Scaling of postural responses to transient and continuous perturbations

Abstract

Scaling of postural ankle-muscle responses was compared for transient (forward and backward acceleration pulses) and continuous (pseudorandom acceleration) platform-translation perturbations. The two types of perturbation were designed to be unpredictable, to have similar frequency content, and to represent similar levels of challenge to stability. A repeated measures experimental design was used to test 12 healthy young (20–45 years) males. Each type of perturbation was administered at three acceleration levels, in random order, and the tests were repeated under eyes-open and blindfolded conditions, while controlling for order of testing. Individual ankle-muscle responses were quantified and, in addition, used to estimate a net response measure (proportional to net ankle torque) and measures of antagonist activation and relative co-contraction level. The results failed to show strong evidence of differences between transient and continuous postural control with regard to the influence of vision, as both types of response were largely unaffected by vision deprivation; however, the results did demonstrate some substantial perturbation-dependent differences in the scaling of the responses to perturbations of differing magnitude. The most pronounced differences, seen in tibialis anterior, appeared to be related to a tendency to lean slightly further forward during continuous perturbations. Substantial co-contraction of antagonistic muscles was frequently observed, for both types of perturbation, and antagonist activation tended to increase at larger perturbation magnitudes. The observed differences in the scaling of the transient and continuous responses raise some concerns about the generalizability of posture control models derived from continuous-perturbation tests.