Do postural responses to transient and continuous perturbations show similar vision and amplitude dependence?

This study compares the influence of vision and perturbation amplitude on postural center-of-pressure responses to transient (acceleration pulse) versus continuous (pseudorandom acceleration) platform-translation perturbations. To control possible confounding effects, efforts were directed at making the perturbations unpredictable and matching the transient and continuous waveforms in terms of frequency content and level of challenge to stability. It was hypothesized that, under these conditions, the same control mechanisms would be activated; hence, responses to transient and continuous perturbations would show similar vision and amplitude dependence. A repeated-measures experimental design was used to test 12 healthy males. The transient and continuous perturbations were each administered at three acceleration amplitude levels, in random order, and the tests were repeated under eyes-open and blindfolded conditions. When order-of-testing effects were taken into account, the results failed to demonstrate any differences between transient and continuous postural control with regard to the influence of vision, as both types of response were unaffected by vision deprivation. This finding is consistent with the experimental hypothesis. Responses to the two types of perturbation did appear to differ, however, with regard to the influence of perturbation amplitude, as the transient-perturbation responses showed greater evidence of nonlinearity. This finding would seem to reject the hypothesis that the responses to transient and continuous perturbations would show similar amplitude dependence. However, it is suggested that the amplitude distribution of the particular continuous waveform that was used, rather than the continuous nature of the perturbation per se, may have led to the greater linearity in the continuous-perturbation responses. Although the present results fail to provide unequivocal support for the hypothesis that the same control mechanisms are utilized in transient and continuous balance recovery, they do demonstrate that distinctions between these two types of postural task are not nearly so clearcut as has been suggested. Nonetheless, from a practical perspective, the findings of substantial nonlinearity indicate that attempts to predict responses to transient perturbations from continuous-perturbation tests should be performed with caution.