Head stabilization on a continuously oscillating platform: the effect of a proprioceptive disturbance on the balancing strategy.

When standing and balancing on a continuously
and predictably moving platform, body equilibrium
relies on both anticipatory control and
proprioceptive feedback. We have vibrated different
postural muscles of the body to assess any effect of
confounding the proprioceptive input on balance during
such unstable conditions. Low and high platform oscillation
frequencies were used, because different strategies
are used to withstand the two perturbations. Eyes open
(EO) and closed (EC) conditions were also tested, to
assess whether the stabilizing effect of vision is independent
from the proprioceptive disturbance. Subjects
(n=14) performed two series of trials, EO and EC: (1)
quiet erect stance, (2) stance on the platform translating
at 0.2 or 0.6 Hz sinusoidally in the anteroposterior (A-P)
direction (dynamic conditions). Continuous bilateral
vibration (90 Hz) was produced by two vibrators fixed
to the following homonymous muscles: dorsal neck,
quadriceps, biceps femoris, tibialis anterior, and triceps
surae. Acquisition of body segments’ displacement began
10 s after the start of platform translation. From
markers fixed to head, hip, and malleolus, we computed
the A-P oscillation of head and hip, body orientation in
space, and cross-correlation (CC) and time-delay between
malleolus and head trajectories. The results were
(a) the head A-P oscillation was smaller with EO than
EC, under both quiet stance and dynamic conditions; (b)
vibration of tibialis and triceps surae, but not of other
muscles, slightly increased head and body A-P oscillation
with EC under dynamic conditions; (c) at 0.2 Hz
but not at 0.6 Hz, for all visual and vibration conditions,
there was a significant association between head and
feet; (d) at 0.2 Hz, EC, neck muscle vibration increased
this association, whereas vibration of the other muscles
induced a major time delay in the oscillation of head
compared with feet; (e) vibration of either neck or tibialis
induced forward body leaning, while vibration of
either triceps surae or biceps femoris induced backward
leaning, with both EO and EC, under both static and
dynamic conditions; (f) the head A-P oscillation, however,
under dynamic conditions was not dependent on
body leaning. The relatively scarce effects of proprioceptive
disturbance on head stabilization and multijoint
coordination (in spite of effects on body orientation
similar to those observed during stance) speak for a
major role of anticipatory control in the dynamic equilibrium
task. However, the significant vibration-induced
time delay in segments’ coordination at low translation
frequency, EC, suggests that the normally patterned
Ia input promotes continuous adjustments of the
feed-forward control mode.