Responses to external repeated perturbations vary with time intervals

While perturbation training is known to be effective in reducing fall risk, it is unclear whether the interval of perturbations affects motor response. We investigate postural responses that could vary with the interval of perturbations, probably leading to different contributions of relevant learning substrates. A total of 12 male volunteers with no neurological deficits (age: 33.33±3.12 S.D.) experienced two sequences of perturbations. Two sequences of perturbations were designed and administered in turn: the first sequence consisted of 24-time repeated perturbations with an interval of 5 seconds, while the second sequence consisted of ones with an interval of 2.5 seconds. A perturbation of a smaller magnitude was inserted into each sequence as a catch trial. Perturbations were given by a force plate moving in the anterior-posterior direction. The magnitude of the excursions of the center of pressure (COP) and ankle angle in response to perturbations with a longer interval is greater in comparison to that with a shorter interval (P < 0.05). A difference in responses to the perturbation following the catch trial appears in COP (P < 0.05), not in ankle angle (P > 0.05). These results suggest that while contraction of agonist muscles and co-contraction of antagonistic muscle pairs across the ankle joint for stability operate independently of each other, the refinement of the neuromotor system for a newly trained response can be modulated with stimulus intervals. The dependency of postural responses on the interval could imply that the strength of the learning effect varies with stimulus intervals.