It has been suggested that a feedforward control mechanism drives the adaptation of the spatial and temporal interlimb locomotion variables. However, the internal representation of limb kinetics during split-belt locomotion has not yet been studied. In hand movements, it has been suggested that kinetic and kinematic parameters are controlled by separate neural processes; therefore, it is possible that separate neural processes are responsible for kinetic and kinematic locomotion parameters. In the present study, we assessed the adaptation of the limb kinetics by analyzing the ground reaction forces (GRFs) as well as the center of pressure (COP) during adaptation to speed perturbation, using a split-belt treadmill with an integrated force plate. We found that both the GRF of each leg at initial contact and the COP changed gradually and showed motor aftereffects during early postadaptation, suggesting the use of a feedforward predictive mechanism. However, the GRF of each leg in the single-support period used a feedback control mechanism. It changed rapidly during the adaptation phase and showed no motor aftereffect when the speed perturbation was removed. Finally, we found that the motor adaptation of the GRF and the COP are mediated by a dual-rate process. Our results suggest two important contributions to neural control of locomotion. First, different control mechanisms are responsible for forces at single- and double-support periods, as previously reported for kinematic variables. Second, our results suggest that motor adaptation during split-belt locomotion is mediated by fast and slow adaptation processes.

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