Postural sway is a result of a complex action-reaction feedback mechanism generated
by the interplay between the environment, the sensory perception, the neural system
and the musculation. Postural oscillations are complex, possibly even chaotic. Therefore
fitting deterministic models on measured time signals is ambiguous. Here we analyse
the response to large enough perturbations during quiet standing such that the resulting
responses can clearly be distinguished from the local postural sway. Measurements
show that typical responses very closely resemble those of a critically damped oscillator.
The recovery dynamics are modelled by an inverted pendulum subject to delayed state
feedback and is described in the space of the control parameters. We hypothesize that
the control gains are tuned such that (H1) the response is at the border of oscillatory
and nonoscillatory motion similarly to the critically damped oscillator; (H2) the
response is the fastest possible; (H3) the response is a result of a combined optimization
of fast response and robustness to sensory perturbations. Parameter fitting shows
that H1 and H3 are accepted while H2 is rejected. Thus, the responses of human postural
balance to "large" perturbations matches a delayed feedback mechanism that is optimized
for a combination of performance and robustness.