8th Annual Meeting of the International Multisensory Research Forum
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Stephen Lord

Multiple Paper Presentation

Stephen Lord
Prince of Wales Medical Research Institute

Annie Butler

Richard Fitzpatrick

     Abstract ID Number: 123
     Last modified: June 1, 2007
     Presentation date: 07/07/2007 2:00 PM in Quad General Lecture Theatre
     (View Schedule)

Introduction: Poor standing balance in weak people may be due to poor proprioceptive input in addition to muscle weakness. We investigated how the mass of an upright body and the available muscle strength affects our ability to balance a load.

Methods: In one study, healthy young subjects (n=12) balanced three different inverted pendula: (i)a small pendulum aligned with the elbow joint, balanced by the elbow flexors, (ii)a large body-sized pendulum aligned with the ankle joint, balanced by the calf muscles, and (iii) the subject’s own body that was balanced around the ankles by standing. Each was tested with different masses, either added to the mechanical pendula or strapped to the body during standing. Sway was measured from the angular movement of the pendula around the axis of rotation during 40s periods using optical sensors, with the eyes shut and open. Root mean square values of sway amplitude and power spectra for each condition were compared.
In a second study, we compared sway (measured with a custom designed sway meter) with eyes open and eyes closed in (i) 17 women with prior-polio and lower-limb weakness and 34 age-matched controls, and (ii) 450 elderly women classified as weak (<60N dorsiflexion force, n=203) or strong (>60N, n=247).

Results: In young subjects, increased load resulted in increased sway but
disproportionately so without vision. As the load increased, sway with open eyes increased linearly. However with eyes closed, sway increased exponentially. That is, subjects relied more and more on vision to stabilise the larger loads. For example, eye closure increased body pendulum sway by 337% for a 1-body load, and at 2 bodies it increased body sway by 9115% (P<0.05). Clinical corroborations of these data were shown in prior-polio and weak elderly women. The women aged 60-69 years swayed no more than their stronger counterparts with their eyes open. However, these weaker subjects had a disproportional increase in sway with eye closure.

Conclusion: We depend more on vision to stabilise balanced loads as their size increases relative to the available muscle strength. The strength of the person and the load they are carrying do not change with the eyes closed. Thus, we propose that proprioceptive control of balance fails with larger loads.

To be Presented at the Following Symposium:
Multisensory mechanisms of posture control
Other papers in this Symposium:

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