast learning behaviors, as reported in the literature, through modifications of a single cerebellar parameter, the adaptation rate. The framework can be extended to test different hypotheses about motor control and adaptations during continuous perturbation tasks.

Author summary

When people walk on a treadmill with the belts running at two different speeds, they initially walk very asymmetrically but gradually decrease certain parameters back toward a symmetric level. We know that the cerebellum is involved in this process, however, the exact neural mechanisms and interdependencies of the numerous interlimb and intralimb adaptation mechanisms remain a topic of ongoing research. We believe that predictive neuromusculoskeletal simulations can advance our understanding of these adaptation processes, as they allow isolating and changing selected arbitrary parameters, which is impossible in human experiments. So far, no models are available in which individual control parameters adapt automatically within the simulation, driven by an embedded physiological process rather than manual adjustments. In our work, we provide a neuromusculoskeletal model extended by a model of the cerebellum, which in turn adapts the gait controller in real-time during the simulation. We found that adapting exclusively the timing of liftoff of the feet can already capture adaptation patterns that are observed in humans with intact cerebellar function. Our model can further be used to test all types of hypotheses about motor adaptation, from adapting individual control parameters to hypotheses about what is stored and adapted during split-belt walking.