Influence of vibration on the equilibrium of patients with unilateral labyrinthine dysfunction]

To determine the role of proprioceptors of different skeletal muscles in posture control in patients with unilateral labyrinthine dysfunction (ULD), we studied the effect of vibration on those muscles by postulography. Subjects were 59 healthy people and 20 ULD patients. We measured the length of the displacement of the center of gravity, maximum sway length, and sway area. Significant differences were observed between healthy and ULD subjects on stimulation of the muscle groups of the upper and lower extremity during vibration. In patients with ULD, vibration to the dorsal neck muscles caused a deviation toward the diseased side. We speculate that the upper dorsal neck muscle plays an important role in maintaining body balance in the frontal plane in patients with ULD. The upper and lower extremity muscles play a significant role in adjusting standing posture.     

Influence of neck vibration on body sway

To investigate the influence of the cervical input to the equilibrium, the effect of neck vibratory stimulation on body sway was analyzed in 49 normal human subjects. Body perturbations during standing posture were recorded by a force platform with or without vibratory stimulus on the upper cervical region, and analyzed by computer. During the neck vibratory stimulation, the center of gravity was shifted to the forward, and the amplitude of the body sway was increased especially along the front-rear axis. These results indicate that the proprioceptive inputs from the cervical receptors largely modifies the body equilibrium in normal subjects.     

Influence of vibration to the neck, trunk and lower extremity muscles on equilibrium in normal subjects and patients with unilateral labyrinthine dysfunction

To investigate the role of proprioceptors of different skeletal muscles in postural control, in normal subjects and patients with unilateral labyrinthine dysfunction (ULD), the effect of vibration on these muscles was studied by postulography. The subjects comprised 59 normal subjects and 12 patients with ULD due to resection of acoustic tumours. Sagittal body sway was observed during vibration to the triceps surae, tibialis anterior and upper dorsal neck muscles. No significant change in sway was observed in the frontal plane in normal subjects. Significant differences between normal subjects and patients were found on stimulation of the muscle groups of triceps surae and biceps femoris during vibration. In patients with ULD, vibration to the dorsal neck muscles caused a deviation towards the diseased side. It can be speculated that the upper dorsal neck muscle plays an important role in maintaining the body balance in the frontal plane in patients with ULD. On the other hand, the lower extremity muscles, especially the muscles on the dorsal side of the body, play a significant role in adjusting the standing posture in the sagittal plane.     

Influence of Vibration to the Neck,Trunk and Lower Extremity Muscles on Equilibrium in Normal Subjects and Patients with Unilateral Labyrinthine Dysfunction

To investigate the role of proprioceptors of different skeletal muscles in postural control, in normal subjects and patients with unilateral labyrinthine dysfunction (ULD), the effect of vibration on these muscles was studied by postulography. The subjects comprised 59 normal subjects and 12 patients with ULD due to resection of acoustic tumours. Sagittal body sway was observed during vibration to the triceps surae, tibialis anterior and upper dorsal neck muscles. No significant change in sway was observed in the frontal plane in normal subjects. Significant differences between normal subjects and patients were found on stimulation of the muscle groups of triceps surae and biceps femoris during vibration. In patients with ULD, vibration to the dorsal neck muscles caused a deviation towards the diseased side. It can be speculated that the upper dorsal neck muscle plays an important role in maintaining the body balance in the frontal plane in patients with ULD. On the other hand, the lower extremity muscles, especially the muscles on the dorsal side of the body, play a significant role in adjusting the standing posture in the sagittal plane.     

Effect of proprioceptor stimulation on postural stability in patients with peripheral or central vestibular lesion

Body sway in upright stance at rest and after inducing proprioceptor stimulation, elicited by vibration applied to the calf or neck muscles, was studied in 11 patients with peripheral lesion and in 17 patients with central vestibular lesion. The responses were compared with those of 20 normal subjects. Vibratory stimulus was applied at five different frequencies, ranging from 32 to 150 Hz, and at a constant amplitude of 2.1 mm. Postural stability was measured with a force platform in terms of average deviation of body position (ADBP) analyzed in relation to the individual maximum support distance in the anterio-posterior direction. In patients with peripheral vestibular lesion ADBP was moderately increased, compared to normal subjects, when the calf muscles were exposed to vibration under eyes closed conditions (i.e. no visual information available); stimulation of neck muscles both under eyes open and eyes closed conditions and stimulation of calf muscles with open eyes produced an ADBP of the same magnitude as in controls. In patients with central vestibular lesion, proprioceptor stimulation of calf and neck muscles caused increased ADBP whether with eyes open or closed. The ADBP induced by stimulation of neck muscles was significantly greater in patients with a central lesion than in those with a peripheral vestibular lesion. The results indicate that patients with peripheral lesion differ from those with central vestibular lesion in their reaction to proprioceptor stimulus; and that in patients with central vestibular lesion proprioceptor stimulation of the neck muscles produces disproportionately powerful cervico-collic reflexes.     

The importance of cervical muscles in responses of galvanic body sway test

Body sway in normal subjects was analyzed by means of various methods to study a role of cervical muscles in galvanic body sway test. Galvanic stimulation through the retro-auricular electrode induced an initial response and a deviation response in body sway. When the anodal stimulation was given through the right retro-auricle during standing, a deviation response toward the right side was observed. While keeping a posture weighted on one foot, the stimulation induced a similar response. The stimulation during squatting produced also a deviation response toward the right side. When the head was rotated to the right, the stimulation produced backward responses. When rotated to the left, it produced forward responses. Even without galvanic stimulation, similar responses were also induced by some other method, for example, inclining the head to one side. Galvanic stimulation while sitting resulted in slight but apparent head inclination. The results suggested that cervical muscles played an important role in galvanic body sway test. Initial and deviation responses appeared to be secondarily produced by changes in the cervical muscular tension.     

Proprioceptive influence on the optokinetic nystagmus

The influence of neck and leg proprioceptive inputs on optokinetic-induced quick phases was studied in humans. Ten subjects received unidirectional horizontal optokinetic stimulation (10-20%/s) during sinusoidal neck, leg and combined neck + leg proprioceptive stimulation. The optokinetic reflex was measured by electro-oculography. Neck stimulation induced a shift in the nystagmus beating field in the opposite direction to body movement (gain 0.3 0.4, phase 140-180 degrees). The beating field shift resulted totally from the amplitude and frequency modulation of optokinetic quick phases, as slow phases were not affected. Leg proprioceptive stimulation induced a similar effect, but the phase of the response lagged by approximately 90 degrees compared with that of neck response. With combined neck + leg stimulation, the amplitude of the effect was a sum of the separate effects, but the phase coincided with that of the leg response. This suggests that neck and leg proprioceptive signals do not add linearly and that the leg signal determines the time of the response.     

Neck Muscle Vibration Alters Visually Perceived Roll in Normals

The objective of this study was to determine whether vibration of dorsal neck muscles or of the mastoid bone or of both modified the perception of visual orientation in the head roll-tilt plane in normal subjects. Measurements of the subjective visual vertical (SVV) were obtained from 26 normal human subjects. Subjects reported the SVV in the upright and in the left and right 30° static head roll-tilt positions. Subjects then reported the SVV while vibration was applied to the left or right dorsal neck or left or right mastoid. Both head position and vibration independently modified settings of the SVV. In head-tilted positions, vibration of the upper dorsal neck muscles (on the side of the head opposite to the head tilt) caused a significantly greater shift of the SVV in the opposite direction of head roll-tilt compared to vibration of the lower dorsal neck muscles or of the mastoid. These results support a role for cervical somatosensory information in perception of visual orientation in the roll plane. Our findings may help explain the differences observed in visual orientation perception in normal subjects between head alone and whole-body roll-tilt. Finally, vibration of neck muscles in the head roll-tilted plane may be a useful method to test cervical somatosensory function possibly by increasing their response to external stimulation. The views expressed in this article are those of the author and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, nor the U. S. Government.     

Influence of subjects' height on the stabilization of posture.

In order to investigate the influence of subjects' height in stabilization of body sway, postural EMG reactions were analysed following perturbation of posture during stance on a force measuring platform. Perturbing momenta of different strengths were unexpectedly applied at the back (level of the center of gravity) after being matched to the body weight of each subject. EMG activity of the antagonistic leg muscles and head, hip and ankle joint movements were recorded. There was a close correlation between displacement amplitude at the ankle joint and height of the subject, with the largest displacements in small subjects. The consequence of this relationship was that 1) The compensatory reactions consisted of larger gastrocnemius responses and a stronger coactivation of the tibialis anterior; 2) Momenta of increasing strength resulted in a larger increment of both ankle joint displacement and gastrocnemius EMG responses in small compared to larger subjects. In analogy to tip-toeing movements, it is concluded that the coactivation pattern is typical for stance conditions with a restricted area of support in order to reduce body sway. On the basis of latency measurements it is suggested that the response pattern is induced by proprioceptive information from the impact site of the momentum.     

Gait disturbance in aging and equilibrium disorders--body sway research no. 45

Initiation of gait in relation to aging in normal adults and patients with equilibrium disorders were examined by a large force platform to analyze trace of the center of foot pressure while walking with eyes open. Sixty normal adults and fifty-one patients with equilibrium disorders underwent examinations. Normal adult subjects were divided into three groups by depending on the age; the young age group (20-39 years), the middle age group (40-64 years), and the aged group (over 65 years). The patients consisted of peripheral vestibular disorders and central equilibrium disorders. Analyzed items were step length (mm), step width (mm), cadence (steps/min), walking speed (m/min), step length/body height (step length ratio), step width/step length, and LG/LS. The aged group showed characteristic gait pattern: marked shortness of step length, broadness of step width, slight decrease in cadence, slowness of walking speed and increased body sway during walking. Instable body sway was expressed by step width/step length and LG/LS. No statistically significant difference was obtained between the normal young and the normal middle age groups. The patients with central equilibrium disorders showed shortness of step length in comparison with contemporary normal generations. Both patients with peripheral and central disorders revealed remarkable lower walking speed. In the patients with central disorders the walking was characterized with increase of body sway during their walking.     

Age-related changes in human posture control: motor coordination tests.

Postural responses to support surface displacements were measured in 214 normal human subjects ranging in age from 7 to 81 y. Motor tests measured leg muscle electromyographic (EMG) latencies, body sway, and the amplitude and timing of changes in center of pressure displacements in response to sudden forward and backward horizontal translations of the support surface upon which the subjects stood. There were small increases in both EMG latencies and the time to reach the peak amplitude of center of pressure responses with increasing age. The amplitude of center of pressure responses showed no change with age if the amplitude measures were normalized by a factor related to subject height. In general, postural responses to sudden translations showed minimal changes with age, and all age-related trends that were identified were small relative to the variability within the population.     

Cervical muscle afferents play a dominant role over vestibular afferents during bilateral vibration of neck muscles

A previous study showed that vibratory stimulation of neck muscles in humans induced short-latency electromyographic (EMG) activation of lower leg muscles, producing postural reactions at the feet. These findings indicated that cervical proprioception contributes to stabilization of stance through rapidly integrated pathways. However, as vibration may excite both proprioceptive and vestibular afferents, and because of the proximity of neck muscles to the vestibular apparatus, neck muscle vibration could also have activated the vestibular system thereby contributing to the effect observed. To investigate any possible contribution of vestibular stimulation, vibratory stimuli were applied bilaterally and separately to the splenius muscles of the neck and the planum mastoideum overlying the vestibular organs. Ten normal subjects, with eyes closed, were exposed to vibratory stimulation of two different amplitudes and frequencies. Responses were assessed by EMG activity recorded from tibialis anterior and gastrocnemius muscles of both legs and by changes in center of pressure as measured by a force platform. Results indicated that vibration induced reproducible EMG and postural responses in the anteroposterior direction, particularly on cessation of vibration. EMG and postural responses were considerably lower and less consistent with mastoid vibration compared with neck muscles vibration. Previous reports suggest that vibratory stimulation could propagate to the vestibular organs and generate a vestibular-induced postural activation. However, our findings indicate that cervical muscles afferents play a dominant role over vestibular afferents when vibration is directed towards the neck muscles.     

Head and body sway in response to vertical visual stimulation

CONCLUSIONS: Postural responses differed according to the stimulus direction, i.e. vertical visual stimulation induced head rather than trunk displacements. Accordingly, it could be that center of foot pressure (COP) responses tended to underestimate the postural sway during visual stimulation. OBJECTIVES: To investigate head and body sway in response to vertical visual surround motion, and to examine the correlation between the displacements of head and body segments derived from video-motion analysis and COP measurements. MATERIAL AND METHODS: Postural sway was assessed in 10 young female subjects by video-motion analysis of four different head and body segments, and by use of force-plate posturography. Head and body sway in the pitch plane was induced by rotating a random pattern of dots about the subject's inter-aural axis at a constant acceleration of 1 degree/s(2) or a constant velocity of 60 degrees/s in darkness. RESULTS: Generally, head displacement was greater than that of other body parts during vertical optokinetic stimulation (OKS). In most subjects, maximum head displacements were induced in the same direction as the visual motion. Downward OKS induced a forward head and body sway. The COP trajectory correlated well with the displacements of each head and body segment during downward OKS. In contrast, postural responses to upward OKS were complicated in terms of their time course. The correlation coefficient between each head and body segment and the COP varied among individuals for upward OKS.     

Paraspinal muscle response to electrical vestibular stimulation

Galvanic (electrical) vestibular stimulation (GVS) has been used to study the role of the vestibular system in postural control by inducing postural sway in standing subjects. The purpose of this study was to determine the timing and pattern of activation in the paraspinal muscles in response to GVS and to compare these responses with those in the muscles of the lower leg. Binaural-bipolar GVS was applied to the skin overlying the mastoid processes of 10 subjects while they stood on a force plate with their eyes closed. The stimulus consisted of a 0.6 mA 5-pulse sequence. Each pulse lasted for 2 s, followed by 4 s of rest. The centre of pressure (COP) vs. time for each trial was calculated from the reaction forces and moments. Surface electromyographic (EMG) signals from the paraspinal and gastrocnemius muscles were recorded bilaterally. The EMG signals were rectified and integrated (iEMG). The iEMG from the muscles on the cathodal side of the body were then subtracted from the iEMG of the anodal side muscles, to yield a differential EMG (dEMG). Both the paraspinal and gastrocnemius muscles became activated in response to the stimulus. The pattern of activation was consistent with the changes observed in the centre of pressure. The primary response in both muscles acted to move the body toward the anode. This primary response began at 74 +/- 20 ms in the paraspinal muscles and at 118 +/- 18 ms in the gastrocnemius. A second component of the response began at 232 +/- 27 ms in the paraspinal muscles and 262 +/- 54 ms in the gastrocnemius muscles. This second phase of the response was opposite in direction to the primary response and was responsible for decelerating the body and maintaining the deviated position of the centre of mass over the base of support. Following the termination of the stimulus, the opposite pattern of muscle activation in both the paraspinal and the gastrocnemius muscles was observed. The results of this study suggest that the paraspinal muscles may play a significant role in the frontal plane response to vestibular stimulation during stance in humans.     

The deviation of the body's sway center with galvanic stimulation

We investigated galvanic body sway of ten healthy adults and eight patients by using the averaging program for measuring T1 and T2, which were latencies at the onset and at the cutoff of galvanic stimulation, respectively, and the plotting program, which adopted 1.73 sec as T1 and 0.39 sec as T2, for calculating the coordinates of the body's sway center. We estimated galvanic body sway with the difference between the coordinate of the body's sway center during stimulation and that during no stimulation. The deviation in lateral direction on the healthy subjects ranged from 0.7 or 0.9 to 2.1 cm at right- or left-side stimulation. One of the patients with peripheral disorders revealed differences between right- and left-side stimulation. The other patients did not reveal any differences. In antero-posterior direction six of the healthy subjects deviated backward at right-side stimulation and nine subjects deviated backward at left-side stimulation. The patients with peripheral disorders did not reveal any differences. The other patients revealed some differences. Waves computed by the averaging program do not always show the quantity of galvanic body sway. The deviation of the body's sway center shows the quantity of body sway consistently. The body's sway center is considered to be a suitable parameter in the galvanic body sway test for statistical analysis.     

Head and body sway in response to vertical visual stimulation

Conclusions. Postural responses differed according to the stimulus direction, i.e. vertical visual stimulation induced head rather than trunk displacements. Accordingly, it could be that center of foot pressure (COP) responses tended to underestimate the postural sway during visual stimulation. Objectives. To investigate head and body sway in response to vertical visual surround motion, and to examine the correlation between the displacements of head and body segments derived from video-motion analysis and COP measurements. Material and methods. Postural sway was assessed in 10 young female subjects by video-motion analysis of four different head and body segments, and by use of force-plate posturography. Head and body sway in the pitch plane was induced by rotating a random pattern of dots about the subject's inter-aural axis at a constant acceleration of 1°/s² or a constant velocity of 60°/s in darkness. Results. Generally, head displacement was greater than that of other body parts during vertical optokinetic stimulation (OKS). In most subjects, maximum head displacements were induced in the same direction as the visual motion. Downward OKS induced a forward head and body sway. The COP trajectory correlated well with the displacements of each head and body segment during downward OKS. In contrast, postural responses to upward OKS were complicated in terms of their time course. The correlation coefficient between each head and body segment and the COP varied among individuals for upward OKS.     

The effects of stochastic monopolar galvanic vestibular stimulation on human postural sway

Galvanic vestibular stimulation (GVS) is a technique in which small currents are delivered transcutaneously to the afferent nerve endings of the vestibular system through electrodes placed over the mastoid bones. The applied current alters the firing rates of the peripheral vestibular afferents, causing a shift in a standing subject's vestibular perception and a corresponding postural sway. Previously, we showed that in subjects who are facing forward, stochastic bipolar binaural GVS leads to coherent stochastic mediolateral postural sway. The goal of this pilot study was to extend that work and to test the hypothesis that in subjects who are facing forward, stochastic monopolar binaural GVS leads to coherent stochastic anteroposterior postural sway. Stochastic monopolar binaural GVS was applied to ten healthy young subjects. Twenty-four trials, each containing a different galvanic input stimulus from among eight different frequency ranges, were conducted on each subject. Postural sway was evaluated through analysis of the center-of-pressure (COP) displacements under each subject's feet. Spectral analysis was performed on the galvanic stimuli and the COP displacement time series to calculate the coherence spectra. Significant coherence was found between the galvanic input signal and the anteroposterior COP displacement in some of the trials (i.e., at least one) in nine of the ten subjects. In general, the coherence values were highest for the mid-range frequencies that were tested, and lowest for the low- and high-range frequencies. However, the coherence values we obtained were lower than those we previously reported for stochastic bipolar binaural GVS and mediolateral sway. These differences may be due to fundamental characteristics of the vestibular system such as lower sensitivity to symmetric changes in afferent firing dynamics, and/or differences between the biomechanics of anteroposterior and mediolateral sway.     

Effects of neck muscle vibration on subjective visual vertical: comparative analysis with effects on nystagmus

In patients with unilateral vestibular dysfunction, vibratory stimulation to the neck muscles not only induces shift of the subjective visual vertical (SVV), but also enhances the generation of nystagmus. In the present study, the effects of neck vibration on the SVV were compared with those on nystagmus in patients with unilateral vestibular schwannoma (14 patients; 6 males and 8 females, mean age 54.2 years). The results indicated that the presence of nystagmus and magnitude of the SVV were generally correlated, neck vibration significantly increased the abnormal shift of the SVV and the presence of nystagmus, and the effects of vibration to the ipsilateral dorsal neck were significantly larger than those to the contralateral dorsal neck on the SVV, whereas no significant difference was observed in slow phase velocity of nystagmus. The present study suggests that both SVV and nystagmus induced by vibration have many similar clinical features and may be important in assessing the unilateral vestibular dysfunction.     

Nonsequential vector analysis of body sway in Menière's disease]

Body sway test which examines the vestibulo-spinal reflex is sensitive for evaluating vestibular dysfunction in patients with vertigo, dizziness or unsteadiness. Body sway is usually evaluated by measuring the length and area of traces on the force platform. However, it is not necessarily possible to evaluate abnormal body sway with these indicators because there is a discrepancy between sway length and area. In the present study, 8 directional vectro-posturography was developed and applied to the patients with Meniere's disease. Averaging of measurements at moving 3 points was used to eliminate high frequency noise over 4.92Hz. The subjects consisted of 66 patients with unilateral Meniere's disease and 31 normal controls. Eight directional vectrogram allowed measurement of shift area and directionality, simultaneously. The patient's group showed statistically significant body sway in the right forward and left backward direction with eyes open, in all the directions with eyes closed, when compared with the control's group. The effect of vision on postural stability was studied by obtaining 8 directional Romberg's quotients. The patients with Meniere's disease had significantly in forward-backward, and left-forward and right-backward direction when compared with controls. However, laterality of the affected side was not detected from the vectrogram. The effect of endolymphatic sac operation on postural stability was evaluated in the patients of unilateral Meniere's disease who showed progressive hearing loss and resisted conservative medication therapy. The results showed unstable body sway in the condition with eyes closed until at least 4 months after the operation. The patients with Meniere's disease presented pathological body sway, even during symptom free periods, when examined with 8 directional vector posturography. Power spectral analysis of body sway indicated that most energy was distributed at frequencies lower than 0.5Hz. Nevertheless, directional preponderance of body sway was characterized by higher frequency components. The findings infers that postural stability at the frequency range over 0.5Hz, may be governed by the input of vestibular organs.     

The role of the labyrinth, proprioception and plantar mechanosensors in the maintenance of an upright posture

The maintenance of an upright posture in man requires information from vision, the labyrinth, proprioception and plantar mechanosensors. In order to evaluate the role of the labyrinth, proprioception and plantar mechanosensors, stabilometry was performed in subjects with closed eyes. Ten patients with bilateral severe or complete labyrinthine paresis were studied, as well as 9 patients with severe proprioceptive disorders and 10 normal healthy persons whose plantar mechanosensors were anesthetized by hypothermia. Both the area of sway and the total locus length (accumulated shift distance length) were evaluated. On closing eyes, in patients with labyrinthine disorders demonstrated that the area of sway increased more than length. On the other hand, in patients with proprioceptive disorders, length increased more than the area. In plantar anesthetized subjects, similar to the labyrinthine disorder cases, the area of sway increased more than length. These findings suggest that the labyrinth is a main monitor of the area of body sway, while proprioception is a principle monitor of the velocity of body movement of sway (or locus length). The plantar mechanosensor monitors the area of body sway similar to the labyrinth, but works less than the labyrinth. The locus length is the distance per minute and reflects the velocity of body sway. Thus, the length per area is a parameter for the velocity of body sway per area. Since proprioceptive disorders increase both the locus length and the length per area, present findings suggest that if proprioception is damaged, the body begins to move faster. Compensated labyrinthine disorders have a tendency to increase the length per area, indicating that if a labyrinthine disorder is compensated, the body adapts and moves faster to maintain an upright posture.