North American Congress on Biomechanics
Canadian Society for Biomechanics - American Society of Biomechanics
University of Waterloo
Waterloo, Ontario, Canada
August 14-18, 1998
Fear of falling often accompanies a balance disorder and it is important to both diagnosis and treatment of the disorder to distinguish psychological and physiological influences on postural control. Standing on an elevated surface can cause a fear of falling in otherwise healthy people and we have been using this manipulation to examine the influence of fear of falling on postural control for maintaining upright stance. Standing on even a moderate elevation (.81 m above ground) causes an increase in joint stiffness during quiet stance (Carpenter et al. 1998) and reduces the velocity of centre of mass displacement when pushed forward off balance (Brown and Frank, 1997).
Voluntary movements performed while standing are accompanied by postural adjustments which stabilize the body centre of mass or move it to a position over the new base of support. Belinkii et al. (19--) was the first to report that the central nervous system anticipates destabilizing forces resulting from voluntary movement. When raising the extended arms forward while standing, activation of the anterior deltoid (a focal muscle) was preceded by activation of the biceps femoris (a postural muscle) which acts to stabilize the hip. The purpose of this investigation was to examine the influence of height-induced fear of falling on postural control during rising to the toes. This task is controlled by an initial backward displacement of the foot centre of pressure (CoP) which allows the body centre of mass (CoM) to fall forward (postural adjustment), followed by a forward displacement of the CoP to arrest the body CoM and move it upwards (focal movement). Changes to the initial postural adjustment will influence the velocity and final position of the CoM.
Twelve healthy university students (20-23 years) participated in the experiment. Participants stood with the feet positioned at a constant width equal to their foot length and arms by their side. The movement task involved rising onto the toes as quickly as possible and holding this position for a count of 3 seconds. Each participant performed 5 blocked trials under 5 experimental conditions which varied: the platform height (.19 m vs .81 m) and stance position (away from the edge, toes at the edge and heels at the edge of the platform). The order of presentation of experimental conditions was counterbalanced across participants.
CoP was recorded from a forceplate (AMTI). The peak backward displacement of the CoP was defined as 0 ms and marked the boundary of the initial postural adjustment and subsequent focal movement. CoP measures included: initial and final position, duration and magnitude of the backward displacement (postural adjustment), and duration and magnitude of the forward displacement (focal movement). A 2-way repeated measures ANOVA (height x stance position) was used to analyse the data.
Initial Position of the CoP. The initial position of the CoP remained constant across all conditions (4.7 cm anterior to the ankle joint) except when participants stood with the heels positioned at the edge of the high platform. In this case, the CoP moved further forward (1.0 cm), away from the edge of the high platform.
Final position of the CoP. The final position of the CoP was constant across all conditions on the low platform and when away from the edge of the high platform (14.5 cm anterior to the ankle joint). However, with the toes at the edge of the high platform, the final CoP position moved anterior .5 cm; positioning the heels at the edge of the high platform moved the final CoP anterior by .5 cm. In both conditions, this shift is away from the platform edge.
Timing of the Postural Adjustment and Focal Movement. The duration of the postural adjustment and the focal movement remained constant across all conditions (Figure 1). The mean durations of the postural adjustment and focal movement were 320 and 580 ms, respectively.
Figure 1. Duration of the initial backward and subsequent forward displacement of the CoP when rising onto the toes. The peak backward displacement of the CoP was defined as 0 ms. The shaded vertical bars represent the performance (1 sd) away from the platform edge. Values present mean plus 1 standard deviation.
Figure 2. Magnitude of the initial backward and subsequent forward displacement of the CoP. The shaded horizontal bars represent performance (1 sd) away from the platform edge. Values present mean plus 1 standard deviation.
Magnitude of the Postural Adjustment. The magnitude of the postural adjustment (initial position Ð peak backward displacement) was constant across all experimental conditions (2.7 cm), except when standing with the toes positioned at the edge of the high platform (Figure 2). In this case, backward displacement of the CoP increased by .6 cm.
Magnitude of the Focal Movement. The magnitude of the focal movement (final position Ð peak backward displacement) was constant across conditions (9.7 cm) except when standing at the edge of the high platform. Both stance directions, toes and heels at the edge, resulted in less forward displacement (.7 cm) of the CoP.
Performance of the rise-to-toes task was altered only when standing with the toes or heels positioned at the edge of the high platform. As these conditions presented the greatest threat to safety, we assume the performance changes were induced by a fear of falling. The postural adjustment was modified differently for the toes versus heels positioned at the edge of the platform: with the toes at the edge the initial backward displacement of the CoP increased and with the heels at the edge, the initial CoP position was moved forward. These strategies likely are adopted to overcome an increased level of joint stiffness which we have demonstrated in earlier studies (Carpenter et al. 1998).
The focal movement also was modified when standing at the edge of the high platform. . For both toes and heels at the edge, the forward displacement of the CoP was decreased. This change in the movement strategy positioned the CoP further from the edge of the platform, thereby allowing a greater margin of safety.
Belinkii V.Ye. et al. Elements of control of voluntary movements. Biofizika, 12, 135-144, 1967.
Brown, L.A. and Frank, J.S. Postural compensations to the potential consequences of instability: kinematics. Gait and Posture, 6, 89-97, 1997.
Carpenter, M.G. et al. Surface height effects on postural control: A hypothesis for a stiffness strategy for stance. J. Vest. Res. submitted, 1998.
Research supported by a grant from NSERC.