Postural control is scaled to level of postural threat.

This study investigated control of posture when standing at different surface heights above ground level. Alterations in surface height were used to modify threat to postural control. Sixty-two healthy adults (mean+/-S.D.=20.3+/-1.3 years) stood quietly on a force plate 40 cm (LOW threat), 100 cm (MEDIUM threat) or 160 cm (HIGH threat) above ground level. Each standing trial was performed with eyes open for 120 s. Postural threat was presented in ascending (n=31) or descending (n=31) order with the first threat condition in each series (LOW threat for ascending group, HIGH threat for descending group) repeated. This manipulation allowed for an examination of set effects (i.e. prior experience of postural threat) on postural control. The results demonstrated scaling of postural control variables to level of postural threat. Amplitude of centre of pressure (COP) displacement decreased and frequency of COP displacement increased linearly as postural threat increased from LOW to HIGH. The central nervous system progressively tightened control of posture as postural threat increased. Initial exposure to the HIGH or LOW threat condition influenced postural control differently. The group who received the HIGH threat condition first (descending) demonstrated increased amplitude of COP displacement in the anterior-posterior direction compared with the group who received the LOW threat condition first (ascending). A 'first trial' effect was observed when standing for two consecutive trials but only at the LOW threat condition. Decreased amplitude and increased frequency of COP displacement were observed on the first trial compared to the second trial. The results of this study demonstrated that control of posture is influenced not only by the threat to posture but also by the order in which the threat to posture is experienced.