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 @entity1 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 @entity1 physiology (what triggers and mediates these FTRs, and what is the relation to @entity427 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 @entity393 leads to @entity6377 due to loss of the normal modulatory influence. Third, we address the question whether @entity427 -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' @entity427 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 @entity1 compared to healthy subjects. We conclude that FTRs are an important source of information about @entity1 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 @entity427 reactions in triggering or modulating FTRs.

Review of first trial responses in balance control: influence of XXXX and Parkinson's disease.

@entity1 :: ('9606', 'Species') :: ['humans', 'human', 'patients'] @entity393 :: ('MESH:D034381', 'Disease') :: ['vestibular loss'] @entity427 :: ('MESH:D012021', 'Disease') :: ['startle'] @entity6377 :: ('MESH:D005549', 'Disease') :: ['excessive upper body FTRs']
@entity393 :: (MESH:D034381,Disease) :: ['vestibular loss']
