Postural perturbation does not reset stepping rhythm in humans, but brief intermission does.

In the present study, we tested a hypothesis that the rhythm generator in humans keeps the rhythm of periodic motor output during brief inactivation of the pattern generator. This investigation was made through testing whether the step rhythm was reset after an interruptive event. A reset of the step rhythm was defined as an observation that the step re-emerges at random timing after an interruptive event regardless of the step rhythm before the interruption. This observation reflects an intermission of rhythm-keeping activity. Healthy participants stepped on a platform that could translate forward or backward. They continued stepping after the platform translation (non-stop session) or stopped briefly after the translation before resuming step with their own timing (stop session). In the non-stop session, the second step after the platform translation appeared at the integer multiple of the pre-existing step period in most participants, indicating that step rhythm was not reset. This finding indicates that postural perturbation does not interfere the rhythm-keeping activity. In the stop session, the step immediately after the intermission of stepping appeared at random time regardless of the step rhythm before the intermission in most participants. The actual side of the first step after the intermission was consistent with the predicted first step side at a 0.5 probability. Those findings indicate that step rhythm is reset after brief intermission of stepping, and contradict with the hypothesis that the activity of the rhythm generator is maintained, while the pattern generator is temporally inactive during a brief intermission of periodic motor output. This analysis could help to determine whether rhythm-keeping activity is inactivated by an interruptive event during periodic motor activity.