Anteroposterior Postural Sway Research Articles

Effects of cycling training on balance and gait in children with Duchenne muscular dystrophy: A randomized controlled study

Background and aimsAlthough aerobic exercises such as cycling and swimming are increasingly being recommended in Duchenne muscular dystrophy (DMD), their effect on gait and balance parameters is unclear. This study was aimed to investigate the effect of cycling training on balance and spatio-temporal gait parameters in children with DMD. MethodsAmbulant children (age range: 6.17–11.33 years) were randomly divided into two groups: home-based exercise training applied in the control group (n = 12) while 12 weeks of supervised submaximal lower extremity cycling training in addition to home-based exercise training performed in the study group (n = 11). Gait and balance parameters were evaluated using the GAITRite electronic walkway system and the Bertec Balance Check Screener™, respectively. Assessments were applied before and after 12 weeks of training. ResultsThe mean ages of the children in the study and control groups were 8.20 (SD:1.34) and 8.86 (SD:1.30) years, consecutively (p > 0.05). Considering the baseline values, the balance and spatio-temporal gait parameters of the children were similar except for the antero-posterior postural sway on the perturbed surface with eyes open (p > 0.05). There was a significant time x group interaction effect in favor of the study group for the antero-posterior postural sway of children on the normal surface with eyes open (F (1,58) = 12.62, p = 0.002). It was found that the antero-posterior postural sway on the normal surface with eyes open was improved in the study group within group comparison (F (1,10) = 8.50, p = 0.015). ConclusionsThe study showed that both the cycling and the home-based exercise training groups may maintain gait and balance parameters during the study. Adding a cycling training to the rehabilitation program can also provide additional contribution to improve antero-posterior balance.

A modified two-dimensional sensory organization test that assesses both anteroposterior and mediolateral postural control.

The Sensory Organization Test (SOT) was designed to measure changes in postural control in response to unreliable visual and/or proprioceptive feedback. However, secondary to the manipulation of sensory cues in only the sagittal plane, the SOT is capable of only describing postural control in a single direction. The present study aimed to characterize postural responses to a modified SOT designed to concurrently challenge both anteroposterior and mediolateral postural control. Twenty-one healthy adult volunteers (30.6 ± 10.2 years) completed the standard anteroposterior one-dimensional (1D) SOT, in addition to a modified SOT with the support surface sway-referenced to both anteroposterior and mediolateral postural sway (two-dimensional, 2D). Our primary analysis concerned a comparison of mediolateral, as well as anteroposterior postural sway measured during the standard one-dimensional (i.e., pitch tilt) and the novel two-dimensional (i.e., roll and pitch tilt) sway-referenced paradigms. Here, postural sway was quantified by calculating the root mean square distance (RMSD) of the center of pressure (CoP) during each trial. Our data showed that the 2D sway-referenced conditions yielded a selective increase in mediolateral postural sway relative to the standard 1D conditions for both wide (η2 = 0.66) and narrow (η2 = 0.78) stance conditions, with anteroposterior postural sway being largely unaffected (η2 = 0.001 to 0.103, respectively). The ratio between mediolateral postural sway in the sway-referenced conditions and postural sway in the corresponding stable support surface conditions was greater for the 2D (2.99 to 6.26 times greater) compared to 1D paradigms (1.25 to 1.84 times greater), consistent with a superior degradation of viable proprioceptive feedback in the 2D paradigm. A modified 2D version of the SOT was shown to provide a greater challenge to mediolateral postural control relative to the standard 1D SOT protocol, putatively as a result of a superior capacity to degrade proprioceptive feedback in the mediolateral direction. Given these positive findings, future studies should investigate the clinical utility of this modified SOT as a means by which to better characterize sensory contributions to postural control in the presence of various sensorimotor pathologies, including vestibular hypofunction.

Comparison of static postural sway characteristics between pregnant and non-pregnant women

The present study aimed to compare the postural sway between pregnant and non-pregnant women during eight different sensory conditions including those in which vision, proprioception, and base of support are compromised. Forty primigravidae at the 32nd week of pregnancy and forty non-pregnant women who were matched for age and anthropometric measurements participated in this cross-sectional comparison study. Static posturography equipment was used to record the anteroposterior sway velocity, mediolateral sway velocity, and velocity moment during normal stance and when vision, proprioception, and base of support were compromised. Pregnant women (mean age: 25.4) demonstrated a larger median velocity moment and mean anteroposterior sway velocity compared to non–pregnant women (mean age:24.4) across all tested sensory conditions (p < 0.05). Although mediolateral sway velocity did not show any statistically significant difference, the ANCOVA results suggested that there was a statistically significant difference in mediolateral sway velocity in Eyes open feet apart condition on the firm surface [F (1,77, p = 0.030, ηp2 = 0.121] and Eyes closed feet apart condition on the firm surface [F (1,77, p = 0.015, ηp2 = 0.15] between pregnant and non-pregnant women. There was a larger velocity moment and anteroposterior postural sway velocity in pregnant women in their third trimester compared to non-pregnant women when exposed to different sensory conditions.Title: Comparison of static postural sway characteristics between pregnant and non-pregnant women.

Modulation of lower limb muscles and trajectory correction in the bipedal stance during visual perturbation.

The ability to actively track posture using visual targets as indicators is important for improving impairments in whole-body coordination, and accurate visual feedback on tasks is considered effective in promoting sensory-motor integration and behavioral success. In the present study, we examined inter- and intramuscular modulation between the two lower limbs in response to visual perturbation. Sixteen healthy young subjects (age: 21.3 ± 0.7 years) were asked to move their weight back and forth while tracking a visual target displayed on a monitor in front of them for 30 s. Three types of target movements were examined: a sinusoidal wave (i.e., a predictable pattern), more complex patterns (random), and no movement (stationary). Electromyography (EMG) was used to assess intra- and intermuscular coherence modulation of the plantar flexor muscles (right and left soleus and right and left medial gastrocnemius). The ability to adjust posture to follow the target signal was assessed using a stabilometer. Inter- and intramuscular coherence increased during the visual perturbation task compared to the stationary task. In addition, left-right differences in lower limb modulation were observed during the visual perturbation task. Furthermore, interlimb coherence was related to the motor accuracy of tracking. The muscles of both lower limbs cooperated in response to visual perturbation, suggesting that these muscles control visually induced anteroposterior postural sway. Since such visual perturbations promote coordination between both lower extremities, this relationship may indicate the potential for rehabilitation training to help individuals acquire and improve the motor functions necessary to efficiently and stably perform activities of daily living.

Modulation of Lower-Limb Muscle Activity in Maintaining Unipedal Balance According to Surface Stability, Sway Direction, and Leg Dominance.

Determining temporal similarity in shape between electromyographic (EMG) and center-of-pressure (COP) signals reflects neuromuscular control in terms of which relevant muscles are involved in maintaining balance. The current study aimed to investigate a cross-correlation between seven lower-limb EMG activities and COP displacements, simultaneously measured in 25 young adults unipedally balancing on stable and multiaxial-unstable surfaces. The effect of surface stability, sway direction, and leg dominance was then tested on two EMG–COP correlation levels: individual muscles and groups (patterns) of multi-muscles involved in postural sway, as determined by principal component analysis (PCA). The results show that two factors demonstrate their effects only at the level of individual muscles: sway direction (p ≤ 0.003) and leg dominance (p = 0.003). Specifically, the semitendinosus, tibialis anterior, peroneus longus, and soleus correlate more with the mediolateral postural sway than with the anteroposterior postural sway, except for the gastrocnemius medialis. Additionally, balancing on the non-dominant leg shows a lower correlation between the semitendinosus and postural sway than on the dominant leg. The current findings suggest that when achieving unipedal equilibrium, the postural control system may be constrained the most in the specific muscles of the least steady conditions, e.g., the frontal plane and non-preferred leg.

Hallux valgus deformity and postural sway: a cross-sectional study

BackgroundHallux valgus deformity has been reported to be associated with increased postural sway. However, the direction and magnitude of postural sway associated with hallux valgus remain inconclusive. We assessed the association between hallux valgus deformity and postural sway using a force plate.MethodsThe subjects were 169 healthy volunteers, > 40 years old (63 males, 106 females, average age: 66.0 ± 12.4 years old), who took part in an annual medical examination. We investigated the photographic hallux valgus angle (°), total trajectory length of the gravity center fluctuation (mm), area of the center of pressure (mm2), mediolateral and anteroposterior postural sway (mm) in a standing position with 2-legged stance and eyes open, hallux pain (Numerical Rating Scale), trunk and lower limb muscle mass (kg). We classified the subjects into a hallux valgus group (n = 44, photographic hallux valgus angle of 1 or both feet ≥ 20°) and a no hallux valgus group (n = 125, photographic hallux valgus angle of both feet < 20°) and analyzed the relationship between hallux valgus and postural sway.ResultsThe anteroposterior postural sway in the hallux valgus group (6.5 ± 2.8) was significantly greater than in the no hallux valgus group (5.4 ± 2.2, p = 0.014), and the lower limb muscle mass in the hallux valgus group (12.4 ± 2.2) was significantly smaller than in the no hallux valgus group (13.5 ± 3.2, p = 0.016). The total value of the photographic hallux valgus angle on both feet was positively correlated with the anteroposterior postural sway (p = 0.021) and negatively correlated with the lower limb muscle mass (p = 0.038). The presence of hallux valgus (p = 0.024) and photographic hallux valgus angle (p = 0.008) were independently related to the magnitude of anteroposterior postural sway.ConclusionsHallux valgus deformity and its severity were positively associated with the magnitude of the anteroposterior postural sway.Trial registration2017 − 135. Registered 22 August 2017.

Association between knee extensor and ankle plantarflexor muscle thickness and echo intensity with postural sway, mobility and physical function in older adults.

The purpose of this study was to assess the association between muscle thickness and echo intensity of the knee extensors and ankle plantarflexors with postural sway, mobility and physical function in older adults. Twenty-one older men and women (age; 69.9±4.3years) were assessed for postural sway (centre of pressure movement), mobility (i.e. Timed-Up and-Go-test [TUG]), physical function (i.e. 5 times sit-to-stand [STS]), and ultrasound derived measures of muscle thickness and echo intensity of the vastus lateralis (VL) and gastrocnemius medialis (GM). Significant inverse correlations were observed between VL and GM thickness with TUG (r=-0.432 to -0.492) and STS (r=-0.473 to -0.596). Significant positive correlations were observed between VL and GM echo intensity with TUG (r=0.459 to 0.518) and STS (r=0.481 to 0.635). Significant positive correlations were also detected between GM echo intensity and anteroposterior sway (r=0.451 to 0.534). Two key findings emerged from the present experiment. First, this study provides novel evidence that ankle plantarflexor echo intensity, but not thickness, was associated with anteroposterior postural sway among older adults. Second, we provide new evidence that muscle thickness and echo intensity of the knee extensors and uniquely, the ankle plantarflexors, presented with similar magnitude associations with TUG and STS performance in older adults.

Postural sway changes in genu recurvatum deformity during standing with manipulation of visual and proprioceptive systems

ObjectivesGenu recurvatum deformity shifts the mechanical axis of lower extremity posteriorly which may disturb the balance and lead to injury. The aim of this research was to compare postural sways in genu recurvatum subjects with healthy controls while manipulating visual and proprioceptive systems. Methods10 female subjects with genu recurvatum (20.04 ± 4.64 years) and 11 female healthy controls (20.11 ± 4.64 years) participated in the study. The back knee angle was measured using a goniometer. Dynamic BIODEX balance system was used to measure the total, anteroposterior, and mediolateral postural sways while manipulating the stability of the base of support (stable versus unstable) and visual system (open versus closed eyes). ResultDuring standing on the stable level, in both visual conditions, genu recurvatum subjects demonstrated greater total and anteroposterior postural sways than controls in both standing on the right and left foot (p<0.05). Similarly, when standing on the unstable base of support, in all conditions, the postural sways of genu recurvatum subjects were significantly greater than controls in both right and left foot (p<0.05). The vision had a main effect on postural control of genu recurvatum group (p=0.0001). There was a significant interaction between vision, the stability of the base of support and group factors (p=0.003). ConclusionThe results indicated that genu recurvatum subjects have a proprioceptive deficiency and they mainly rely on visual system inputs to control and maintain their balance. Thus, the proprioceptive system and balance control should be considered in assessment and designing an appropriate treatment regimen for genu recurvatum.

Characterization of Balance Control After Moderate to Severe Traumatic Brain Injury: A Longitudinal Recovery Study.

Balance impairments after traumatic brain injury (TBI) are common and persist after injury. Postural asymmetries in balance have been reported, but not quantified, across recovery. The objective of this study was to characterize balance recovery after moderate to severe TBI, with a focus on postural asymmetry. A secondary analysis of prospectively collected data was used in this study. Data were from 45 participants with moderate to severe TBI. Participants' balance in 2 bipedal stances and 2 unipedal stances was assessed with force plates at approximately 2, 5, and 12 months after injury. Single-visit data from participants who were matched for age and served as healthy controls were collected for visual comparison using 95% confidence intervals. Spatial and temporal center-of-pressure (COP) measures were calculated from force plates in the anteroposterior (AP) and mediolateral (ML) directions. Despite improvements in net ML COP postural sway from 2 to 5 months after injury, there were no changes in AP postural sway across recovery. Postural sway in individuals with TBI was higher than normative values at all time points in both directions. Interlimb synchrony did not change across recovery in either direction. TBI weight-bearing asymmetry was lower than normative values at all time points and did not change across recovery. The characteristics of unipedal stance differed between limbs. Sample size was reduced as a result of the inclusion and exclusion criteria; future studies will benefit from a larger sample size. The absence of recovery in ML COP postural sway, interlimb synchrony, and weight-bearing symmetry indicated that reduced ML control may contribute to balance impairments after TBI. These impairments may extend to dynamic balance tasks and may also place individuals with TBI at a higher risk for falls.

Immediate effects of kinesio taping on pain and postural stability in patients with chronic low back pain

ObjectivePostural control of patients with chronic low back pain (CLBP) is usually impaired. Effects of treatment applications on postural stability have not been well investigated. The aim of this study was to investigate the immediate effects of kinesio taping on pain and postural stability in patients with CLBP. MethodsOne hundred and one patients with CLBP (age: 53.00 (10.69) years, body mass index (BMI): 31.52 (5.57) kg/m2) were included in this study. Kinesio taping was applied on paravertebral muscles and sacrum with muscle and ligament techniques. Postural stability was assessed with Biodex Balance System® (USA) both at static and dynamic mode in bilateral standing position. The base was set “static” for static mode, and set between“12-1” for dynamic mode. Pain intensity was evaluated with the pain section (0–5 points) of Oswestry Disability Index, including ten items (pain, personal care, lifting, walking, sitting, standing, sleeping, sex life, social life, travelling), pre- and post-application. ResultsThe static overall (pre: 1.49 (1.15), post: 1.25 (1.11)) and antero-posterior (pre: 0.96 (0.86), post: 0.74 (0.59)), dynamic overall (pre: 3.12 (2.26), post: 2.73 (2.44)) and medio-lateral postural sway scores (pre: 1.62 (1.12), post: 1.24 (0.75)), and pain (pre: 2.00 (0.00–5.00), post: 1.00 (0.00–4.00)) decreased after the application (p < .05). However, no differences were observed in static medio-lateral (pre: 0.84 (0.78), post: 0.82 (1.01)) and dynamic antero-posterior postural sway scores (pre: 2.33 (1.99), post: 2.12 (2.33)), (p > .05). ConclusionsKinesio taping may immediately improve postural stability and decrease pain of patients with CLBP.

Maturation of the postural control in healthy children

Introduction The multitest platform (MT) framiral is used in the evaluation and rehabilitation of balance disorders in adults. Normative data are missing for children. The aims of the study were to collect, in normal children, MT normative data usable for clinical diagnosis and to explore the maturation of the postural control. Material and method In this prospective study, 103 healthy children aged from 3 to 15 years were tested on MT in the 6 classical conditions: eyes open (condition A), eyes closed (B), vision disturbed by optokinetic stimulation (C), and the same conditions (A, B and C) with a mobile platform disturbing the proprioceptive inputs (D, E and F). The effect of age and gender were studied on the index of postural instability (IPI). Results The IPI of children was higher (worse) than those observed in adults. They were logically better in condition A and worse in the conditions with disturbed proprioceptive inputs (D, E and F). The scores improved with age but only for the conditions A, B and C. The younger children use more energy for their postural control than adults in the easier conditions (A and B). They were less efficient in the use of sensory inputs, especially with disturbed proprioceptive inputs (D, E and F). They were more disturbed by visual motion. Girls presented with a more rapid maturation of their postural control than boys. Boys demonstrated a trend to a deterioration of their postural control at adolescence. Even in oldest children (13–15 years), the IPI scores did not reach the adults values in the more difficult conditions (D, E and F). Conclusion MT normative data for IPI were recorded for clinical use. Younger healthy children need more energy for maintaining an efficient control of their antero-posterior postural sways. This more energy-consuming postural control is still observed in adolescents.

Factors influencing referrals to physiotherapy and subsequent follow-up for management of low back pain: preliminary results from a cohort study

standardized mean differences for analysis of continuous variables. Risk ratios were derived and 95% confidence intervals calculated. Results: Six studieswere included. Three trials (64 participants) compared Wii and conventional rehabilitation versus conventional rehabilitation alone. Three trials (102 participants) compared Wii with other exercise interventions. The addition of Wii to conventional rehabilitation resulted in significantmean differences in favor of additionalWii compared to standard care for Timed Up and Go test (TUG) (0.81 points, CI 0.29 to 1.33, P= 0.002), but not for other mobility and functional outcomes: Functional Independence Measure (FIM) score (0.45, CI−0.21 to 1.11, P= 0.18), Berg Balance Score (−0.64, CI −3.66 to 2.39, P= 0.68), antero-posterior postural sway (0.23, CI −0.38 to 0.84, P= 0.46). No serious adverse events were reported and whenWii was compared to exercise alone we demonstrated a decreased risk of participants dropping out of follow up (RR 0.40, CI 0.20 to 0.78, P= 0.007). Conclusion(s): The addition of Wii gaming to conventional rehabilitation in chronic stroke patients significantly improved performance in TUG and not in the other physical measures. The pooled effect was small and not beyond the minimal detectable change. However, Wii can be used safely in stroke patients and participants were less likely to drop out in the Wii group. This review highlights the need for further high quality studies to demonstrate the efficacy of Wii in stroke rehabilitation. Implications: Nintendo Wii can be use as an adjunct in stroke rehabilitation to allowmore training time outside therapy hours. It also makes therapy more interactive, engaging and enjoyable for the stroke survivors.

Relation between the Sensory and Anthropometric Variables in the Quiet Standing Postural Control: Is the Inverted Pendulum Important for the Static Balance Control?

The aim of this study was to evaluate the relation between the sensory and anthropometric variables in the quiet standing. Methods. One hundred individuals (50 men, 50 women; 20–40 years old) participated in this study. For all participants, the body composition (fat tissue, lean mass, bone mineral content, and bone mineral density) and body mass, height, trunk-head length, lower limb length, and upper limb length were measured. The center of pressure was measured during the quiet standing posture, the eyes opened and closed with a force platform. Correlation and regression analysis were run to analyze the relation among body composition, anthropometric data, and postural sway. Results. The correlation analysis showed low relation between postural sway and anthropometric variables. The multiple linear regression analyses showed that the height explained 12% of the mediolateral displacement and 11% of the center of pressure area. The length of the trunk head explained 6% of displacement in the anteroposterior postural sway. During eyes closed condition, the support basis and height explained 18% of mediolateral postural sway. Conclusion. The postural control depends on body composition and dimension. This relation is mediated by the sensory information. The height was the anthropometric variable that most influenced the postural sway.

Relationship between Postural Sway and Dynamic Balance in Stroke Patients.

[Purpose] The purpose of the current study was to investigate the relationship between postural sway and dynamic balance in post stroke patients. [Subjects] Thirty-one stroke patients (20 men and 11 women; age 64.25 years; stroke duration 12.70 months; MMSE-K score 26.35) participated in this study. [Methods] This study applied a cross-sectional design. A Good Balance system was used for measurement of the postural sway velocity (anteroposterior and mediolateral) and velocity moment of subjects under the eyes open and eyes closed conditions in a standing posture. The postural sway of subjects was measured under two surface conditions (stable and unstable surfaces). [Results] On the unstable surface (foam), no significant correlation was observed between postural sway and dynamic balance except for the berg balance scale (BBS) score and anteroposterior postural sway velocity under the eyes open condition, anteroposterior postural sway velocity under the eyes closed condition, and postural sway velocity moment. In addition, in the stable condition, no significant correlation was observed between postural sway and dynamic balance. [Conclusion] Our results indicate that a decrease in postural sway does not necessarily reflect improvement of dynamic balance ability. We believe that this finding may be useful in balance rehabilitation for prevention of falls after a stroke.

The Influence of Cervical Spine Flexion-Rotation Range-of-Motion Asymmetry on Postural Stability in Older Adults

Cross-sectional design. The purpose of this study was to isolate the contribution of cervical spine range-of-motion (ROM) asymmetry to postural control in a group of older adults with neck pain. Previous research has suggested that the upper cervical spine plays an important role in postural stability. However, it remains unclear whether the upper cervical spine rotation ROM asymmetry is associated with postural stability. Using the cervical range-of-motion device to assess upper cervical spine ROM via the cervical flexion-rotation test, we classified 54 older adults with neck pain (30 females; mean [standard deviation] age, 66 [5] yr) into the (1) symmetrical group (SYM; n = 20; ≤5° side-to-side difference) or (2) asymmetrical group (ASYM; n = 34; >5° difference). Standing postural control was characterized by the center-of-pressure (CoP) movements measured using a Balance Board. Other measures included habitual, fast-paced gait speed and neck pain. Both groups were compared on the various measures using Welch t tests. Although the ASYM group had 26% greater anteroposterior postural sway than the SYM (P < 0.01), both groups did not differ on postural sway velocity, gait speed, and neck pain intensity. Analyzing the frequency content of the postural sway using wavelet analysis (a modern, nonlinear signal processing method) shed further light: the standing postural sway in the ASYM group was skewed toward lower frequency movement (ultralow [< 0.10Hz] frequency content, anteroposterior: 6.7% in ASYM, 4.7% in SYM, P = 0.01; medial-lateral: 4.2% in ASYM, 3.4% in SYM, P = 0.045). The ASYM group seemed to have compensated for their altered somatosensory input to achieve similar functional levels as the SYM group. Given what is known about the association between ultralow frequency postural sway and visual input, we speculated that the postural strategy adopted by the ASYM group was adaptive and that this group may be relying on the visual system to achieve these compensations.

Biomechanics of Ankle Instability. Part 2

To test the hypothesis that ankles with functional instability will demonstrate greater single-limb postural sway (PS) than their contralateral stable joint and stable healthy controls and to examine the relationship between single-limb postural sway and muscular reaction time to a simulated ankle sprain mechanism. Nineteen male volunteers with a history of unilateral ankle sprain and functional ankle instability (FAI) and 19 healthy male controls performed 12 single-limb PS tests, 3 on each leg with and without vision. Participants provided informed consent. Postural sway data are reported on the FAI group's unstable (UA) and stable ankles (SA), and the control group's dominant (DA) and nondominant ankles (NDA). With vision, the UA and SA revealed similar postural control; however, the UA showed greater (P < 0.05) anteroposterior PS than the DA (0.46 cm) and the NDA (0.51 cm). Without vision, the UA showed greater (P < 0.05) medial (2.41 cm) and lateral (2.59 cm) PS than the SA and also showed greater (P < 0.05) medial (2.05 and 2.10 cm, respectively) and lateral (2.28 and 2.26 cm, respectively) than the DA and NDA. The relationship between PS and muscle reaction times, derived from the previous article was calculated. Significant correlations (P < 0.05) were found between the unstable ankle peroneus longus (PL) and peroneus brevis (PB) reaction time and lateral (r = 0.63 and r = 0.81, respectively), medial (r = 0.74 and r = 0.76, respectively), and anterior PS (r = 0.56 and r = 0.55, respectively; P < 0.01). Results reveal postural sway deficits in ankles with FAI. They also demonstrate a significant relationship between PL and PB reaction times and postural sway in UA. Individuals who sustain an acute ankle sprain and those with FAI require rehabilitation that improves proprioception, strengthens the evertors and dorsiflexors, and restores peroneal reaction time.

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.

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

Galvanic vestibular stimulation serves to modulate the continuous firing level of the peripheral vestibular afferents. It has been shown that the application of sinusoidally varying, bipolar galvanic currents to the vestibular system can lead to sinusoidally varying postural sway. Our objective was to test the hypothesis that stochastic galvanic vestibular stimulation can lead to coherent stochastic postural sway. Bipolar binaural stochastic galvanic vestibular stimulation was applied to nine healthy young subjects. Three different stochastic vestibular stimulation signals, each with a different frequency content (0-1 Hz, 1-2 Hz, and 0-2 Hz), were used. The stimulation level (range 0.4-1.5 mA, peak to peak) was determined on an individual basis. Twenty 60-s trials were conducted on each subject - 15 stimulation trials (5 trials with each stimulation signal) and 5 control (no stimulation) trials. During the trials, subjects stood in a relaxed, upright position with their head facing forward. Postural sway was evaluated by using a force platform to measure the displacements of the center of pressure (COP) under each subject's feet. Cross-spectral measures were used to quantify the relationship between the applied stimulus and the resulting COP time series. We found significant coherency between the stochastic vestibular stimulation signal and the resulting mediolateral COP time series in the majority of trials in 8 of the 9 subjects tested. The coherency results for each stimulation signal were reproducible from trial to trial, and the highest degree of coherency was found for the 1- to 2-Hz stochastic vestibular stimulation signal. In general, for the nine subjects tested, we did not find consistent significant coherency between the stochastic vestibular stimulation signals and the anteroposterior COP time series. This work demonstrates that, in subjects who are facing forward, bipolar binaural stochastic galvanic stimulation of the vestibular system leads to coherent stochastic mediolateral postural sway, but it does not lead to coherent stochastic anteroposterior postural sway. Our finding that the coherency was highest for the 1- to 2-Hz stochastic vestibular stimulation signal may be due to the intrinsic dynamics of the quasi-static postural control system. In particular, it may result from the effects of the vestibular stimulus simply being superimposed upon the quiet-standing COP displacements. By utilizing stochastic stimulation signals, we ensured that the subjects could not predict a change in the vestibular stimulus. Thus, our findings indicate that subjects can act as "responders" to galvanic vestibular stimulation.