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Vestibular-evoked balance responses are facilitated when faced with threats to stability. However, the extent to which these sensorimotor adaptations covary with changes in emotional and autonomic state remains unclear. This study repeatedly exposed individuals to the same postural threat while vestibular-motor responses were probed using stochastic vestibular stimulation (SVS; 2-25 Hz). This allowed emotional and autonomic state to be manipulated within the same threat environment to determine if vestibular-evoked balance responses are coupled with the emotional/autonomic changes induced by the threat or are facilitated in a strictly context-dependent manner. Twenty-three young adults stood with their head turned 90° while receiving SVS at LOW (0.8 m above ground, away from edge) and HIGH (3.2 m above ground, at edge) conditions. LOW trials were completed before and after a block of 10 HIGH trials. Ground reaction forces (GRFs) and plantar flexor (soleus and medial gastrocnemius (MG)) EMG were recorded. Vestibular-evoked responses recorded from GRFs and EMG were quantified in terms of signal coupling (coherence and cumulant density) and gain, and emotional and autonomic state were assessed from self-reports and electrodermal activity. Vestibular-evoked balance responses were facilitated with initial threat exposure. After repeated exposure, there was significant habituation of the emotional response to threat, which was accompanied by reductions in vestibular-evoked balance responses, most notably for GRFs and MG-EMG. This suggests that threat-related changes in vestibular-motor function are tightly coupled with the emotional and autonomic changes induced by the threat, and not an invariant response to context-specific features of the threat. KEY POINTS: Balance corrective responses mediated through vestibular-motor pathways are facilitated when stability is threatened; however, the extent to which these sensorimotor adaptations covary with changes in emotional state remains unclear. By having young adults repeatedly stand at the edge of an elevated surface, this study examined how vestibular-evoked balance responses, probed using stochastic vestibular stimulation and recorded from ground reaction forces and plantar flexor EMG, changed alongside estimates of emotional state. Vestibular-evoked responses were facilitated when individuals were first exposed to the postural threat, but demonstrated marked habituation alongside estimates of emotional state after repeated exposure. This suggests that threat-related changes in vestibular-motor function are coupled to the emotional response to threat, and are not an invariant response to context-specific features of the threat. These changes in vestibular-motor function are likely part of a multisensory adaptation process that primes the nervous system to respond to sudden destabilizing forces when fearful of falling., (© 2025 The Author(s). The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Our ability to balance upright provides a stable platform to perform daily activities. Balance deficits associated with various clinical conditions may affect activities of daily living, highlighting the importance of quantifying standing balance in ecological environments. Although typically performed in laboratory settings, the growing availability of low-cost inertial measurement units (IMUs) allows the assessment of balance in the real world. However, it is unclear how many IMUs are required to adequately estimate linear displacements of the centre of mass (CoM) at stance widths associated with daily activities. While wearing IMUs on their head, sternum, back, right thigh, right shank, and left shank, 16 participants stood quietly on a force platform in narrow, hip-width, and shoulder-width stances, each for three two-minute trials. Using a multi-segment biomechanical model, we estimated CoM displacements from all possible combinations of the IMUs. We then calculated the correlation between the IMU- and force platform- CoM estimates to determine the minimal number of IMUs needed to estimate CoM sway. Four IMUs were necessary to accurately estimate anteroposterior (AP) and mediolateral (ML) CoM displacements across stance widths. Using IMUs on the back, right thigh, and both shanks, we found strong correlations between the IMU CoM estimation and the force platform CoM estimation in narrow stance (AP: r = 0.92±0.04, RMSE = 2.39±2.08 mm; ML: r = 0.97±0.02, RMSE = 1.16±0.77 mm), hip-width stance (AP: r = 0.93±0.04, RMSE = 2.00±1.18 mm; ML: r = 0.92±0.06, RMSE = 0.92±0.70 mm), and shoulder-width stance (AP: r = 0.93±0.03, RMSE = 1.95±1.66 mm; ML: r = 0.86±0.13, RMSE = 1.39±1.46 mm). These results indicate that IMUs can be used to estimate CoM displacements during quiet standing and that four IMUs are necessary to do so. Using an algorithm based on a simple biomechanical model, researchers and clinicians can estimate whole-body CoM displacements accurately during unperturbed quiet standing. This approach can improve the ecological validity of standing balance research and opens the possibility for assessing/monitoring patients with standing balance deficits., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2025 Foulger et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The relative contributions of proprioceptive, vestibular, and visual sensory cues to balance control change depending on their availability and reliability. This sensory reweighting is classically supported by nonlinear sway responses to increasing visual surround and/or surface tilt amplitudes. However, recent evidence indicates that visual cues are reweighted based on visual tilt velocity rather than tilt amplitude. Therefore, we designed a study to specifically test the hypothesized velocity dependence of reweighting while expanding on earlier findings for visual reweighting by testing proprioceptive reweighting for standing balance on a tilting surface. Twenty healthy young adults stood with their eyes closed on a toes-up/-down tilting platform. We designed four pseudorandom tilt sequences with either a slow (S) or a fast (F) tilt velocity and different peak-to-peak amplitudes. We used model-based interpretations of measured sway characteristics to estimate the proprioceptive sensory weight ( W prop ) within each trial. In addition, root-mean-square values of measured body center of mass sway amplitude (RMS) and velocity (RMSv) were calculated for each tilt sequence. W prop , RMS, and RMSv values varied depending on the stimulus velocity, exhibiting large effects (all Cohen's d >1.10). In contrast, we observed no significant differences across stimulus amplitudes for W prop (Cohen's d : 0.02-0.16) and, compared with the differences in velocity, there were much smaller changes in RMS and RMSv values (Cohen's d : 0.05-0.91). These results confirmed the hypothesized velocity, rather than amplitude, dependence of sensory reweighting. NEW & NOTEWORTHY This novel study examined the velocity dependence of sensory reweighting for human balance control using support surface tilt stimuli with independently varied amplitude and velocity. Estimates of the proprioceptive contribution to standing balance, derived from model-based interpretations of sway characteristics, showed greater sensitivity to changes in surface tilt velocity than surface tilt amplitude. These results support a velocity-based mechanism underlying sensory reweighting for human balance control.

Background : Falls and related injuries are critical issues in several disease states, as well as aging, especially when interactions between vestibular and visual sensory inputs are involved. Slow support surface tilt (0.6 deg/s) followed by subjective postural horizontal (SPH) assessments have been proposed as a viable method for assessing otolith contributions to balance control. Previous assessments of perceived body alignment to vertical, including subjective visual vertical, have suggested that visual inputs are weighted more when vestibular information is near the threshold and less reliable during slow body tilt. To date, no studies have examined the influence of visual stimuli on slow roll-tilt postural responses and the SPH. Therefore, this study investigated how dynamic visual cues, in the form of circular vection (CV), influence postural responses and the perception of the horizontal during and after support surface tilt. Methods : Ten healthy young adults (6 female, mean age 23) wore a head-mounted display while standing on a tilting platform. Participants were asked to remain upright for 30 s, during which (1) the visual scene rotated, inducing roll CV clockwise (CW) or counter-clockwise (CCW) at 60°/s; (2) the platform only (PO) rotated in roll to test SPH (0.6°/s, 2°, CW or CCW); (3) a combination of both; or (4) neither occurred. During SPH trials, participants used a hand-held device to reset the position of the platform to 0.8°/s to their perceived SPH. The angular motion of body segments was measured using pairs of light-emitting diodes mounted on the head, trunk and pelvis. Segment motion, prior to platform motion, was compared to that at peak body motion induced by platform motion and when SPH had been set. Results : When the support surface was tilted 2°, peak upper body tilt significantly increased for congruent CV and platform tilt and decreased at the pelvis for incongruent CV when compared to PO, leading to significant differences across body segments for congruent and incongruent conditions ( p ≤ 0.008). During PO, participants' mean SPH deviated from horizontal by 0.2°. The pelvis deviated 0.2°, the trunk 0.3°, and the head 0.5° in the direction of initial platform rotation. When platform tilt and CV directions were congruent or incongruent, only head tilt at SPH reset under congruent conditions was significantly different from the PO condition (1.7° vs. 0.5°). Conclusions : Roll CV has a significant effect on phasic body responses and a less significant effect on tonic body responses to lateral tilt. The SPH of the support surface was not altered by CV. Responses during tilt demonstrated enhanced reactions for congruent and reduced reactions for incongruent CV, both different from responses to CV alone. Tonic body displacements associated with SPH were changed less than those during tilt and were only slightly larger than displacements for CV alone. This study supports the hypothesis of weighted multisensory integration during dynamic postural tasks being highly dependent on the direction of visual cues during tilt and less dependent on tonic SPH offsets. These techniques could be used to examine vestibular and visual interactions within clinical populations, particularly those with visual vertigo and dizziness.

Introduction: The ability to scale anticipatory postural adjustments (APAs) according to the predicted size of the upcoming movement is reduced with aging. While age-related changes in central set may be one reason for this effect, an individual's emotional state might also contribute to changes in anticipatory postural control. Therefore, the purpose of this study was to determine whether an altered emotional state, as elicited through postural threat, alters the scaling of APAs during a handle pull movement in young and older adults. It was hypothesized that the presence of postural threat would lead to more homogenous APAs (i.e., less scaling of APAs) across a range of pulling forces., Methods: Young ( n  = 23) and older adults ( n  = 16) stood on top of a force plate that was mounted to a motorized platform. From this position, participants performed a series of handle pull trials without (no threat) or with (threat) the possibility of receiving a postural perturbation in the form of an unpredictable surface translation. Handle pulls were performed at force levels between 50 and 90% of maximum force. For each trial, the magnitude and timing of the APA were quantified from center of pressure (COP) recordings as well as electromyographic (EMG) activity of the soleus and medial gastrocnemius. The scaling of APAs with respect to force exertion was then determined through regression analyses and by comparing APAs during pulls of lower versus higher force., Results and Discussion: As evidenced by their smaller slope of the regression line between various dependent measures (i.e., COP velocity, soleus EMG onset latency, and soleus EMG amplitude) and the pulled forces, older adults demonstrated less scaling of APAs than the young. However, increases in arousal, anxiety and fear of falling due to postural threat, only minimally altered the scaling of APAs. Regardless of age, the slope of the regressions for none of the measures were affected by threat while only the soleus and medial gastrocnemius EMG onsets demonstrated significant force × threat interaction effects. These results suggest that the decreased ability to scale APAs with aging is unlikely to be due to changes in emotional state., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Phanthanourak, Adkin, Carpenter and Tokuno.)

It has been proposed that sensory force/pressure cues are integrated within a positive feedback mechanism, which accounts for the slow dynamics of human standing behavior and helps align the body with gravity. However, experimental evidence of this mechanism remains scarce. This study tested predictions of a positive torque feedback mechanism for standing balance, specifically that differences between a "reference" torque and actual torque are self-amplified, causing the system to generate additional torque. Seventeen healthy young adults were positioned in an apparatus that permitted normal sway at the ankle until a brake on the apparatus was applied, discreetly "locking" body movement during stance. Once locked, a platform positioned under the apparatus remained in place (0 mm) or slowly translated backward (3 mm or 6 mm), tilting subjects forward. Postural behavior was characterized by two distinct responses: the center of pressure (COP) offset (i.e., change in COP elicited by the surface translation) and the COP drift (i.e., change in COP during the sustained tilt). Model simulations were performed using a linear balance control model containing torque feedback to provide a conceptual basis for the interpretation of experimental results. Holding the body in sustained tilt positions resulted in COP drifting behavior, reflecting attempts of the balance control system to restore an upright position through increases in plantar flexor torque. In line with predictions of positive torque feedback, larger COP offsets led to faster increases in COP over time. These findings provide experimental support for a positive torque feedback mechanism involved in the control of standing balance. NEW & NOTEWORTHY Using model simulations and a novel experimental approach, we tested behavioral predictions of a sensory torque feedback mechanism involved in the control of upright standing. Torque feedback is thought to reduce the effort required to stand and play a functional role in slowly aligning the body with gravity. Our results provide experimental evidence of a torque feedback mechanism and offer new and valuable insights into the sensorimotor control of human balance.

Height-induced postural threat affects emotional state and standing balance behaviour during static, voluntary, and dynamic tasks. Facing a threat to balance also affects sensory and cortical processes during balance tasks. As sensory and cognitive functions are crucial in forming perceptions of movement, balance-related changes during threatening conditions might be associated with changes in conscious perceptions. Therefore, the purpose of this study was to examine the changes and potential mechanisms underlying conscious perceptions of balance-relevant information during height-induced postural threat. A combination of three experimental procedures utilized height-induced postural threat to manipulate emotional state, balance behavior, and/or conscious perceptions of balance-related stimuli. Experiment 1 assessed conscious perception of foot position during stance. During continuous antero-posterior pseudorandom support surface rotations, perceived foot movement was larger while actual foot movement did not change in the High (3.2 m, at the edge) compared to Low (1.1 m, away from edge) height conditions. Experiment 2 and 3 assessed somatosensory perceptual thresholds during upright stance. Perceptual thresholds for ankle rotations were elevated while foot sole vibrations thresholds remained unchanged in the High compared to Low condition. This study furthers our understanding of the relationship between emotional state, sensory perception, and balance performance. While threat can influence the perceived amplitude of above threshold ankle rotations, there is a reduction in the sensitivity of an ankle rotation without any change to foot sole sensitivity. These results highlight the effect of postural threat on neurophysiological and cognitive components of balance control and provide insight into balance assessment and intervention., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Cleworth, Peters, Chua, Inglis and Carpenter.)

Introduction: Postural threat elicits modifications to standing balance. However, the underlying neural mechanism(s) responsible remain unclear. Shifts in attention focus including directing more attention to balance when threatened may contribute to the balance changes. Sample entropy, a measure of postural sway regularity with lower values reflecting less automatic and more conscious control of balance, may support attention to balance as a mechanism to explain threat-induced balance changes. The main objectives were to investigate the effects of postural threat on sample entropy, and the relationships between threat-induced changes in physiological arousal, perceived anxiety, attention focus, sample entropy, and traditional balance measures. A secondary objective was to explore if biological sex influenced these relationships., Methods: Healthy young adults (63 females, 42 males) stood quietly on a force plate without (No Threat) and with (Threat) the expectation of receiving a postural perturbation (i.e., forward/backward support surface translation). Mean electrodermal activity and anterior-posterior centre of pressure (COP) sample entropy, mean position, root mean square, mean power frequency, and power within low (0-0.05 Hz), medium (0.5-1.8 Hz), and high-frequency (1.8-5 Hz) components were calculated for each trial. Perceived anxiety and attention focus to balance, task objectives, threat-related stimuli, self-regulatory strategies, and task-irrelevant information were rated after each trial., Results and Discussion: Significant threat effects were observed for all measures, except low-frequency sway. Participants were more physiologically aroused, more anxious, and directed more attention to balance, task objectives, threat-related stimuli, and self-regulatory strategies, and less to task-irrelevant information in the Threat compared to No Threat condition. Participants also increased sample entropy, leaned further forward, and increased the amplitude and frequency of COP displacements, including medium and high-frequency sway, when threatened. Males and females responded in the same way when threatened, except males had significantly larger threat-induced increases in attention to balance and high-frequency sway. A combination of sex and threat-induced changes in physiological arousal, perceived anxiety, and attention focus accounted for threat-induced changes in specific traditional balance measures, but not sample entropy. Increased sample entropy when threatened may reflect a shift to more automatic control. Directing more conscious control to balance when threatened may act to constrain these threat-induced automatic changes to balance., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Fischer, Missen, Tokuno, Carpenter and Adkin.)

During unperturbed bipedal standing, postural control is governed primarily by subcortical and spinal networks. However, it is unclear if cortical networks begin to play a greater role when stability is threatened. This study investigated how initial and repeated exposure to a height-related postural threat modulates cortical potentials time-locked to discrete centre of pressure (COP) events during standing. Twenty-seven young adults completed a series of 90-s standing trials at LOW (0.8 m above the ground, away from edge) and HIGH (3.2 m above the ground, at edge) threat conditions. Three LOW trials were completed before and after 15 consecutive HIGH trials. Participants stood on a force plate while electroencephalographic (EEG) activity was recorded. To examine changes in cortical activity in response to discrete postural events, prominent forward and backward peaks in the anterior-posterior COP time series were identified. EEG data were waveform-averaged to these events and the amplitude of event-related cortical activity was calculated. At the LOW condition, event-related potentials (ERPs) were scarcely detectable. However, once individuals stood at the HIGH condition, clear ERPs were observed, with more prominent potentials being observed for forward (edge-directed), compared to backward, COP events. Since forward COP peaks accelerate the centre of mass away from the platform edge, these results suggest there is intermittent recruitment of cortical networks that may be involved in the detection and minimization of postural sway toward a perceived threat. This altered cortical engagement appears resistant to habituation and may contribute to threat-related balance changes that persist following repeated threat exposure. KEY POINTS: While standing balance control is regulated primarily by subcortical and spinal processes, it is unclear if cortical networks play a greater role when stability is threatened. This study examined how cortical potentials time-locked to prominent peaks in the anterior-posterior centre of pressure (COP) time series were modulated by exposure to a height-related postural threat. While cortical potentials recorded over the primary sensorimotor cortices were scarcely detectable under non-threatening conditions, clear cortical potentials were observed when individuals stood under conditions of height-related threat. Cortical potentials were larger in response to COP peaks directed toward, compared to away from, the platform edge, and showed limited habituation with repeated threat exposure. Since forward COP peaks accelerate the centre of mass away from the platform edge, these findings suggest that when balance is threatened, there is intermittent recruitment of cortical networks, which may minimize the likelihood of falling in the direction of a perceived threat., (© 2023 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Sensory perturbations are a valuable tool to assess sensory integration mechanisms underlying balance. Implemented as systems-identification approaches, they can be used to quantitatively assess balance deficits and separate underlying causes. However, the experiments require controlled perturbations and sophisticated modeling and optimization techniques. Here we propose and validate a virtual reality implementation of moving visual scene experiments together with model-based interpretations of the results. The approach simplifies the experimental implementation and offers a platform to implement standardized analysis routines. Sway of 14 healthy young subjects wearing a virtual reality head-mounted display was measured. Subjects viewed a virtual room or a screen inside the room, which were both moved during a series of sinusoidal or pseudo-random room or screen tilt sequences recorded on two days. In a between-subject comparison of 10 [Formula: see text] 6 min long pseudo-random sequences, each applied at 5 amplitudes, our results showed no difference to a real-world moving screen experiment from the literature. We used the independent-channel model to interpret our data, which provides a direct estimate of the visual contribution to balance, together with parameters characterizing the dynamics of the feedback system. Reliability estimates of single subject parameters from six repetitions of a 6 [Formula: see text] 20-s pseudo-random sequence showed poor test-retest agreement. Estimated parameters show excellent reliability when averaging across three repetitions within each day and comparing across days (Intra-class correlation; ICC 0.7-0.9 for visual weight, time delay and feedback gain). Sway responses strongly depended on the visual scene, where the high-contrast, abstract screen evoked larger sway as compared to the photo-realistic room. In conclusion, our proposed virtual reality approach allows researchers to reliably assess balance control dynamics including the visual contribution to balance with minimal implementation effort., (© 2023. The Author(s).)

Exposure to postural threat has been documented to influence the sensory contributions of proprioceptive and vestibular information in standing balance control. Contributions from the visual system to balance are also crucial, yet the degree to which postural threat may modulate visual control of balance is not well characterized. Therefore, the aims of this study were to assess the feasibility of eliciting visual evoked postural responses (VEPRs) using head-mounted virtual reality (VR) and use this method to examine the potential influence of virtual postural threat on the visual control of balance. Thirty-six healthy young adults were exposed to a pseudorandom, translational visual stimulus of a real-world environment in VR. The visual stimulus was presented in virtual conditions of LOW and HIGH postural threat in which participants stood at ground level, and on a 7 m elevated platform, respectively. VEPRs were successfully produced in both postural threat conditions. When exposed to the visual stimulus while at an elevated surface height, participants demonstrated significant changes to their physiological arousal and emotional state. Despite significant coherence across the stimulus' frequency range, stimulus correlated VEPRs were not significantly modulated during exposure to the visual stimulus under virtual postural threat. This study supports the future utility of VR head-mounted displays in examining emotional influences on the visual control of balance., (Copyright © 2022. Published by Elsevier B.V.)

Threats to stability elicit context-specific changes in balance control; however, the underlying neural mechanisms are not fully understood. Previous work has speculated that a shift toward greater supraspinal control may contribute to threat-related balance changes. This study investigated how neural correlates of cortical and subcortical control of balance were affected by initial and repeated exposure to a height-related postural threat. Corticomuscular coherence (CMC) between EEG recorded over the sensorimotor cortex and EMG recorded from the soleus (SOL) provided an estimate of cortical control, while intermuscular coherence (IMC) between bilateral SOL provided estimates of both cortical and subcortical control. These outcomes, along with measures of psychological and arousal state and standing balance control, were examined in 28 healthy young adults during a series of 90-s quiet standing trials completed at LOW (0.8 m above ground; away from edge) and HIGH (3.2 m above ground, at edge) threat conditions. Initial exposure to threat significantly increased gamma-band CMC (31-40 Hz) and IMC at frequencies thought to be mediated by cortical (21-40 Hz) and subcortical (5-20 Hz) substrates. Following repeated threat exposure, only estimates of cortical control (gamma CMC and 21-40 Hz IMC) demonstrated significant habituation. Estimates of cortical control changed in parallel with high-frequency centre of pressure power (>0.5 Hz) and plantar-dorsiflexor coactivation, but not other threat-related balance changes which did not habituate. These results support the hypothesis that postural threat induces a shift toward more supraspinal control of balance, and suggests this altered neural control may contribute to specific threat-related balance changes., (Copyright © 2022 IBRO. Published by Elsevier Ltd. All rights reserved.)

We investigated the impairment of position sense associated with muscle fatigue. In Experiment 1 , participants performed learned eccentric extension (22°/s) movements of the elbow as the arm was pulled through the horizontal plane without vision of the arm. They opened their closed right hand when they judged it to be passing through a target. Dynamic position sense was assessed via accuracy of limb position to the target at the time of hand opening. Eccentric movements were performed against a flexion load [10% of flexion maximum voluntary contractions (MVCs)]. We investigated performance under conditions with and without biceps vibration, as well as before and after eccentric exercise. In Experiment 2 , a motor was used to extend the participant's limb passively. We compared conditions with and without vibration of the lengthening but passive biceps, before and after exercise. In Experiment 1 , vibration of the active biceps resulted in participants opening their hands earlier [mean, [Formula: see text] (95% confidence interval, CI) -5.52° (-7.40, -3.63)] compared with without vibration. Exercise reduced flexion MVCs by ∼44%, and participants undershot the target more [-5.51° (-9.31, -1.70)] in the post-exercise block during control trials. Exercise did not influence the persistence of the vibratory illusion. In Experiment 2 , vibration resulted in greater undershooting [-2.99° (-3.99, -1.98)] compared with without vibration, before and after exercise. Although exercise reduced MVCs by ∼50%, the passive task showed no effects of exercise. We suggest that the central nervous system continues to rely on muscle spindles for limb position sense, even when they reside in a muscle exposed to fatiguing eccentric contractions. NEW & NOTEWORTHY Dynamic position errors were examined in an eccentric and a passive elbow extension proprioceptive-targeting task, before and after eccentric exercise, with and without muscle vibration. Participants actively undershot the target more when fatigued while fatigue did not exacerbate task accuracy during passive movement. Vibration caused undershoots regardless of fatigue state during active and passive movements. We propose that the central nervous system continues to rely on muscle spindles for kinesthesia, even when they reside in a fatigued muscle.