Elsevier

Gait & Posture

Volume 12, Issue 2, 1 October 2000, Pages 87-93
Gait & Posture

Postural control is scaled to level of postural threat

https://doi.org/10.1016/S0966-6362(00)00057-6Get rights and content

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.

Introduction

It is well documented that falls most often result from an interaction of multiple factors including age-related changes to the postural control system [1]. Psychological factors, such as fear of falling, may also constrain control of posture leading to falls [2]. Although elderly [3], [4], [5], [6], [7], [8], [9] and patients with balance disorders [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 posture characterized by smaller amplitude and higher frequency postural sway compared to normals. The results of these two studies illustrate the possible confounding 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 problem [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 instability, would be greater when standing on a high (0.81 m above ground level) compared to low platform height (0.19 m above ground level). Carpenter et al. [15] showed that participants adopted smaller amplitude and higher frequency postural sway and leaned back away from the platform edge when standing on a high platform. Brown and Frank [16] demonstrated that participants limited displacement and velocity of center of mass (COM) movement in response to a destabilizing push applied to the upper back when standing at the edge of a high platform. These findings suggest that in fearful situations the central nervous system (CNS) controls posture to limit the chances of the COM moving outside the base of support.

The results of these four studies provide evidence that fear of falling has observable effects on postural control. However, each of the studies discussed examined a dichotomous fearful versus non-fearful situation. An issue still unresolved is whether modifications to postural control vary with intensity of fear of falling or threat to posture. To investigate this question, we extended the work of Carpenter et al. [15] and examined changes to control of posture when individuals stood under multiple levels of postural threat. In particular, we were interested in whether or not modifications to postural control were scaled to level of postural threat.

Previous research has shown that prior experience and expectation of an external perturbation can influence control of posture [17], [18], [19]. Thus, a second question we examined focused on the postural response to a specific level of threat and whether or not this response was influenced by prior experience in a more or less threatening condition. To investigate this question, we altered presentation order of postural threat to determine whether the influence of threat was larger or smaller when preceded by a more threatening condition compared to a less threatening condition. We were also interested in whether postural control was influenced when standing at a particular level of postural threat if prior experience at that same level of threat was available, and whether or not this effect was observed in both more or less threatening conditions. Two consecutive trials of both, the least threatening condition and the most threatening condition, were performed to examine this issue of ‘first trial’ effects.

Section snippets

Participants

Sixty-two healthy young adults (mean±S.D. age=20.3±1.3 years) volunteered for this study. Each participant completed a medical history and physical activity questionnaire. Participants were free from any neurological or musculoskeletal disorder. Each participant, informed of the experimental procedures, provided written consent prior to the testing session. All experimental procedures were approved by the University of Waterloo Office of Research Ethics.

Manipulation of postural threat

Postural threat was modified through

Postural threat effects

Significant modifications to COP control in both A–P and M–L directions were observed as postural threat was increased from LOW to HIGH.

A–P S.D. showed a significant main effect of postural threat (F(2,119)=7.85, P=0.0006). As threat increased, A–P S.D. values linearly decreased (P=0.0002) (Fig. 2). The percent change decrease in mean A–P S.D. values from the LOW threat condition was 8.3% for the MEDIUM and 15.7% for the HIGH threat condition.

A–P MPF showed a significant main effect of postural

Postural control is scaled to level of postural threat

The results of this study show that the CNS precisely adjusts control of posture in response to different levels of postural threat. Amplitude of COP displacement decreased linearly and frequency of COP displacement increased linearly as postural threat increased from LOW to HIGH (Fig. 2, Fig. 3). The finding that postural control is scaled to level of postural threat extends the work of Carpenter et al. [15] who observed similar changes in postural control but only between two levels of

Conclusions

The observations for healthy young adults suggest the CNS adopted tighter control of posture under conditions of increased postural threat and that this control was precisely scaled to the level of postural threat. The control of posture was also influenced by the order in which the threat to posture was experienced.

These results emphasize the importance of identifying both psychological and physiological influences on postural control when assessing individuals with balance disorders or

Acknowledgements

Research supported by Natural Sciences and Engineering Research Council of Canada (NSERC) funding to J.S. Frank and A.L. Adkin.

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