Variability in a dynamic postural task attests ample flexibility in balance control mechanisms

When humans stand upright on a platform that
sinusoidally translates in the anterior-posterior direction,
the movements of upper and lower body segments are
appropriately coordinated, in order to keep the body
within its limits of stability. A significant fluctuation in
this behaviour is evident across subjects and perturbation
conditions. The inter- and intrasubject variability in the
body segment kinematics, as occurs during repeated trials
across different conditions, is quantitatively described
here. Twenty normal subjects stood upright with
eyes open (EO) or eyes closed (EC) on a platform moving
to-and-fro in the horizontal plane for 30 s, at a frequency
of 0.2 and 0.6 Hz, with a peak-to-peak amplitude
of 6 cm. Each subject made two trial repetitions for each
visual and frequency condition. The last 20 s of each
trial was acquired. The displacement of markers fixed on
the lateral malleolus, hip and head was sampled at a frequency
of 50 Hz. An index of the ‘average’ displacement
of each marker during the trial was the standard deviation
(SD) of its anterior-posterior displacements, calculated
across the acquired trial cycles. The cross-correlation
(CC) between pairs of marker displacement traces
gave an indication of the degree of coupling of the body
segments. All subjects showed two basic modes of coping
with the perturbation, depending on the availability
of the visual input: with EO, they tended to stabilize the
head in space; with EC, the head oscillated in the anteriorposterior
direction more than hip and platform. Within
this general behaviour, the values of the SD of horizontal
displacement of head and hip marker traces varied within
an ample range during different trials of the same perturbation
condition. Even within a single trial there was an
ample variability of the body segments’ position. In spite
of this, neither head nor hip ever bypassed the anterior or
posterior limits of stability. Remarkably, the range of
variability of the whole population of normal subjects,
both across and within trials, was analogous to the range
of variability of single subjects across numerous trials.
This large variability notwithstanding, it appeared that
the relationship of head to hip SD across trials was
almost constant, independent of visual and perturbation
frequency condition. The results show that there exist a
large variety of dynamic postures, rather than one particular
configuration, which assure stability. The findings
also suggest a neural or biomechanical constraint underlying
the operations of the equilibrium control strategy.