Responses to External Repeated Perturbations Vary With Time Intervals

It is generally accepted that stimuli with different time intervals lead to different contributions of relevant learning substrates. We investigated postural responses in men to trains of perturbations with different time intervals. A total of 12 male volunteers with no neurological deficits (age: 33.33±3.12 S.D.) experienced a sequence of perturbations. Two sequences of perturbations by a translational plate to still standing participants in the anterior-posterior direction were designed and administered: the first sequence consisted of 24 repeated perturbations with an interval of 5 s, while the second sequence consisted of ones with an interval of 2.5 s. A perturbation of a smaller magnitude was inserted into each sequence as a catch trial. We found that while a shorter interval and a longer interval both led to the learning effect (P < 0.05), a shorter interval results in a stiff strategy, presumably through muscle cocontraction, minimizing the degree of body sway. The learned motor response continued after experiencing the catch trial (P < 0.05). The results imply that stimulus intervals could lead to a different adaptation mechanism in the neuromotor system in the way to regain postural stability. Also, our results suggest that training for male individuals with repetition of postural perturbations with a shorter time interval leads to a stiff strategy with a greater degree. A stiff strategy could lower the ability to cope with unexpected postural threats, possibly leading to falls.