Gait and Posture 12 (2000) 87–93 Postural control is scaled to level of postural threat Allan L. Adkin *, James S. Frank, Mark G. Carpenter, Gerhard W. Peysar Gait and Posture Laboratory, Department of Kinesiology, 200 Uniersity Aenue West, Uniersity of Waterloo, Waterloo, Ont., Canada N2L 3G1 Accepted 7 March 2000 Abstract 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. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Fear of falling; Postural threat; Surface height; Postural control; Centre of pressure; Scaling; Prior experience www.elsevier.com/locate/gaitpost 1. Introduction It is well documented that falls most often result from an interaction of multiple factors including age-re- lated changes to the postural control system [1]. Psy- chological factors, such as fear of falling, may also constrain control of posture leading to falls [2]. Al- though elderly [3 – 9] and patients with balance disor- ders [10,11] frequently report fear of falling, few studies have directly examined the relationship between this fear and postural control. Maki et al. [12,13] observed an association between fear of falling and control of posture. For example, elderly, who reported a fear of falling, demonstrated larger amplitude of postural sway when blindfolded and poorer scores when timed on a one-leg stance test compared to those who did not report a fear of falling. Krafczyk et al. [14] have shown that patients with phobic postural vertigo adopt a tighter control of pos- ture characterized by smaller amplitude and higher frequency postural sway compared to normals. The results of these two studies illustrate the possible con- founding effects of fear of falling on postural control. However, it is difficult to distinguish psychological and physiological influences on postural control in elderly and patients with balance disorders, as fear of falling may compound an already existing physiological prob- lem [11]. To address this issue, Carpenter et al. [15] and Brown and Frank [16] examined postural control of healthy young adults when faced with a threat to their posture. These authors argued that fear of falling, based on perceived risk of injury as a result of instabil- * Corresponding author. Tel.: +1-519-8884567, ext. 2601; fax: +1-519-7466776. E-mail address: adkin@nclab.uwaterloo.ca (A.L. Adkin). 0966-6362/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 6 6 - 6 3 6 2 ( 0 0 ) 0 0 0 5 7 - 6