Frantisek Hlavacka

Modification of human postural response to leg muscle vibration by electrical vestibular stimulation

In order to understand proprioceptive and vestibular contributions to human stance posture, the effect of electrical vestibular stimulation on body lean induced by leg muscle vibration was investigated. The magnitudes and directions of postural responses were registered as changes in the center of foot pressure (COP) with a force platform. Vestibular stimulation consisted of 1 mA, binaural, bipolar galvanic current and proprioceptive input from tibialis anterior or soleus muscles was altered vibratory stimulation. The body lean induced by combined vibratory and galvanic stimulation could be largely considered as a summation of responses evoked by the galvanic and vibratory stimulation alone. The results of the present study showed that both vestibular and proprioceptive signals play important roles in the estimation of internal representation of the body vertical.

The timing of galvanic vestibular stimulation affects responses to platform translation

We compared the effects of galvanic vestibular stimulation applied at 0, 0.5, 1.5 and 2.5 s prior to a backward platform translation on postural responses. The effect of the galvanic stimulation was largest on the final equilibrium position of the center of pressure (CoP). The largest effects occurred for the 0.5 and 0-s pre-period, when the dynamic CoP pressure changes in response to both the galvanic stimulus and the platform translation coincided. The shift in the final equilibrium position was also larger than the sum of the shifts for the galvanic stimulus and the platform translation alone for the 0.5 and 0-s pre-periods. The initial rate of change of the CoP response to the platform translation was not significantly affected in any condition. Changes in the peak CoP position could be accounted for by local interaction of CoP velocity changes induced by the galvanic and translation responses alone, but the changes in final equilibrium position could only be accounted for by a change in global body orientation. These findings suggest that the contribution of vestibulospinal information is greatest during the dynamic phase of the postural response, and that the vestibular system contributes most to the later components of the postural response, particularly to the final equilibrium position. These findings suggest that a nonlinear interaction between the vestibular signal induced by the galvanic current and the sensory stimuli produced by the platform translation occurs when the two stimuli are presented within 1 s, during the dynamic phase of the postural response to the galvanic stimulus. When presented at greater separations in time, the stimuli appear to be treated as independent events, such that no interaction occurs.

The age-related changes of trunk responses to Achilles tendon vibration

The contribution of different sensory modalities to balance control is modified by age. Postural responses to Achilles tendon vibration were investigated in order to understand the influence of age on proprioceptive input from lower legs in human stance. Postural responses to bilateral vibrations of Achilles tendon with 10s duration were recorded at three frequencies (40, 60 and 80 Hz) in 9 healthy young (range, 24-27 years) and in 9 healthy older adults (59-70 years). Subjects were instructed to keep standing on firm surface with eyes closed. They performed three trials in each of three vibration frequencies. Postural responses were characterized by displacement of the centre of foot pressure (CoP) and by kinematics of body segments in the anterior-posterior direction. Bilateral vibrations of Achilles tendon induced backward body lean increasing with frequency of vibration and with age. The leg angle response to vibration was found similar in both groups of subjects. Slight trunk tilts from vertical position were induced by vibration in young subjects while in older subjects the trunk tilted backward together with the whole body. This observation was supported also by the minimal change of hip angle in older subjects contrary to increased hip activity in young subjects. The findings showed that the trunk and hip angle responses to proprioceptive stimulation might be a good indicator of age-related destabilization in balance control.

Effectiveness of different visual biofeedback signals for human balance improvement

The aim of this study was to examine the effectiveness of visual biofeedback (VBF) signals from a force platform and accelerometer sensors placed on different body segments. The study was performed on 20 young subjects during standing on a firm and foam support surface with a VBF signal sensed from CoP, lower trunk (L5) and upper trunk (Th4). The VBF signal was controlled by 2D-movement of chosen body segment, which was presented as a red point on a monitor screen. Location of VBF signal had a significant effect on each postural parameter of CoP and trunk segments. RMS and amplitudes of postural sway in medial-lateral and anterior-posterior directions decreased during standing on both types of support surface due to VBF. L5-VBF and CoP-VBF significantly reduced CoP displacements and lower trunk tilts. Th4-VBF reduced upper trunk tilts. Frequency analysis of postural sway revealed a decrease of power spectral density (PSD) values in low frequency range (0.02-0.3Hz) and an increase of PSD values in higher frequency range (0.5-1.4 Hz) in the VBF conditions during the stance on the firm surface in anterior-posterior direction. Reduction of body sway was the most significant in the body segment from which the VBF signal was sensed. The CoP position and L5 position provided the best signals for VBF. Changes in frequency ranges of body sway suggest voluntary activation of balance control. The results open new opportunities to optimize VBF system for balance improvement using accelerometers.

Effects of Linear versus Sigmoid Coding of Visual or Audio Biofeedback for the Control of Upright Stance

Although both visual and audio biofeedback (BF) systems for postural control can reduce sway during stance, a direct comparison between the two systems has never been done. Further, comparing different coding designs of audio and visual BF may help in elucidating how BF information is integrated in the control of posture, and may improve knowledge for the design of innovative BF systems for postural control. The purpose of this paper is to compare the effects of linear versus sigmoid coding of trunk acceleration for audio and visual BF on postural sway in a group of eight, healthy subjects while standing on a foam surface. Results showed that sigmoid-coded audio BF reduced sway acceleration more than did a linear-coded audio BF, whereas a linear-coded visual BF reduced sway acceleration more than a sigmoid-coded visual BF. In addition, audio BF had larger effects on reducing center of pressure (COP) displacement whereas visual BF had larger effects on reducing trunk sway. These results suggest that audio and visual BF for postural control benefit from different types of sensory coding and each type of BF may encourage a different type of postural sway strategy

Static posturography and infraclinical postural instability in early-stage Parkinson's disease.

We read with interest the recent article by Chastan et al. on the discordance between postural instability measured by posturography and the absence of clinical symptoms in patients with early-stage Parkinson’s disease (PD). They compared nine very mildly affected PD patients (mean Hoehn and Yahr (H&Y) stage was 1.22 and the mean motor score of the Unified PD Rating Scale (UPDRS) was 10.3) and 18 age-matched control subjects. The PD patients swayed significantly over a greater area and with higher mediolateral energy during static balance conditions, both with open (OE) and with closed eyes (CE). Stance on a mobile platform (dynamic condition) revealed only slight differences: PD subjects had a greater sway area in the anteriorposterior direction when their eyes were closed. These results were interpreted to be a sign of infraclinical postural instability in early-stage PD. In order to verify this surprising finding of infraclinical postural instability, we tested static balance of early-stage PD patients and healthy controls. Eighteen consecutive PD patients without a history of falling or other balance problems were included. Their average age was 63.4 6 7.2 years, duration of PD was 2.8 6 1.4 years, H&Y score was 1.7 6 0.5, and the UPDRS motor score was 14.4 6 5.8. The control group consisted of 17 ageand sex-matched healthy subjects (average age was 64.1 6 6.0). All subjects underwent static balance measurements using a posturographic platform (Kistler, type 9281B). Their sway activity was recorded in a 30-second-long trial, once with OE and once with CE. They were instructed to remain in upright stance (feet next to each other, splayed at an angle of 308, arms hanging by the sides) and to refrain from any voluntary movements during the recording. Signals from the platform were amplified and digitized at a sampling rate of 40 Hz. The root mean square, sway path, and velocity values for mediolateral (X) and anterior-posterior (Y) displacements of the center of foot pressure were calculated and statistically evaluated on the basis of the raw data. Table 1 gives the mean 6 standard deviation of the tested variables. The results of statistical analysis by Student’s t-test are given as ‘‘P value.’’ Unlike Chastan et al., we did not find any significant difference in any of the tested variables between early-stage PD patients (even more clinically affected than Chastan’s patients) and healthy age-matched controls. Our study demonstrates that the issue of the clinical utility of static posturography for diagnosing early balance problems in PD is still open.

Velocity dependence of vestibular information for postural control on tilting surfaces

Vestibular information is known to be important for postural stability on tilting surfaces, but the relative importance of vestibular information across a wide range of surface tilt velocities is less clear. We compared how tilt velocity influences postural orientation and stability in nine subjects with bilateral vestibular loss and nine age-matched, control subjects. Subjects stood on a force platform that tilted 6 deg, toes-up at eight velocities (0.25 to 32 deg/s), with and without vision. Results showed that visual information effectively compensated for lack of vestibular information at all tilt velocities. However, with eyes closed, subjects with vestibular loss were most unstable within a critical tilt velocity range of 2 to 8 deg/s. Subjects with vestibular deficiency lost their balance in more than 90% of trials during the 4 deg/s condition, but never fell during slower tilts (0.25-1 deg/s) and fell only very rarely during faster tilts (16-32 deg/s). At the critical velocity range in which falls occurred, the body center of mass stayed aligned with respect to the surface, onset of ankle dorsiflexion was delayed, and there was delayed or absent gastrocnemius inhibition, suggesting that subjects were attempting to actively align their upper bodies with respect to the moving surface instead of to gravity. Vestibular information may be critical for stability at velocities of 2 to 8 deg/s because postural sway above 2 deg/s may be too fast to elicit stabilizing responses through the graviceptive somatosensory system, and postural sway below 8 deg/s may be too slow for somatosensory-triggered responses or passive stabilization from trunk inertia.

Vestibular-Somatosensory Interactions for Human Posture

The postural control system is organized around two main behavioral goals: equilibrium and orientation. The task of controlling equilibrium requires balancing all the forces acting on the body to control the position of the body’s center of mass. The task of maintaining orientation requires interpretation of sensory information from many sources to align body parts with reference to gravitoinertial forces and other environmental features. Automatic postural responses that restore equilibrium in response to external perturbations in stance return the body to a particular orientation; that is, a particular body position which is selected based on particular orientation references (Massion, 1992).

Parkinson's disease versus ageing: different postural responses to soleus muscle vibration

BACKGROUND Impairments of postural stability occur with increasing age and in neurodegenerative diseases like the Parkinsons disease (PD). While changes in balance have been described in many studies under steady-state conditions, less is known about the dynamic changes in balance following sudden transition to different sensory inputs. RESEARCH QUESTION The aim was to clarify different effects of age and Parkinsons disease on dynamic postural responses immediately after lower leg muscle stimulation offset. Sudden removing of active sensory input represents a transient period in balance control. METHODS Postural responses of 13 young, 13 healthy elderly and 13 PD patients to proprioceptive bilateral vibration of soleus muscles during stance were assessed by a force platform and two accelerometers attached on the upper and the lower trunk. The experimental protocol consisted of 2 conditions of soleus muscle vibration with 1) eyes open and 2) eyes closed randomly repeated four times. RESULTS During vibration period before stimulus offset, postural responses were similar in elderly and PD patients. Contrary, immediately after vibration offset significantly larger backward amplitude of centre of foot pressure (CoP) displacement and trunk tilts were observed in PD patients compared to healthy peers. In returning to vertical position, peak-to-peak amplitudes, maximal velocity of CoP and trunk tilts significantly increased in PD patients. Without vision, their postural responses were more enhanced. The differences between young and elderly were found in most parameters in transient period after vibration offset and also during vibration. SIGNIFICANCE The PD patients showed more unstable transient postural responses to selective sensory stimulation switch off, which may reflect impairment of sensory reweighting in balance control. Understanding how early stages PD patients differ in balance control from neurologically intact peers may help researchers and clinicians to refine their intervention and fall prevention programs.BACKGROUND Impairments of postural stability occur with increasing age and in neurodegenerative diseases like the Parkinsons disease (PD). While changes in balance have been described in many studies under steady-state conditions, less is known about the dynamic changes in balance following sudden transition to different sensory inputs. RESEARCH QUESTION The aim was to clarify different effects of age and Parkinsons disease on dynamic postural responses immediately after lower leg muscle stimulation offset. Sudden removing of active sensory input represents a transient period in balance control. METHODS Postural responses of 13 young, 13 healthy elderly and 13 PD patients to proprioceptive bilateral vibration of soleus muscles during stance were assessed by a force platform and two accelerometers attached on the upper and the lower trunk. The experimental protocol consisted of 2 conditions of soleus muscle vibration with 1) eyes open and 2) eyes closed randomly repeated four times. RESULTS During vibration period before stimulus offset, postural responses were similar in elderly and PD patients. Contrary, immediately after vibration offset significantly larger backward amplitude of centre of foot pressure (CoP) displacement and trunk tilts were observed in PD patients compared to healthy peers. In returning to vertical position, peak-to-peak amplitudes, maximal velocity of CoP and trunk tilts significantly increased in PD patients. Without vision, their postural responses were more enhanced. The differences between young and elderly were found in most parameters in transient period after vibration offset and also during vibration. SIGNIFICANCE The PD patients showed more unstable transient postural responses to selective sensory stimulation switch off, which may reflect impairment of sensory reweighting in balance control. Understanding how early stages PD patients differ in balance control from neurologically intact peers may help researchers and clinicians to refine their intervention and fall prevention programs.

Breathing changes accompanying balance improvement during biofeedback

The aim of this study was to determine whether respiration would be altered during visual biofeedback condition while standing on a foam surface. Fifty young, healthy subjects (24 men, 26 women) were divided into a spirometry group, in which additional spirometry analysis was performed, and a control group. All subjects were tested in two conditions: 1) standing on a foam surface and 2) standing on a foam surface with visual biofeedback (VF) based on the centre of pressure (CoP). CoP amplitude and velocity in anterior-posterior (Aap, Vap) and medial-lateral (Aml, Vml) directions were measured by the force platform. Breathing movements were recorded by two pairs of 3D accelerometers attached on the upper chest (upper chest breathing - UCB) and the lower chest (lower chest breathing - LCB). Results showed that significant decreases of CoP amplitude and velocity in both directions were accompanied by a significant decrease of lower chest breathing, and an increase of LCB frequency was seen during VF condition compared to control condition in both groups. Moreover, a significant decrease in tidal volume and increased breathing frequency during VF condition were confirmed by spirometric analysis. Reduced breathing movements and volumes as well as increased breathing frequency are probably part of an involuntary strategy activated to maximize balance improvement during VF condition.