Do Head Position and Active Head Movements Influence Postural Stability?
Do Head Position and Active Head Movements Influence Postural Stability?
Abstract and Keywords
The differential effects of voluntary head movements (horizontal oscillations about the z axis) and varying head positions on postural sway are investigated in this chapter. The methods of the experiments are presented here. Ten subjects took part in the experiments, where their head movements were recorded using a head-fixed angular accelerometer. The main finding of the experiment is that the differential effects of varying head positions were surprisingly small. Moreover, the body sway does not significantly increase with head rotation and that indicated the precise reevaluation of head sway with respect to the head position relative to the trunk.
Most studies on postural sway in man have been performed under standard conditions (i.e., head upright and gaze straight ahead) (Edwards, 1946; Paulus et al., 1984). Under natural conditions, however, stabilization of upright stance involves different head positions relative to the trunk as well as head movements, [n this context, we investigated the differential effects of voluntary head movements (horizontal oscillations about the z axis) and varying head positions on postural sway.
Ten healthy subjects (3 women and 7 men; mean age 32 ± 5.4 years) took part in both experiments. Measurements of body sway were recorded with a piezoelectric force measuring platform (Kistler, type 9281b). These measurements were used to calculate the change in position of the subject's center of pressure independent of the weight of the subject. The sway path was calculated by a microcomputer for 25.6-second segments in meters/minute (burst acquisition of multiple channel data; sampling interval 25 ms; analog data bandwidth from DC to 25 Hz). The subjects stood on the platform at a distance of 1 m from a cylindrical structured screen of 2 m in diameter. The subjects stood on a slab of foam rubber (10 cm thickness, density 50 g/dm3) that was covered by a second rigid foot support in order to increase postural sway by reducing the reliability of the somatosensory liferents. Head movements were recorded by a head-fixed angular accelerometer. The accelerometer did not provide somatosensory feedback about head position.
Student's t test was used to test the data for significance.
The following conditions were tested in a randomized sequence:
1. Eyes open, head straight ahead
2. Eyes closed, head straight ahead
3/4. Eyes open/closed, head maximally retroflected (head extension)
5/6. Eyes open/closed, head maximally antefiected
7/8. Eyes open/closed, head tilted toward the right ear (40° roll)
9/10. Eyes open/closed, head tilted toward the left ear (40° roll)
11/12. Eyes open/closed, head turned to the right (40° yaw)
13/14. Eyes open/closed, head turned to the left (40° yaw)
The subjects performed active head oscillations about the z axis at a constant amplitude of ± 10° either while fixating a central laser spot [use of the vestibulo-ocular reflex VOR)] or by pursuing an oscillating laser spot (fixation suppression of the VOR). In both conditions an oscillating laser spot served to adjust head movement amplitude. The following conditions were tested:
1. Eyes open (no head movement)
2. Eyes closed (no head movement)
3. Active head rotation during pursuit of an oscillating laser spot at a frequency of 0.02, 0.05, 0.1, 0.2, 0.5, 1.0, or 1.5 Hz (VOR suppression)
4. Active head rotation during fixation of a laser spot straight ahead with the same frequencies as in condition 3 (VOR).
For all conditions tested, sway path values (fore-aft and lateral body sway path in meters/minute) with the eyes closed were three to four times higher than with the eyes open (Romberg quotient) (Figs. 87–1 and 87–2). There was a slight but statistically insignificant increase of sway path for the conditions with lateral tilt of the head and with anteflection of the head. Similarly, the sway path increased only insignificantly when the head was turned either to the right or to the left (Fig. 87–3). Only with the head in retroflection did the sway path show a statistically significant increase in comparison with normal upright head position
Active head oscillation while fixating straight ahead (thus minimizing the retinal slip of the environment in comparison to the condition without fixating a stationary laser spot) increased sway path only at head rotation frequencies above 0.5 Hz (Fig. 87–4). With head oscillation at a frequency of 1.0 Hz (±10° amplitude) the sway path increased from 0.59 m/min with straight-ahead fixation (no head movements, eyes open) to 1.04 m/min without fixation.
When tracking the laser spot with the head—and thus inducing concurrent retinal slip of the visual scene—sway path increase was somewhat greater than when fixating straight ahead (Fig. 87–5). The difference of the sway path was 0.13 m/min at a frequency of 1.0 Hz (1.17 m/min when pursuing the oscillating laser spot versus 1.04 m/min when fixating a stationary target straight ahead). Even with a head oscillation of 1.5 Hz, stabilization was better (1.7 ± 1.02 m/min) than with eyes closed (2.1 ± 0.58 m/min).
The differential effects of varying head positions on postural sway were measured in normal subjects and were surprisingly small. This was unexpected because changes in head position considerably alter the spatial coordinates for two of the three stabilizing sensory systems, vision and equilibrium. For example, horizontal accelerations as well as retinal slips from right to left indicate fore-aft movements of the body when the head is turned sideways (yaw). The awareness of the head position relative to the trunk is based mainly on the force necessary to maintain its position rather than on afferent input from the neck. That body sway does not significantly increase with head rotation indicates the precise reevaluation of head sway with respect to the actual position of the head relative to the trunk. With prolonged.
Changes of head position in pitch, particularly with head extension, significantly increased the postural sway in the fore-aft direction Brandt et al., 1981). With the head maximally extended, the saccular maculae (approximately parallel to the ipsilateral anterior semicircular canal) are rotated, which causes a corresponding rotation of shear forces. In this position the utriclar otoliths are beyond their optimal working range since the plane of the utricular maculae (approximately parallel to the horizontal semicircular canal) is elevated relative to its normal horizontal (20° flexion) orientation. Thus when the head is maximally extended the two sensors of the vestibular system—the semicircular canals and the otoliths—that transduce accelerations of the head as well as the gravitational force, are out of their working range. Visual cues, which also correct posture, change their direction—vertical retinal slip of the stationary target viewed with the head elevated now indicates fore-aft sway. From other experiments there is some evidence that different head positions affected latencies and muscular activities of the vestibulospinal reflexes (Brandt et al., 1988; Fries et al., 1988; Tokita et al., 1989) as well as the compensation patterns of body perturbations (Diener et al., 1986).
Active sinusoidal oscillations of the head with a constant amplitude of ± 10° increase body sway only above a frequency of 0.5 Hz. With increasing frequency of head oscillation mechanical contamination of body sway must be suspected, preventing an evaluation of the net sensory effects. The lack of visual stabilization is reflected by comparison of the conditions during activation of the VOR versus VOR suppression (Straube et al., 1989). The major goal of these experiments, however, was to quantify the effects in relation to the frequency for a constant amplitude. The increase in sway path was approximately logarithmic from 0.51 (0.02 Hz) to 1.7 (1.5 Hz), with a correlation factor r = .98.
A possible explanation for the small effects of head oscillations and the change of head position is that in healthy subjects the vestibular system contributes only a little to the modification of the fast, reflex-like motor compensation of posture (Dichgans and Diener, 1989). However, the vestibular systems may contribute more to the choice of motor strategies and the compensation of slow, continuous displacements of the body.