Article

The reaction to an unexpected balance disturbance is unpracticed, often startling
and frequently associated with falls. This everyday situation can be reproduced
in an experimental setting by exposing standing humans to sudden, unexpected and
controlled movements of a support surface. In this review, we focus on the
responses to the very first balance perturbation, the so-called first trial
reactions (FTRs). Detailed analysis of FTRs may have important implications, both
for clinical practice (providing new insights into the pathophysiological
mechanisms underlying accidental falls in real life) and for understanding human
physiology (what triggers and mediates these FTRs, and what is the relation to
startle responses?). Several aspects of the FTRs have become clear. FTRs are
characterized by an exaggerated postural reaction, with large EMG responses and
co-contracting muscles in multiple body segments. This balance reaction is
associated with marked postural instability (greater body sway to the
perturbation). When the same perturbation is repeated, the size of the postural
response habituates and the instability disappears. Other issues about FTRs
remain largely unresolved, and these are addressed here. First, the functional
role of FTRs is discussed. It appears that FTRs produce primarily increased trunk
flexion during the multi-segmental response to postural perturbations, thus
producing instability. Second, we consider which sensory signals trigger and
modulate FTRs, placing specific emphasis on the role of vestibular signals.
Surprisingly, vestibular signals appear to have no triggering role, but
vestibular loss leads to excessive upper body FTRs due to loss of the normal
modulatory influence. Third, we address the question whether startle-like
responses are contributing to FTRs triggered by proprioceptive signals. We
explain why this issue is still unresolved, mainly because of methodological
difficulties involved in separating FTRs from 'pure' startle responses. Fourth,
we review new work about the influence of perturbation direction on FTRs. Recent
work from our group shows that the largest FTRs are obtained for toe-up support
surface rotations which perturb the COM in the posterior direction. This
direction corresponds to the directional preponderance for falls seen both in the
balance laboratory and in daily life. Finally, we briefly touch upon clinical
diagnostic issues, addressing whether FTRs (as opposed to habituated responses)
could provide a more ecologically valid perspective of postural instability in
patients compared to healthy subjects. We conclude that FTRs are an important
source of information about human balance performance, both in health and
disease. Future studies should no longer discard FTRs, but routinely include
these in their analyses. Particular emphasis should be placed on the link between
FTRs and everyday balance performance (including falls), and on the possible role
played by startle reactions in triggering or modulating FTRs.
CI - Copyright (c) 2011 Elsevier B.V. All rights reserved.

Langue : ANGLAIS