To study quiescent stance without applying external disturbances is no
t a theoretically appealing way to unveil the dynamic properties of hu
man equilibrium. Methods to disturb equilibrium range from standing on
foam surface, attaching vibrators to the calves to interfere with som
atosensation, and exposure to body-position tracking environments, as
in dynamic posturography (EquiTest). The EquiTest apparatus was modifi
ed by a menu-driven software to allow arbitrary movements of the suppo
rt surface and visual surround, and force data were recorded for subse
quent analysis. The support surface was randomly moved in the antero-p
osterior direction. First equilibrium was studied on the stable suppor
t surface, then low (RMS 1.3 cm) and high (RMS 2.6 cm) amplitude movem
ents were used. Vision was either present or absent at all test amplit
udes. Equilibrium was evaluated by the confidence (61%) ellipse sway a
rea and average sway velocity during 45 s. Eleven healthy subjects age
d 23-36 years (mean 29) were sedated with a short acting sedative, mid
azolam 0.1 mg/kg. Randomized perturbed posturography was conducted at
baseline, and at about 60, 120 and 180 min after injection. Psychomoto
r tests were conducted at baseline, and at 30, 90, 150 and 210 min. La
rge interindividual variations were found. One subject could not be te
sted at all at 60 min due to sleepiness, whereas some subjects felt ne
arly full awake at 30 min. Sway areas were larger at 60 min, but not s
ubsequently. At 60 min, sway velocities with open eyes were higher, ju
st as when vision was absent and low amplitude movements were used. La
ter no effects could be shown. All psychomotor tests had deteriorated
at 30, and some also at 90 min. In conclusion, the randomized perturbe
d posturography evaluates the dynamics of disturbed equilibrium, and t
he modified EquiTest apparatus allows the study of several aspects of
dynamic equilibrium control. Balance disturbances due to midazolam sed
ation were quantified and correlated to psychomotor tests.