Human body-sway steady-state responses to small amplitude tilts and translations of the support surface-Effects of superposition of the two stimuli

Background: For humans, control of upright standing posture is a prerequisite for many physical activities. Experimentally, this control is often challenged by the motion of the support surface presented as tilt or trans-lation, or some combination thereof. In particular, we have investigated subjects balancing in situations where tilt and translation stimuli were presented in isolation and compared it to a situation where such stimuli occurred simultaneously.Research questions: Is the human posture control system in the case of two or more superimposed external dis-turbances responding to these as if it were dealing with one disturbance? Or does it identify the disturbances individually and as such and respond to them specifically, as suggested in a current concept of disturbance-specific estimations and compensations?Methods: We had healthy human subjects controlling their balancing of upright stance on a motion platform while we presented them with different combinations of pseudorandom support surface tilt and translation stimuli alone or in superposition (with peak-to-peak amplitude of 0.5 degrees and 1 degrees for tilt, and 0.8 cm and 1.5 cm for translation). In one set of trials they kept their eyes closed and in a second set open. Furthermore, a simulation was performed to qualitatively evaluate the impact of sensory non-linearities and joint stiffness modulation.Results: We found that the experimental conditions 'eyes open' vs. 'eyes closed' always created significant dif-ferences (p < 0.05) between the frequency response functions. In contrast to this, with different combinations of the tilt and translation stimuli, significant differences between the responses were observed only in 5 cases over the 24 that have been tested. SignificanceThe superposition of translation and tilt can be used to characterize the responses to both stimuli with one trial. When the amplitude of the stimuli is unbalanced (e.g. very small tilt superimposed with a larger translation) the effect of stiffness modulation can be studied.