Cervical spine ROM measurements: optimizing the testing protocol by using a 3D ultrasound-based motion analysis system.

The aim of this study was to evaluate the intra- and inter-examiner reliability and validity of neck range of motion (ROM) measurements. Thirty-five healthy subjects were assessed in all neck movements from two initial positions, sitting and standing, actively (open and closed eyes) and passively by using a 3D ultrasound-based motion analysis device (Zebris). Three tests were employed to assess intra-examiner reliability and two examiners used for the inter-examiner reliability. X-rays in neck flexion and extension were used to validate the Zebris system. The standing position yielded higher intraclass correlation coefficient (ICC) values (>0.86) with less error [smallest detectable difference (SDD)<13.8%] than sitting (ICC>0.79, SDD<14%). Passive assessment of neck ROM presented better reproducibility than active assessment with open or closed eyes in both positions. The inter-examiner reliability was moderate (ICC=0.43-0.68). The correlation between the Zebris system and X-rays was high in both flexion and extension movements. The results showed that the most reliable protocol for assessment of neck ROM is a passive measurement in the standing position. The measurements were well validiated against X-rays and the experience of the investigators must be considered before any comparison among studies is employed.

Influence of motion parallax in the control of spontaneous body sway.

Visual control of postural sway during quiet standing was investigated in normal subjects to see if motion parallax cues were able to improve postural stability. In experiment 1, six normal subjects fixated a fluorescent foreground target, either alone or in the presence of full room illumination. The results showed that subjects reduced body sway when the background was visible. This effect, however, could be mediated not only by parallax cues but also by an increase in the total area of visual field involved. In experiment 2, other parameters such as image angular size and target distance were controlled for. Twelve subjects fixated a two light-emitting diode (LED) target placed at 45 cm from their eyes in a dark room. A second similar two-LED target was placed either at 170 cm (maximum parallax) or at 85 cm (medium parallax) from the fixated target, or in the same plane of the fixated target (0 cm, no parallax). It was found that the amplitude of sway was reduced significantly, by approximately 20%, when the two targets were presented in depth (parallax present) as compared to when they were in the same plane (no parallax). The effect was only present in the lateral direction and for low frequency components of sway (up to 0.5 Hz). We confirmed in experiment 3 on eight subjects with a design similar to that used in experiment 2 that the effect of motion parallax on body sway was of monocular origin since observed with monocular and binocular vision. Geometrical considerations based on these results support the existence of two modes of visual detection of body sway, afferent (retinal slippage) and efferent (extra-retinal or eye-movement based).

The effects of hyperventilation on postural control mechanisms.

The effect of hyperventilation on postural balance was investigated. Voluntary hyperventilation increased body sway in normal subjects, particularly in the sagittal plane. The possibility that this hyperventilation-induced unsteadiness is due to interference with lower limb somatosensory input, vestibular reflexes or cerebellar function was assessed. (i) The effect of hyperventilation on peripheral compound sensory action potentials (SAPs) and somatosensory evoked potentials (SEPs) (recorded centrally, from the scalp) elicited by electrical stimulation of the sural nerve was measured in six normal adults. A reduction in the scalp SEP amplitude and an increase in the peripheral SAP amplitude were observed during hyperventilation, which reversed during the recovery period. These changes indicate increased peripheral neural excitability which could lead to a higher level of ectopic activity; the latter would interfere with central reception of peripheral input. (ii) The click-evoked vestibulo-collic reflex was recorded to study the effect of hyperventilation on vestibulo-spinal activity. EMG recordings from both sternocleidomastoid muscles of six healthy subjects were made in response to loud clicks presented to either ear. Neither the amplitude nor the latency of the response were altered significantly by hyperventilation. (iii) Eye-movement recordings were obtained in the six normal subjects to assess the effect of hyperventilation on the vestibulo-ocular reflex and its visual suppression, the latter being a function largely mediated by the cerebellum; no changes were detected. (iv) Three-dimensional eye-movement recordings and body-sway measurements were obtained in six patients with longstanding unilateral vestibular loss in order to evaluate if hyperventilation disrupts vestibular compensation. In all patients, a horizontal nystagmus either appeared or was significantly enhanced for > or = 60 s after voluntary hyperventilation. Sway was also enhanced by hyperventilation in these patients, particularly in the frontal plane. This study suggests that hyperventilation disrupts mechanisms mediating vestibular compensation. The increase in sway may be, at least partly, mediated by deranged peripheral and central somatosensory signals from the lower limbs. Hyperventilation seems to spare vestibular reflex activity and cerebellar-mediated eye movements.

Hyperventilation effect on postural sway.

OBJECTIVE: To examine the effect of voluntary hyperventilation (HV) on postural sway. DESIGN: Crossover controlled, experimental study. SETTING: Human movement and balance clinical research unit. SUBJECTS: Four different groups of normal subjects (n = 6, 6, 7, and 9) and patients with bilateral absence of vestibular function (n = 9). INTERVENTION: Partial carbon dioxide pressure (tc-PCO2) was measured transcutaneously with surface electrodes. Body sway was measured with a force platform immediately after maximal voluntary HV for 30 to 90 seconds. Recordings were obtained with eyes open and eyes closed, standing on the platform and on foam-rubber, and after head or body movements. MAIN OUTCOME MEASURE: Postural sway. RESULTS: HV increased body sway in all conditions, but the effects were more intense when subjects were standing directly on the platform surface with their eyes closed. Recordings after HV of 30, 60, and 90 sec in normal subjects showed that although CO2 levels were inversely related to the duration of HV, body sway did not increase further. HV also increased sway after active movements by the subjects. The main sway increase was in sway area and mean and maximal deviations but less for mean sway velocity. HV preferentially increased low-frequency sway oscillations. These effects were also present in labyrinthine-defective subjects. CONCLUSIONS: HV increases body sway, but the relationship between CO2 levels and degree of unsteadiness is not linear. The dizziness reported by patients with HV syndrome may be partly caused by objective unsteadiness. The presence of HV-induced unsteadiness in patients with absent vestibular function indicates that the effects of HV are not mediated by the labyrinth.

Reorientation of visually evoked postural responses by different eye-in-orbit and head-on-trunk angular positions.

We examined the question of whether the position of the eyes in the head and of the head on the trunk influence the direction of visually elicited postural reactions. Normal subjects stood on a force platform viewing a large disc, rotating in the roll plane, always maintained orthogonal to the line of sight. The disc was presented at 0 degree, 30 degrees and 90 degrees to the right or left with respect to the mid-frontal plane of the subject's body and was viewed with various combinations of horizontal eye-in-orbit and head-on-trunk deviations. It was found that the main direction of body sway was always reoriented to be parallel to the disc (e.g. viewing the disc at 30 degrees oriented sway responses at a mean angle of 33 degrees). The largest sway responses were obtained when the disc was parallel to the sagittal plane of the body and was viewed with an ipsilateral eye-neck deviation totalling 90 degrees (head-on-trunk 60 degrees+eye-in-orbit 30 degrees). When eye and head deviations cancelled each other (i.e. eye-in-orbit +30 degrees combined with head-on-trunk -30 degrees), directional effects on sway also cancelled each other out. This result demonstrates that signals of eye-in-orbit and head-on-trunk position have the capability to redirect visuo-motor commands to the appropriate postural muscles. This allows vision to regulate postural balance whatever the position of the eyes in space. We speculate that this function is mediated by eye and neck proprioceptive signals (or alternatively by efference copy) with access to gain control mechanisms in the visuo-postural system.

Auditory and visual interactions in postural stabilization.

The interaction and subsequent interpretation of sensory feedback from different modalities are important determinants in the regulation of balance. The importance of sound in this respect is not, as yet, fully understood. The aim of the present study was to determine the interaction of specific auditory frequencies and vision on postural sway behaviour. The frequencies employed represent the geometrical mean of 23 of the 25 critical bandwidths of sound, each presented at two loudness levels (70 and 90 phones). Postural sway was recorded using a biomechanical measuring platform. As expected vision had a highly significant stabilizing effect on most sway parameters. The frequency of the sound, however, appeared to influence the regulation of anteroposterior sway, while increasing loudness tended to increase mediolateral sway. At some frequencies the sound appeared to compensate for the lack of visual feedback. The interaction of sound and vision, particularly in combinations that lead to increased sway behaviour, may have implications in the occurrence, and possible prevention, of industrial accidents.

The effect of eye/head deviation and visual conflict on visually evoked postural responses.

Three interrelated experiments on visually evoked postural responses (VEPR) are presented to investigate the effect of lack of coplanarity between retinal and body coordinates (Experiment I) and the effect of directionally conflicting information in the visual stimulus. Experiment I showed that the direction of VEPR is modified by eye-in-orbit and head-on-trunk position signals, presumably of proprioceptive origin. Experiments II and III showed that VEPR can be critically suppressed by the presence of conflict within the visual stimulus (Experiment II: a linear, tangential component of visual motion acting in the opposite direction to the main angular component of a roll-motion display; Experiment III: a non congruent "improbable" visual motion parallax linear motion stimulus). A conceptual model of the postural system is presented, incorporating a gain control unit for the visuo-postural loop with inputs from the ocular/cervical proprioceptive system and from intra- and inter-sensory conflict detectors (comparators).