Locomotor adaptation and retention to gradual and sudden dynamic perturbations.

Motor learning principles are increasingly used in robotic rehabilitation. These principles are mainly derived from reaching studies and it is currently unknown whether the same principles apply in locomotion. The aim of this study was to determine the effect of a gradually and suddenly introduced dynamic perturbation on locomotor adaptation and on recalling the adaptation when being re-exposed to the same perturbation. Subjects walked on a treadmill and adapted to a viscous force field that was applied during the swing phase. In one group the strength of the force field was gradually increased over different steps whereas in a second group it was introduced suddenly at full strength. The gradual group showed less adaptation and a faster decay of the adaptation during the washout than the sudden group. Strikingly, when both groups were being re-exposed to the perturbation at full strength, the gradual group showed no adaptation whereas the sudden group showed a faster occurring adaptation than during the first exposure. In conclusion, in contrast to the reported beneficial effects of a gradual introduction of a perturbation on adaptation in reaching, it seems to have a detrimental effect on locomotor adaptation. These results indicate that caution should be taken when applying motor learning principles solely derived from reaching studies to improve robotic gait rehabilitation.