The effects of the proximity of an object on human stance

The present study tested the possibility that center of pressure (COP) behaviour is changed because of the proximity of an object (visible or not) from the body. Main effects of distance were expected for COP sway and main effects of location were expected for COP position when the object stands within a security margin (four centimetres; near distance) and within a personal space rather than without it (fifteen percent within and without arm extension; middle and far distances, respectively). This would be so to protect the body against an object entering into an intimate environment. Twelve standing younger adults kept their eyes open or closed toward a target located three meters in front of them. A big object set at trunk height was around the participants in a combination of distance (near, middle, far) and location (behind them, in front of them, away invisible). Consistent with the security margin hypothesis, the participants leaned away from the near object and on their left. Other results (correlations, ANOVAs, post hoc) were unexpected and showed that the closer the visible object, the more stable the participants. As an exception, the participants swayed quicker in the middle conditions in which they were the most centered. The participants were also the most stable when leaning on their left, and the proposition is made that postural control is modulated by lateral body inclinations. At a practical level, if unstable people have to lean away from a near object, it may cause more instability and falls. (C) 2010 Elsevier B.V. All rights reserved.