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1 Behavioural Brain Sciences Centre, University of Birmingham, Birmingham, United Kingdom
2 Department of Physical Education and Sport Sciences, Aristotle University of Thessaloniki, Thessaloniki, Greece
* To whom correspondence should be addressed. E-mail: a.m.wing{at}bham.ac.uk.
Light touch contact between the body and an environmental referent reduces fluctuations of centre of pressure (CoP) in quiet standing although the contact forces are insufficient to provide significant forces to stabilize standing balance. Maintenance of upright standing posture (with light touch contact) may include both predictive and reactive components. Recently, Dickstein et al. (2003) demonstrated that reaction to temporally unpredictable displacement of the support surface was affected by light touch raising the question whether light touch effects also occur with predictable disturbance to balance. We examined the effects of shoulder light touch on standard deviation (SD) of CoP rate (dCoP) during balance perturbations associated with forward sway induced by pulling on (voluntary), or being pulled by (reactive), a hand-held horizontal load. Prior to perturbation, SD dCoP was lower with light touch, corresponding to previous findings. Immediately after perturbation, SD dCoPAP was greater with light touch in the case of voluntary pull whereas no difference was found for reflex pull. However, in the following time course light touch contact again resulted in a significantly lower SD dCoP and faster stabilization of SD dCoP. We conclude that shoulder light touch contact affects immediate postural responses to voluntary pull but also stabilization after voluntary and reflex perturbation. We suggest that in voluntary perturbation CoP fluctuations are differentially modulated in AP and ML directions to maintain light touch, which not only provides augmented sensory feedback about body self-motion, but may act as a 'constraint' to the postural control system when preparing postural adjustments.
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