To what extent can increasing the magnification of visual feedback of the centre of pressure position change the control of quiet standing balance?

Previous research has shown that standing sway can be reduced when real-time visual feedback of the centre of pressure (COP) position is provided and that it can be further reduced when the visual feedback is magnified. The objective of this study was to determine the magnification beyond which there was no further change in the control of standing sway, as indicated by measures of the root mean square (RMS) and mean power frequency (MPF) of the COP and COM positions. Participants stood with as little movement as possible on a force platform for 2 min while being provided with visual feedback of their COP position at seven different magnifications (1 x, 4 x, 8 x, 16 x, 32 x, 48 x and 64 x) in two support surface conditions: standing on a foam surface and on a non-compliant surface. The RMS of the COM position decreased while the MPF of the COP position increased with increasing magnification. In the non-compliant surface conditions, these changes reached a plateau when visual feedback was magnified 8 x. When balance was made more difficult by standing on a foam surface, plateaus occurred at larger magnifications and, in some measures, did not reach a plateau at all. These data suggest that: (1) with increasing magnification of visual feedback, small movements of the COP were more easily detected, allowing corrective postural adjustments to be made before accelerations of the COM could lead to large deviations in postural sway and (2) the visual feedback was relied upon to a greater extent when standing on a foam surface.