Analysis of short- and long-term effects of adaptation in human postural control

Investigation of Head Shake Sensory Organization Test (HS-SOT) in three planes: Test-retest reliability and age-related differences

BACKGROUND

In recent years, it has been determined that SOT sensitivity is insufficient in patients who develop vestibular compensation and therefore the Head Shake Sensory Organization Test (HS-SOT) has been developed.

RESEARCH QUESTION

How differs the balance performance of healthy adults that is tested with HS-SOT according to age and test planes? What is the test-retest reliability level of the HS-SOT in three planes?

METHODS

Our prospective study, which has a methodological research design, included 80 participants divided into three groups by age range (Group 1: 20-39 years (n = 30); Group 2: 40-49 years (n = 30) and Group 3: 50-64 years (n = 20)). SOT and HSSOT ( yaw, pitch, roll) were performed to all participants. To investigate the testretest reliability of the HS-SOT, a total of 27 participants were re-evaluated one week later. The HS-SOT performance of the participants was compared between age groups and test planes. Intra-class correlation coefficient and minimum detectable change values (MDC) was calculated to test-retest reliability of HS-SOT.

RESULTS

HS-SOT scores (HS-2 and HS-5) did not differ significantly between age groups. The balance performance of individuals for the pitch plane was lower than other planes. Only the HS-5 score showed a significant difference between the sessions. HS-5 scores were higher in the re-test; for the first group in the pitch plane and for the third group in the yaw plane. The test-retest reliability level of these conditions was "moderate-good" for both groups. The corresponding MDC value was highest (14.01) for the HS-5 (yaw) score of the elderly group.

SIGNIFICANCE

The findings from this study demonstrated that the test plane influences the HS-SOT, a learning/practice effect may occur because of repeated HS-SOT evaluation, and this effect is more explicit in the elderly. This study provides a perspective for the evaluation and follow-up processes of patients with balance problems.

Immediate effect of lower extremity joint manipulation on a lower extremity somatosensory illusion: a randomized, controlled crossover clinical pilot study

Objective: This study explored the influence of lower extremity manipulation on the postural after-effects of standing on an inclined surface.

Methods: Eight healthy individuals (28.0 ± 4.1 years) were recruited for this open-label, crossover study. Participants stood on an incline board for 3 min to develop a known form of somatosensory illusion. After randomization to either a lower-extremity joint manipulation or no intervention, participants immediately stood on a force plate for 3 min with eyes closed. After a 24-h washout period, participants completed the remaining condition. Center of pressure (CoP) position data was measured by a force plate and evaluated using statistical parametric mapping. Pathlength, mean velocity, and RMS were calculated for significant time periods and compared with corrected paired t-tests.

Results: Parametric maps revealed that CoP position of control and intervention conditions differed significantly for two time periods (70–86 s—control: 0.17 ± 1.86 cm/intervention: −1.36 ± 1.54 cm; 141–177 s—control: −0.35 ± 1.61 cm/intervention: −1.93 ± 1.48 cm). CoP pathlength was also significantly decreased for the second period (control: 6.11 ± 4.81 cm/intervention: 3.62 ± 1.92 cm).

Conclusion: These findings suggest that extremity manipulation may be a useful intervention for populations where CoP stability is an issue. This study contributes to the growing body of evidence that manipulation of the extremities can drive global postural changes, as well as influence standing behavior. Further, it suggests these global changes may be driven by alterations in central integration.

Clinical Trial Registration:ClinicalTrials.gov, NCT Number: NCT05226715.

Long-term postural control in elite athletes following mild traumatic brain injury

Background

Traumas to the head and neck are common in sports and often affects otherwise healthy young individuals. Sports-related concussions (SRC), defined as a mild traumatic brain injury (mTBI), may inflict persistent neck and shoulder pain, and headache, but also more complex symptoms, such as imbalance, dizziness, and visual disturbances. These more complex symptoms are difficult to identify with standard health care diagnostic procedures.

Objective

To investigate postural control in a group of former elite athletes with persistent post-concussive symptoms (PPCS) at least 6 months after the incident.

Method

Postural control was examined using posturography during quiet stance and randomized balance perturbations with eyes open and eyes closed. Randomized balance perturbations were used to examine motor learning through sensorimotor adaptation. Force platform recordings were converted to reflect the energy used to maintain balance and spectrally categorized into total energy used, energy used for smooth corrective changes of posture (i.e., <0.1 Hz), and energy used for fast corrective movements to maintain balance (i.e., >0.1 Hz).

Results

The mTBI group included 20 (13 males, mean age 26.6 years) elite athletes with PPCS and the control group included 12 athletes (9 males, mean age 26.4 years) with no history of SRC. The mTBI group used significantly more energy during balance perturbations than controls: +143% total energy, p = 0.004; +122% low frequency energy, p = 0.007; and +162% high frequency energy, p = 0.004. The mTBI subjects also adapted less to the balance perturbations than controls in total (18% mTBI vs. 37% controls, p = 0.042), low frequency (24% mTBI vs. 42% controls, p = 0.046), and high frequency (6% mTBI vs. 28% controls, p = 0.040). The mTBI subjects used significantly more energy during quiet stance than controls: +128% total energy, p = 0.034; +136% low-frequency energy, p = 0.048; and +109% high-frequency energy, p = 0.015.

Conclusion

Athletes with previous mTBI and PPCS used more energy to stand compared to controls during balance perturbations and quiet stance and had diminished sensorimotor adaptation. Sports-related concussions are able to affect postural control and motor learning.

Reliability and learning effects of repeated exposure to the Bertec Balance Advantage sensory organisation test in healthy individuals

BACKGROUND

The Sensory Organisation Test (SOT) of computerised dynamic posturography (CDP) is a well-established clinical test used to measure postural control. Advances in technology have enabled new CDP systems to use immersive virtual reality, such as the Bertec® Balance Advantage®. While the Bertec provides an innovative approach to posturography, the reliability and learning effects of the Bertec in administering the SOT has not been thoroughly investigated.

RESEARCH QUESTION

To evaluate the reliability and performance during repeated administration of the Bertec® Balance Advantage® SOT.

METHODS

Fourteen healthy adults (age 27.17 ± 5.5years; 10 females) participated. Each participant performed five SOTs over three sessions. The first two sessions were approximately two days apart and the third one month later. In the first two sessions, two SOTs were conducted, and in the third session, one was performed. Composite, equilibrium, and ratio scores were used for analysis.

RESULTS

Poor within-session reliability was found in the first session for the composite score (ICC: 0.73, 95% CI: 0.32-0.91), which improved by the second session (ICC: 0.84, 95% CI: 0.58-0.94). Poor within-session reliability (ICC <0.5) was found for all ratio and equilibrium scores, except for the equilibrium score of condition 3, which demonstrated moderate reliability (ICC: 0.84, 95% CI: 0.57-0.95). Poor between-session reliability was found for all outcomes. There was an increase in the composite and equilibrium scores for conditions 5 and 6 over the 5 tests, which plateaued after the fourth test, and were retained at 1 month.

SIGNIFICANCE

The data demonstrate a steady increase in performance with repeated exposure to the Bertec SOT, which was maintained one month later, indicating a learning effect. We recommend that a minimum of two familiarisation sessions should be administered to establish baseline performance and improve reliability.

Deep brain stimulation in the subthalamic nuclei alters postural alignment and adaptation in Parkinson's disease

Parkinson’s disease (PD) can produce postural abnormalities of the standing body position such as kyphosis. We investigated the effects of PD, deep brain stimulation (DBS) in the subthalamic nucleus (STN), vision and adaptation on body position in a well-defined group of patients with PD in quiet standing and during balance perturbations. Ten patients with PD and 25 young and 17 old control participants were recruited. Body position was measured with 3D motion tracking of the ankle, knee, hip, shoulder and head. By taking the ankle as reference, we mapped the position of the joints during quiet standing and balance perturbations through repeated calf muscle vibration. We did this to explore the effect of PD, DBS in the STN, and vision on the motor learning process of adaptation in response to the repeated stimulus. We found that patients with PD adopt a different body position with DBS ON vs. DBS OFF, to young and old controls, and with eyes open vs. eyes closed. There was an altered body position in PD with greater flexion of the head, shoulder and knee (p≤0.042) and a posterior position of the hip with DBS OFF (p≤0.014). With DBS ON, body position was brought more in line with the position taken by control participants but there was still evidence of greater flexion at the head, shoulder and knee. The amplitude of movement during the vibration period decreased in controls at all measured sites with eyes open and closed (except at the head in old controls with eyes open) showing adaptation which contrasted the weaker adaptive responses in patients with PD. Our findings suggest that alterations of posture and greater forward leaning with repeated calf vibration, are independent from reduced movement amplitude changes. DBS in the STN can significantly improve body position in PD although the effects are not completely reversed. Patients with PD maintain adaptive capabilities by leaning further forward and reducing movement amplitude despite their kyphotic posture.

Association between Anthropometric Variables, Sex, and Visual Biofeedback in Dynamic Postural Control Assessed on a Computerized Wobble Board

Anthropometrics and sex influence balance performances, and visual information can change anthropometrics’ relation and the postural sway. Therefore, the aim of the present study was to evaluate the effect of anthropometric characteristics, sex, and visual biofeedback and/or their interaction on a computerized wobble board. Twenty-seven (14 females, 13 males) young adults performed three 30-s double leg stance trials on a wobble board during two conditions: with visual and without visual biofeedback. Visual biofeedback improved (p = 0.010) balance on a wobble board with respect to the condition without visual biofeedback. Regardless of sex, no differences between conditions were found (p = 0.088). When investigating the effect of anthropometrics variables, sex, and their interactions on conditions, a significant main effect of the lower limb/height ratio, sex, and their interaction on the condition without visual biofeedback was found (p = 0.0008; R2 = 0.57). For the visual biofeedback condition, significant effects for sex and body mass (p = 0.0012; R2 = 0.43) and sex and whole-body moment of inertia (p = 0.0030; R2 = 0.39) were found. Results from the present study showed (1) visual biofeedback improved wobble board balance performance; (2) a significant main effect of lower limb/height ratio, sex, and their interaction on the wobble board performances without visual biofeedback emerged; (3) significant effects were found for sex and body mass and sex and moment of inertia in the visual biofeedback condition. Findings from the present study could have an impact on training and evaluations protocols, especially when several populations such as children, athletes, older adults and people with balance disorders are involved.

Correlational study of the center of pressure measures of postural steadiness on five different standing tasks in overweight adults

This study applied the posturography framework on five static standing tasks from the Berg Balance Scale (BBS). Thirteen participants were recruited and the trajectory data of the center of pressure (CoP) were collected. To analyze the postural performance, two approaches were taken: the scores from the BBS and statistical analysis. For the statistical analysis, Spearman's method was applied to determine the correlation of CoP parameters. The results revealed the correlations between CoP parameters in the anterior-posterior (AP) and medial-lateral (ML) directions, and on the statokinesgram (SK) plane for all tasks. To obtain the in-depth detail between normal weight and overweight groups, the differences in the postural control mechanism were defined by correlations of CoP parameters. The Mann-Whitney U test was conducted to define the difference in postural control in terms of difference in weight gain and standing task factors, while Cohen's d was used to investigate the influence of the difference in standing tasks and weight gain on postural control. The results showed that the correlations of CoP parameters could distinguish the balance impairment in the overweight condition from the normal postural control. Otherwise, the scores of BBS, the Mann-Whitney U test and Cohen's d did not separate this slightly compensatory movement during equilibrium. Therefore, the correlations of CoP parameters could provide more information to analyze the balance function in each individual, especially in terms of slight compensation.

Subthalamic Deep Brain Stimulation Modulates Proprioceptive Integration in Parkinson's Disease During a Postural Task

Defective proprioceptive integration may play a role in the pathophysiology of motor symptoms in Parkinson's disease (PD). Dysfunction related to proprioceptively-evoked postural reactions in PD patients is still a controversial issue, with only a limited number of studies to date and mostly discordant results. The aims of the present study were (1) to determine whether or not the proprioceptive defect in PD underlies postural impairment and (2) whether or not deep brain stimulation of the subthalamic nucleus (STN-DBS) affects proprioceptive integration. We examined proprioceptive integration during a postural task in 13 PD patients and 12 age-matched control subjects, using a muscle-tendon vibration paradigm. Analysis of the center of pressure displacement and kinematic data indicates a greater degree of postural destabilization and a reduced ability to maintain a vertical orientation in PD. We found a significant positive effect of STN-DBS on these postural features. Our findings indicate that Parkinson patients, even in the absence of any clinical evidence of instability, falls, or freezing, use proprioceptive information for postural control less efficiently than healthy subjects. Furthermore, STN-DBS was found to improve proprioceptive integration, with positive impacts on postural orientation and balance.

Learning effect of dynamic postural stability evaluation system

BACKGROUND

Repeated exposure to a given perturbation of the postural control system has been shown to cause learning of more efficient postural strategies for maintaining balance both within a session and over time. It is important to show whether outcomes from treatment strategies are related to the effectiveness of training or are the result of the learning of the test process.

OBJECTIVE

To investigate the learning effect of the dynamic postural stability evaluation system.

METHODS

We studied 20 healthy young subjects (13 females and 7 males), with a mean age of 22.3 ± 1.9 years. Limits of stability and postural sway were assessed. All participants completed the standardized dynamic postural stability evaluation test (Bertec, Bertec Corporation, Columbus, OH, USA) seven times. The test was performed in both eyes open and eyes closed conditions.

RESULTS

There were differences in the limits of stability scores for backward (p= 0.042), left side (p= 0.05), and the total score (p= 0.04). There were significant differences in postural sway anteroposterior direction in perturbed surface with eyes closed condition (p= 0.004) and total limits of stability scores of perturbed surface with eyes closed condition (p= 0.046).

CONCLUSIONS

The study showed that balance test scores stabilized at different sessions from 1st to 3rd assessment period. Maximum normalized scores were reached at the third trial.

Attenuation of the evoked responses with repeated exposure to proprioceptive disturbances is muscle specific

In response to repetitive proprioceptive disturbances (vibration) applied to postural muscles, the evoked response has been shown to decrease in amplitude within the first few trials. The present experiment investigated whether this attenuation of the response to vibration stimulation (90Hz, 5s) was muscle specific or would be transferred to the antagonist muscles. Sixteen participants stood upright with eyes closed. One half of the participants practiced 15 tibialis vibrations followed by 15 calf vibrations (TIB-CALF order), while the other half practiced the opposite order (CALF-TIB order). Antero-posterior trunk displacements were measured at the level of C7 and centre of foot pressure (COP). EMG activity of the tibialis anterior (TA) and gastrocnemius lateralis (GL) was also measured. Results showed that evoked postural responses as well as EMG activity decreased with practice when vibration was applied to either calf or tibialis muscles. However, such attenuation of the response appeared muscle specific since it did not generalise when the same vibration stimulus was later applied onto the antagonist muscles.

Variations in linear and nonlinear postural measurements under achilles tendon vibration and unstable support-surface conditions

Reduced support-surface stability has been shown to attenuate the effect of Achilles tendon vibration on backward body displacement. In the present study, 20 participants performed a quiet, upright standing task on a stable and sway-referenced support, with and without vibration. The authors calculated equilibrium scores (ES), approximate entropy (ApEn), and mean and peak power spectral density frequencies of center-of-pressure variations. It was found that ES values decreased with the addition of vibration and in the sway-referenced support condition. ApEn values decreased with the addition of vibration but only with a stable support. Conversely, mean and peak frequencies increased with the addition of vibration, independent of support stability. These results suggest that the role of ankle proprioceptive input changes depending on support-surface characteristics and demonstrate the value of using both linear and nonlinear measures of postural sway.

Reduced postural differences between phobic postural vertigo patients and healthy subjects during a postural threat

Phobic postural vertigo is characterized by subjective imbalance and dizziness while standing or walking, despite normal values for clinical balance tests. Patients with phobic postural vertigo exhibit an increased high-frequency sway in posturographic tests. Their postural sway, however, becomes similar to the sway of healthy subjects during difficult balance tasks. Posturographic recordings of 30 s of quiet stance was compared to recordings of 30 s of quiet stance during a postural threat, which consisted of the knowledge of forthcoming vibratory calf muscle stimulation, in 37 consecutive patients with phobic postural vertigo and 24 healthy subjects. During quiet stance without the threat of forthcoming vibratory stimulation, patients with phobic postural vertigo exhibited a postural sway containing significantly more high-frequency sway than the healthy subjects. During the quiet stance with forthcoming vibratory stimulation, i.e., anticipation of a postural threat, the significant differences between groups disappeared for all variables except sagittal high-frequency sway. During postural threat, healthy subjects seemed to adopt a postural strategy that was similar to that exhibited by phobic postural vertigo patients. The lack of additional effects facing a postural threat among phobic postural vertigo patients may be due to an already maximized postural adaptation. Deviant postural reactions among patients with phobic postural vertigo may be considered as an avoidant postural response due to a constant fear of losing postural control.

Effects of proprioceptive vibratory stimulation on body movement at 24 and 36h of sleep deprivation

OBJECTIVE

To investigate whether postural stability and adaptation differed after a normal night of sleep, after 24h (24 SDep) and 36h (36 SDep) of sleep deprivation while subjected to repeated balance perturbations. Also, to determine whether there was any correlation between subjective alertness scores and objective posturographic measurements. Lastly, to investigate the effects of vision on the stability during sleep deprivation.

METHODS

Body movements at five locations were recorded in 18 subjects (mean age 23.8years) using a 3D movement measurement system while subjected with eyes open and closed to vibratory proprioceptive calf stimulation after a normal night of sleep, 24 and 36 SDep.

RESULTS

The clearest sleep deprivation effect was reduced ability to adapt head, shoulder and hip movements, both with eyes open and eyes closed. Additionally, several near falls occurred after being subjected to balance perturbations for 2-3min while sleep deprived. Unexpectedly, postural performance did not continue to deteriorate between 24 and 36h of sleep deprivation, but showed some signs of improvement. Subjective scores of sleepiness correlated poorly with actual changes in postural control performance.

CONCLUSIONS

Sleep deprivation might affect postural stability through reduced adaptation ability and lapses in attention. Subjective alertness might not be an accurate indicator of the physiological effects of sleep deprivation.

SIGNIFICANCE

Sleep deprivation could increase the risk of accidents in attention demanding tasks. There is a need for objective evaluation methods to determine actual performance capacity during sleep deprivation.

Learning Effects of Repetitive Administrations of the Sensory Organization Test in Healthy Young Adults

OBJECTIVES

To evaluate the learning effect of multiple administrations of the Sensory Organization Test (SOT) on performance and to begin to establish clinical meaningful change scores for the SOT.

DESIGN

Descriptive case series.

SETTING

University-affiliated clinic.

PARTICIPANTS

Healthy young adults (6 men, 7 women; mean age, 24+/-4y).

INTERVENTION

All subjects performed the standardized SOT using the SMART EquiTest 5 times over a 2-week period, and 1 month later.

MAIN OUTCOME MEASURE

Composite and individual SOT test condition standardized equilibrium scores.

RESULTS

Test-retest reliability (intraclass correlation coefficient model 2,3) of the composite (.67) and equilibrium score (range, .35-.79) were fair to good. Repeated-measures analysis of variance revealed a significant (P<.05) increase in the composite and equilibrium scores for conditions 4, 5, and 6 over the 5 sessions that plateaued after the third session, and were retained at 1 month. The 95% confidence interval for the composite score change from session 1 to session 4, the plateau of the learning effect, was 3.9 to 8.1.

CONCLUSIONS

Although the findings of this study would indicate that multiple baseline measures are desirable for the more challenging conditions, a composite change of greater than 8 points would indicate change due to rehabilitation.

Postural after-effects of stepping on an inclined surface

In previous studies, blindfolded, healthy subjects exhibited an after-effect of leaning while standing on a horizontal surface after a period of standing on an inclined surface. We investigated whether this kinesthetic after-effect would transfer from one task to another by asking blindfolded subjects to stand on a horizontal surface after stepping-in-place on an incline. Results showed that all subjects demonstrated a forward trunk leaning after-effect lasting from half a minute to over 6 min after stepping on a 10 degrees -toes-up incline for 2.5 min. For 5/7 subjects, the amplitude of the leaning after-effect was very similar following stepping or standing on the inclined surface. The similarity of the post-incline lean between the standing and stepping conditions suggests a common underlying mechanism for the after-effect following standing and walking on a gradient and suggests that prolonged maintenance of a constant ankle or leg posture is not a prerequisite condition for the after-effect. The transfer of a postural effect built-up during a locomotor task to a postural after-effect during a standing task is consistent with a central adaptive mechanism that adjusts the surface-referenced set point for whole body postural orientation for both gait and posture.

Retention of Adaptive Control Over Varying Intervals: Prevention of Slip- Induced Backward Balance Loss During Gait

Stability improvements made in a single acquisition session with merely five slips in walking are sufficient to prevent backward balance loss (BLOB) at the end of session, but not after 12 mo. The purpose of this study was to determine whether the effect of an enhanced single acquisition session would be retainable if tested sooner, at intervals of ≤4 mo. Twenty-four young subjects were exposed to blocks of slip, nonslip, and both types of trials during walking at their preferred speed in the acquisition session. In each of the four follow-up sessions around 1 wk, 2 wk, 1 mo, and 4 mo later, these same subjects experienced only a single slip after eight to 13 unperturbed walking trials in an otherwise identical setup. Gait stability was obtained as the shortest distance between the measured center of mass (COM) state (position and velocity) and the mathematically predicted threshold for BLOB at pre- and postslip, corresponding to the instants of touchdown of the slipping limb and liftoff of the contralateral limb, respectively. During the acquisition session, pre- and postslip stability improved significantly, resulting in a reduction of BLOB from 100% in the first slip (S1) to 0% in the last slip (S24), with improvements converging to a steady state, that enabled all of the subjects to avoid BLOB, regardless of whether a slip occurred. During retest sessions, subjects' preslip stability was not different from that in S24, but was greater than that in S1. Their postslip stability was also greater than that in S1 but less than that in S24, resulting in BLOB at a 40% level. No difference was found in any of these aspects between each follow-up session. These adaptive changes were associated with a range of individual differences, varying from no detectable deterioration in all aspects ( n = 8) to a consistent BLOB in all follow-ups ( n = 3). Our findings demonstrated the extent of plasticity of the CNS, characterized by rapid acquisition of a stable COM state under unpredictable slip conditions and retention of such improvements for months, resulting in a reduced occurrence of unintended backward falling.

TENS to the posterior aspect of the legs decreases postural sway during stance

The purpose of this study was to examine the effect of Transcutaneous Electrical Nerve Stimulation (TENS) applied to the posterior aspect of the legs, on postural sway during stance. Thirty healthy subjects were tested while standing on a force platform under four stimulation conditions: no TENS, bilateral TENS, and unilateral left and right TENS. Thirty-second long tests, employing detection threshold amplitudes, were performed in three blocks. In each block, the four conditions were applied both with and without vision in a random order. The results indicate that the application of TENS brought about a decrease in postural sway as expressed by average sway velocity, in addition to a decrease in the absolute values of maximal and minimal medio-lateral and anterior-posterior velocity. Thus, similar to sub-threshold random electrical noise, it appears that the application of low-amplitude TENS to the lower limbs decreases postural sway during stance. Considering the ease of TENS application and the high prevalence of postural disorders, the potential clinical significance of this observation is to be determined by further studies.

Comparison of different methods to identify and quantify balance control

The goal of this paper is to clarify the methodological aspects of studies of human balance during quiet standing and perturbed standing. Centre of mass (CoM), centre of pressure (CoP) and electromyogram (EMG) or similar measures are commonly recorded to quantify human balance control. In this paper we show that to identify the rigid body dynamics and the physiological mechanism that controls the body separately, one has to externally perturb the body with known perturbations and to use the indirect (IA) or joint input-output approach (JA) for identification. However, in many balance control studies the direct approach (DA) have been used, which is well suited to study open-loop systems but will give erroneous results when applied to a closed-loop system, as in human balance control. The cross-correlation function and linear regression are examples of the erroneous application of the DA approach in human balance control studies. The consequences of this erroneous DA are given. In addition a new application of the JA is presented that identifies physiological mechanisms that control balance, including passive and active feedback pathways. This new method is compared with existing identification schemes that use the IA and an existing JA that estimates the active pathway. Also it is shown how descriptive measures such as the power spectral densities (PSD) or the stabilogram diffusion plot (SDP) of the CoP and/or CoM depends on the PSD of internal perturbations and sensor noise, which are not measured. Although descriptive measures can be used to describe the state of the balance control system for a particular situation, it does not separate the dynamics of unknown processes that perturb balance from the dynamics of the active and passive feedback mechanisms that controls balance. Only the IA and the preferred JA can give estimates of the passive and active passive feedback mechanisms that control balance.

Differences in preferred reference frames for postural orientation shown by after-effects of stance on an inclined surface

This study reports a postural after-effect of leaning that follows a period of stance on an inclined surface with eyes closed. This leaning after-effect maintained the body-to-surface relationship as if subjects still stood on the incline. We examined the incidence and robustness of the leaning after-effect in 51 healthy subjects. The location of the center of pressure (CoP) under the feet and the alignment of the trunk and legs were measured before, during and after blindfolded subjects stood on a 5 degrees toes-up inclined surface for 2.5 min. When the surface was inclined, all subjects stood with their trunk and legs aligned near to gravity-vertical, similar to the alignment adopted in the pre-incline period. When the surface returned to horizontal in the post-incline period, there was a continuum of postural alignment strategies across subjects. At one extreme, subjects leaned forward, with an average trunk lean near 5 degrees . The leaned posture decayed exponentially toward baseline postural alignment across a period of up to 5 min. At the other extreme, subjects did not lean in the post-incline period, but instead, stayed aligned near upright with respect to gravity. Subjects were highly consistent in their post-incline postural behaviors upon repeated testing over days to months and across different directions of surface inclination. Our results suggest that individuals have well-established, preferred, sensory strategies for controlling postural orientation when vision is not available. Subjects who leaned in the post-incline period appear to depend more on the geometry of the support surface as a reference frame and to rely more on proprioceptive information to extract kinematic relationships, whereas subjects who did not lean appear to depend more on gravity as a reference frame and to rely more on sensory information related to forces and load.

Balance control and adaptation during vibratory perturbations in middle-aged and elderly humans

The objective was to investigate if healthy elderly people respond and adapt differently to postural disturbances compared to middle-aged people. Thirty middle-aged (mean age 37.8 years, range 24-56 years) and forty healthy elderly subjects (mean age 74.6 years, range 66-88 years) were tested with posturography. Body sway was evoked by applying pseudorandom vibratory stimulation to the belly of the gastrocnemius muscles of both legs simultaneously. The tests were performed both with eyes open and eyes closed. The anteroposterior body sway was measured with a force platform and analyzed with a method that considers the adaptive changes of posture and stimulation responses. The results showed that middle-aged people generally used a different postural control strategy as compared to the elderly. The elderly responded more rapidly to vibratory perturbation, used more high-frequency (>0.1 Hz) motions and the motion dynamics had a higher degree of complexity. Moreover, the elderly had diminished ability to use visual information to improve balance control. Altogether, despite having an effective postural control adaptation similar to that of middle-aged people, the elderly had more difficulty in withstanding balance perturbations. These findings suggest that the balance control deterioration associated with aging cannot be fully compensated for by postural control adaptation.

Postural control adaptation during galvanic vestibular and vibratory proprioceptive stimulation

The objective for this study was to investigate whether the adaptation of postural control was similar during galvanic vestibular stimulation and during vibratory proprioceptive stimulation of the calf muscles. Healthy subjects were tested during erect stance with eyes open or closed. An analysis method designed to consider the adaptive adjustments was used to evaluate the motion dynamics and the evoked changes of posture and stimulation response. Galvanic vestibular stimulation induced primarily lateral body movements and vibratory proprioceptive stimulation induced anteroposterior movements. The lateral body sway generated by the galvanic stimulation was proportionally smaller and contained more high-frequency movements (> 0.1 Hz) than the anteroposterior body sway induced by the vibratory stimulation. The adaptive adjustments of the body sway to the stimulation had similar time course and magnitude during galvanic and vibratory stimulation. The perturbations induced by stimulation were gradually reduced within the same time range (15-20 s) and both kinds of stimulation induced a body leaning whose direction was dependent on stimulus. The similarities in the adjustment patterns suggest that postural control operates in the same way independent of the receptor systems affected by the disturbance and irrespective of whether the motion responses were induced in a lateral or anteroposterior direction.