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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.

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.

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).)

Introduction: The StartReact (SR) effect is the accelerated release of a prepared movement when a startling acoustic stimulus is presented at the time of the imperative stimulus (IS). SR paradigms have been used to study defective control of balance and gait in people with neurological conditions, but differences in emotional state (e.g. fear of failure) may be a potential confounder when comparing patients to healthy subjects. In this study, we aimed to gain insight in the effects of postural threat on the SR effect by manipulating surface height during a postural (lateral step) task and a non-postural (wrist extension) task., Methods: Eleven healthy participants performed a lateral step perpendicular to the platform edge, and 19 participants performed a wrist extension task while standing at the platform edge. Participants initiated the movement as fast as possible in response to an IS that varied in intensity across trials (80 dB to 121 dB) at both low and high platform height (3.2 m). For the lateral step task, we determined anticipatory postural adjustments (APA) and step onset latencies. For the wrist extension task, muscle onset latencies were determined. We used Wilcoxon signed-rank tests on the relative onset latencies between both heights, to identify whether the effect of height was different for IS intensities between 103 and 118 dB compared to 121 dB., Results: For both tasks, onset latencies were significantly shortened at 121 dB compared to 80 dB, regardless of height. In the lateral step task, the effect of height was larger at 112 dB compared to 121 dB. The absolute onset latencies showed that at 112 dB there was no such stimulus intensity effect at high as seen at low surface height. In the wrist extension task, no differential effects of height could be demonstrated across IS intensities., Conclusions: Postural threat had a significant, yet modest effect on shortening of RTs induced by a loud IS, with a mere 3 dB difference between standing on high versus low surface height. Interestingly, this effect of height was specific to the postural (i.e. lateral stepping) task, as no such differences could be demonstrated in the wrist extension task. This presumably reflects more cautious execution of the lateral step task when standing on height. The present findings suggest that applying stimuli of sufficiently high intensity (≥115 dB) appears to neutralize potential differences in emotional state when studying SR effects., (Copyright © 2021 The Authors. Published by Elsevier B.V. All rights reserved.)

Background: Anxiety and arousal have been shown to influence balance control and, therefore, have the potential to confound balance assessment. It has been suggested that the 'first-trial' effect, where performance on the first trial of a balance task differs from subsequent trials, may be a result of participants being more anxious during their first experience of having their balance assessed. However, this remains speculative since limited work has simultaneously examined emotional state and balance control during repeated assessment of the same balance task., Research Question: Determine how emotional state and standing balance control change over the course of repeated assessment., Methods: Seventy-five healthy young adults completed five 120-s quiet standing trials. Psychological state was probed at each trial using self-report measures that assessed confidence, anxiety, and attention focus. Arousal was estimated from electrodermal activity and balance control was assessed from centre of pressure (COP) measures derived from forceplate data. Repeated measures ANOVAs were conducted to determine how each of these estimates changed with repeated testing., Results: There were significant changes in emotional state with repeated testing; self-report and autonomic measures indicated that participants were most anxious and physiologically aroused during the first trial. This emotional response diminished with repeated testing, although the greatest changes occurred from the first to second trial. Despite these changes in emotional state, only some COP outcomes significantly changed. Individuals leaned further forward during only the first trial and demonstrated higher frequency and velocity mediolateral COP oscillations during the first two trials., Significance: When balance is assessed for the first time in an unfamiliar laboratory setting, there is a transient emotional response which appears sufficient to influence some aspects of balance control. It is critical to control for these confounds when designing experiments or interventions involving balance assessment., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Background: How people with Parkinson's disease habituate their postural response to unpredictable translation perturbation is not totally understood. We compared the capacity to change the postural responses after unexpected external perturbation and investigated the habituation plateaus of postural responses to non-sequential perturbation trials in people with Parkinson's disease and healthy older adults., Methods: In people with Parkinson's disease (n = 37) and older adults (n = 20), sudden posterior support-surface translational were applied in 7 out of 17 randomized trials to ensure perturbation unpredictability. Electromyography and center of pressure parameters of postural response were analyzed by ANOVAs (Group vs. Trials). Two simple planned contrasts were performed to determine at which trial the responses first significantly habituate, and by which trials the habituation plateaus., Findings: Older adults demonstrated a first response change in trial 5 and habituation plateaus after trial 4, while for people with Parkinson's disease, the first change occurred in trial 2 and habituation plateau after trial 5 observed by center of pressure range. People with Parkinson's disease demonstrated a greater center of pressure range in trial 1 compared to older adults. Independent of trial, people with Parkinson's disease vs. older adults demonstrated a greater ankle muscle co-activation and recovery time., Interpretation: Despite the greater center of pressure range in the first trial, people with Parkinson's disease can habituate to unpredictable perturbations. This is reflected by little, to no difference in the time-course of adaptation for all but 2 parameters that showed only marginal differences between people with Parkinson's disease and older adults., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Background: Social anxiety caused by the presence of an evaluator can impair balance performance in older women. However, it is unknown whether co-performing balance tasks with a partner mitigates this effect., Research Question: Does the presence of a partner mitigate the effect of social anxiety on static and dynamic balance assessment in older women?, Methods: Twenty-one older women (mean age 66.5 (SD = 5.2) years) performed nine balance tasks under three conditions: (a) Alone (no evaluator present); (b) Evaluator (male evaluator present); (c) Partner (evaluator + performing tasks in parallel with partner). Participants were split into two groups post-hoc: Affected (n = 10) and Unaffected (n = 11), based on their emotional response to the presence of the evaluator (increased self-reported anxiety and fear)., Results: The affected group took a longer time to complete tandem walking with eyes open in the Evaluator vs. Alone condition, but not in the Partner condition. Both groups increased anterior-posterior trunk angular velocity during tandem walking with eyes closed in the Evaluator vs. Alone condition, but not in the Partner condition., Significance: Social anxiety impairs the balance performance of older women, particularly in those most affected by the evaluator, and during more dynamic modified gait tasks that challenge balance while walking. However, co-performing balance tasks with a partner reduced the effects of social anxiety, suggesting that social support may help to mitigate some of the potential 'white coat' effects experienced during clinical balance assessments., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Humans exhibit changes in postural control when confronted with threats to stability. This study used a prolonged threat exposure protocol to manipulate emotional state within a threatening context to determine if any threat-induced standing behaviours are employed independent of emotional state. Retention of balance adaptations was also explored. Thirty-seven adults completed a series of 90-s standing trials at two surface heights (LOW: 0.8 m above ground, away from edge; HIGH: 3.2 m above ground, at edge) on two visits 2-4 weeks apart. Psychological and autonomic state was assessed using self-report and electrodermal measures. Balance control was assessed using centre of pressure (COP) and lower limb electromyographic recordings. Upon initial threat exposure, individuals leaned backward, reduced low-frequency centre of pressure (COP) power, and increased high-frequency COP power and plantar/dorsiflexor coactivation. Following repeated exposure, the psychological and autonomic response to threat was substantially reduced, yet only high-frequency COP power and plantar/dorsiflexor coactivation habituated. Upon re-exposure after 2-4 weeks, there was partial recovery of the emotional response to threat and few standing balance adaptations were retained. This study suggests that some threat-induced standing behaviours are coupled with the psychological and autonomic state changes induced by threat, while others may reflect context-appropriate adaptations resistant to habituation.

The purpose of this study was to determine whether the type and direction of postural perturbation threat differentially affect anticipatory postural control. Healthy young adults stood on a force plate fixed to a translating platform and completed a series of rise-to-toes movements without (No Threat) and with (Threat) the potential of receiving a postural perturbation to either their feet (15 participants) or torso (16 participants). Each type of perturbation threat was presented along the anteroposterior (A-P) or mediolateral (M-L) axis. For each condition, the A-P center of pressure (COP) signal and tibialis anterior (TA) and soleus (SOL) electromyographical (EMG) recordings were used to quantify the anticipatory postural adjustment (APA). Results indicated that across both threat types and directions, postural threat induced a 40.2% greater TA activation (p < 0.001), a 18.5% greater backward COP displacement (p < 0.001) and a 23.9% greater backward COP velocity (p < 0.001), leading to larger and faster APAs than the No Threat condition. Subsequently, a 7.7% larger forward COP displacement (p = 0.001), a 20.4% greater forward COP velocity (p < 0.001) and 43.2% greater SOL activation (p = 0.009) were observed during the execution phase of the rise-to-toes for the Threat compared to the No Threat condition. Despite these threat effects, there were no differences in the magnitude or velocity of APAs between the threat directsion conditions. Since the type and direction of perturbation-induced postural threat had minimal differential effects on anticipatory postural control, these factors are unlikely to explain the discrepancy of previous findings., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Purpose: Landing involves a tuned anticipatory control to allow for soft and safe contact with the ground. Fearful situations are known to affect postural control strategies during standing, but it is still unclear how fear interferes with the control of a voluntary dynamic task requiring coordination between posture and movement., Methods: Ground reaction forces, limb movements, physiological arousal, and perceived levels of confidence and fear of falling were recorded when hopping off a box to a platform situated 0.8 m above ground and 3.2 m above ground., Results: Height induced a perceived threat as arousal was augmented by the elevated surface for all subjects. Threat induced by height modifies the way participants land, leading to a stiffer landing, as evidenced by an increased loading rate at touchdown during high threat conditions. Greater psychological and physiological changes are associated with greater changes in the control of landing: individuals that are less confident/more fearful appear to compensate for this stiffer landing, by slowing down their landing., Conclusion: Threatening conditions induces a harder contact to the ground, but the strategy is dependent of the level of confidence/fear. Less confident/more fearful participants are more focused on coping strategy and adopt a more cautious behaviour.

To walk safely in their environment, people need to select adequate movement strategies during gait. In situations that are perceived as more threatening, older adults adopt more cautious strategies. For individuals with excessive fear, selecting adequate strategies might be troubling. We investigated how a postural threat affects the selection of strategies within and between older adults by using a stepping-down paradigm. In twenty-four older adults we determined the height at which they switched in stepping-down strategies from a less demanding but more balance threatening heel landing to a more demanding yet safer toe landing. We expected that this switching height would be lower in the high (0.78 m elevation) compared to low threat (floor level) condition. Furthermore, we investigated if older adults, for which the postural threat evoked an increase in the perceived fear, presented a different stepping down strategy due to the postural threat. Our results indicated that the postural threat changed older adults' strategies selection towards a more conservative toe landing. Hence, despite the additional effort, older adults prefer more cautious strategies during a postural threat. No effects of perceived fear on strategy selection between individuals were observed, potentially due to relatively small differences in fear among participants.

Introduction: Poor lighting has been associated with stair falls in young and older adults. However, current guidelines for illuminating stairs seem arbitrary, differ widely between sources, and are often difficult to interpret., Aims: Here we examined the influence of real-world bulb illumination properties on stair descent safety in young and older adults, with a view to generating preliminary evidence for appropriate lightbulb use/stair illumination., Methods: Stair tread illumination (lx) was measured in a standard UK home (2.23 m ceiling) from a low (50 W; 630 lm) and a high (103 W, 1450 lm) power compact fluorescent lamp (CFL) bulb from the time they were turned on until they reached full brightness. This enabled modelling of their illumination characteristics during warm up. Illumination was also measured from a low (40 W, 470 lm) and a high (100 W, 1521 lm) power LED bulb at first turn-on. Computer-controlled custom lighting then replicated these profiles, in addition to a Bright control (350 lx), on an instrumented staircase descended (3 × trials per light condition) by 12 young (25.3 ± 4.4 years; 5 males), 12 higher ability older (HAOA: 69.6 ± 4.7 years; 5 males) and 13 lower ability older (LAOA: 72.4 ± 4.2; 3 males) healthy adults. Older adults were allocated to ability groups based on physiological and cognitive function. Stair-specific confidence was assessed prior to the first descent in each new lighting condition, and whole-body 3D kinematics (Vicon) quantified margins of stability and foot clearances with respect to the step edges. Mixed ANOVAs examined these measures for within-subject effects of lighting (×5), between-subject effects of age (×3) and interactions between lighting and age., Results: Use of CFL bulbs led to lower self-reported confidence in older adults (20.37%, p = .01), and increased margins of stability (12.47%, p = .015) and foot clearances with respect to the step edges (10.36%, p = .003). Importantly, using CFL bulbs increased foot clearance variability with respect to the bottom step (32.74%, p = .046), which is where a high proportion of falls occur., Conclusion: Stair-tread illumination from CFL bulbs at first turn on leads to less safe stair negotiation. We suggest high powered LED bulbs may offer a safer alternative., Competing Interests: Declaration of competing interest None., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Research indicates that threat-induced changes in standing balance are associated with shifts in attention focus. This study investigated whether distracting attention modifies threat-induced changes in standing balance. Twenty-five healthy young adults stood without (No Threat) and with (Threat) the possibility of receiving a temporally unpredictable anteroposterior support surface translation. In both conditions, participants completed a distractor task that consisted of counting how often a pre-selected letter occurred in an auditory sequence, or no distractor task. Emotional responses to threat were quantified using electrodermal activity and self-report measures, while attention focus was quantified using self-report. Centre of pressure (COP) was measured to assess changes in standing balance. Results indicate that postural threat induced an emotional response, as well as broad shifts in attention focus and changes in standing balance. Distracting attention with a cognitive task mitigated threat-induced increases in medium-frequency COP displacements (0.5-1.8 Hz). These results provide support for a relationship between threat-related changes in balance control and attention focus., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Height-induced postural threat influences standing balance control. However, it is unknown if minimizing individuals' emotional response to threat moderates this relationship. This study repeatedly exposed individuals to height-induced postural threat to determine if reducing the emotional response to threat influences standing balance control. Sixty-eight young adults completed a series of standing trials at LOW (0.8 m above ground, away from edge) and HIGH (3.2 m above ground, at edge) postural threat conditions. Emotional state was assessed using self-report and electrodermal measures. Standing balance was assessed through analysis of centre of pressure (COP) movement and lower leg electromyographic activity. Individuals' emotional response to threat was attenuated following repeated threat exposure. However, threat-induced changes in standing balance were largely preserved. When initially threatened, individuals leaned backward and demonstrated smaller amplitude and higher frequency of COP adjustments; these balance outcomes did not change following repeated threat exposure. Only high frequency COP oscillations (>1.8 Hz) and ankle muscle co-contraction showed any adaptation; regression analyses showed that these behavioural adaptations were accounted for by a combination of emotional and cognitive state changes. This suggests that some threat-induced standing balance changes are more closely linked with the emotional response to threat than others, and are therefore amendable to intervention.

Threat-related changes in postural control and their associations with changes in emotional and cognitive states are influenced by postural threat experience, however, limited work has explored individuals' capacity to adapt threat-related responses over longer periods of threat exposure. This study examined the effects of initial and repeated postural threat exposure on emotional, cognitive, and postural responses. Twenty-seven young and twenty-seven older adults stood on a force plate fixed to a translating platform. Threat was manipulated through expectation of a temporally and directionally (left or right) unpredictable platform perturbation. Participants completed one 60s stance trial with no expectation of perturbation (No Threat) followed by 24 trials with threat of perturbation (Threat). The stance period before each perturbation varied (5-60s) except on an early Threat trial and the last Threat trial (60s), which were used for analysis. Postural threat elicited similar emotional, cognitive, and postural changes in young and older adults. With initial threat exposure, participants reported increases in self-reported anxiety and physiological arousal, as well as broad changes in attention focus. Participants also significantly increased centre of pressure (COP) amplitude and frequency, and COP power within medium and high frequencies. With repeated threat exposure, anxiety, arousal, and some threat-induced changes in attention focus significantly adapted. These changes were accompanied by significant reductions in COP frequency and COP power within medium frequencies. Some emotional and cognitive outcomes returned to no threat levels while postural outcomes did not. This study suggests that some threat-related changes in standing postural control may be closely linked with one's emotional response to threat, while others may be context-dependent., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Postural threat decreases center of pressure displacements yet increases the magnitude of movement-related conscious sway perception during quiet standing. It is unknown how these changes influence perception of whole body movement during dynamic stance. The aim of this study was to examine how postural threat influences whole-body movements and conscious perception of these movements during continuous pseudo-random support surface perturbations to stance. Sixteen healthy young adults stood on a moveable platform with their eyes closed for 7 min in a low threat (1.1 m above ground, away from edge) then high threat (3.2 m above ground, near edge) condition. Continuous pseudorandom roll platform rotations (± 4.5°, < 0.5 Hz) evoked large amplitude sway in the medio-lateral (ML) direction. Participants were asked to remain upright and avoid a fall at all times while tracking their ML body movements using a hand-held rotary encoder. Kinematic data was recorded using three markers placed on the upper trunk. Questionnaires assessed anxiety, fear and confidence. Electrodermal activity (EDA) was recorded as an indicator of arousal. Height-induced threat increased fear, anxiety and EDA, and decreased confidence. Trunk sway amplitude remained constant, while tracked movement amplitude increased at height. The gain for perceived to trunk movement was significantly increased at height across frequencies. Threat-related increases in sensitivity of sensory systems related to postural control and changes in cognitive and attention processes may lead to misperceptions of actual movement amplitudes, which may be important when examining increased fall risk in those with a fear of falling., (Copyright © 2019 IBRO. Published by Elsevier Ltd. All rights reserved.)

Individuals report directing attention toward and away from multiple sources when standing under height-related postural threat, and these changes in attention focus are associated with postural control modifications. As it is unknown whether these changes generalize to other types of threat situations, this study aimed to quantify changes in attention focus and examine their relationship with postural control changes in response to a direct threat to stability. Eighty young adults stood on a force plate fixed to a translating platform. Three postural threat conditions were created by altering the expectation of, and prior experience with, a postural perturbation: no threat of perturbation, threat without perturbation experience, and threat with perturbation experience. When threatened, participants were more anxious and reported directing more attention to movement processes, threat-related stimuli, and self-regulatory strategies, and less to task-irrelevant information. Postural sway amplitude and frequency increased with threat, with greater increases in frequency and smaller increases in amplitude observed with experience. Without experience, threat-related changes in postural control were accounted for by changes in anxiety; larger changes in anxiety were related to larger changes in sway amplitude. With experience, threat-related postural control changes were accounted for by changes in attention focus; increases in attention to movement processes were related to greater forward leaning and increases in sway amplitude, while increases in attention to self-regulatory strategies were related to greater increases in sway frequency. Results suggest that relationships between threat-related changes in anxiety, attention focus, and postural control depend on the context associated with the threat.

Key Points: Threats to standing balance (postural threat) are known to facilitate soleus tendon-tap reflexes, yet the mechanisms driving reflex changes are unknown. Scaling of ramp-and-hold dorsiflexion stretch reflexes to stretch velocity and amplitude were examined as indirect measures of changes to muscle spindle dynamic and static function with height-induced postural threat. Overall, stretch reflexes were larger with threat. Furthermore, the slope (gain) of the stretch-velocity vs. short-latency reflex amplitude relationship was increased with threat. These findings are interpreted as indirect evidence for increased muscle spindle dynamic sensitivity, independent of changes in background muscle activity levels, with a threat to standing balance. We argue that context-dependent scaling of stretch reflexes forms part of a multisensory tuning process where acquisition and/or processing of balance-relevant sensory information is continuously primed to facilitate feedback control of standing balance in challenging balance scenarios., Abstract: Postural threat increases soleus tendon-tap (t-) reflexes. However, it is not known whether t-reflex changes are a result of central modulation, altered muscle spindle dynamic sensitivity or combined spindle static and dynamic sensitization. Ramp-and-hold dorsiflexion stretches of varying velocities and amplitudes were used to examine velocity- and amplitude-dependent scaling of short- (SLR) and medium-latency (MLR) stretch reflexes as an indirect indicator of spindle sensitivity. t-reflexes were also performed to replicate previous work. In the present study, we examined the effects of postural threat on SLR, MLR and t-reflex amplitude, as well as SLR-stretch velocity scaling. Forty young-healthy adults stood with one foot on a servo-controlled tilting platform and the other on a stable surface. The platform was positioned on a hydraulic lift. Threat was manipulated by having participants stand in low (height 1.1 m; away from edge) then high (height 3.5 m; at the edge) threat conditions. Soleus stretch reflexes were recorded with surface electromyography and SLRs and MLRs were probed with fixed-amplitude variable-velocity stretches. t-reflexes were evoked with Achilles tendon taps using a linear motor. SLR, MLR and t-reflexes were 11%, 9.5% and 16.9% larger, respectively, in the high compared to low threat condition. In 22 out of 40 participants, SLR amplitude was correlated to stretch velocity at both threat levels. In these participants, the gain of the SLR-velocity relationship was increased by 36.1% with high postural threat. These findings provide new supportive evidence for increased muscle spindle dynamic sensitivity with postural threat and provide further support for the context-dependent modulation of human somatosensory pathways., (© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society.)

Background: Height-related changes in postural control can alter feedback used to control balance, which may lead to a mismatch in perceived and actual sway changes during quiet stance. However, there is still a need to examine how these changes affect the ability to detect limits of stability (and movements related to base of support limits)., Research Question: The aim of this study was to examine how changes in height-related threat influence conscious perceptions of body position during voluntary balance tasks., Methods: Twenty young healthy adults, fitted with kinematic markers on the right side of the body, stood on a forceplate mounted to a hydraulic lift placed at two heights (0.8 m and 3.2 m). At height (completed first), participants leaned as far forward as possible, at the ankle joint, while trying to remain as an inverted pendulum. Then, at each height, participants stood with eyes open, and voluntarily leaned to one of ten targets (10%-100% maximum lean) displayed visually as angular displacement of body segments on a screen. Once on target, participants reported a perceived position relative to their maximum lean. Balance confidence, fear and anxiety, and physiological arousal (hand electrodermal activity, EDA) were recorded and statistically tested using paired sample t-tests. Actual and perceived body positions were tested using repeated measures ANOVAs (height x target)., Results: Height significantly increased EDA, fear and anxiety, and decreased balance confidence. Participants voluntarily leaned to all target positions equally across heights. However, at any given target position, the perceived lean changed with height. When participants are asked to lean to a target in at height, their amount of perceived lean was larger by 4.9%, on average (range: 1.8%-9.7%)., Significance: This modulation in perceived limits of stability may increase the risk of falls in those who have an increased fear of falling., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Cortical excitability increases during the performance of more difficult postural tasks. However, it is possible that changes in postural threat associated with more difficult tasks may in themselves lead to alterations in the neural strategies underlying postural control. Therefore, the purpose of this study was to examine whether changes in postural threat are responsible for the alterations in corticospinal excitability and short-interval intracortical inhibition (SICI) that occur with increasing postural task difficulty. Fourteen adults completed three postural tasks (supported standing, free standing, or standing on an unstable board) at two surface heights (ground level or 3 m above ground). Single- and paired-pulse magnetic stimuli were applied to the motor cortex to compare soleus (SOL) and tibialis anterior (TA) test motor-evoked potentials (MEPs) and SICI between conditions. SOL and TA test MEPs increased from 0.35 ± 0.29 to 0.82 ± 0.41 mV (SOL) and from 0.64 ± 0.51 to 1.96 ± 1.45 mV (TA), respectively, whereas SICI decreased from 52.4 ± 17.2% to 39.6 ± 15.4% (SOL) and from 71.3 ± 17.7% to 50.3 ± 19.9% (TA) with increasing task difficulty. In contrast to the effects of task difficulty, only SOL test MEPs were smaller when participants stood at high (0.49 ± 0.29 mV) compared with low height (0.61 ± 0.40 mV). Because the presence of postural threat did not lead to any additional changes in the excitability of the motor corticospinal pathway and intracortical inhibition with increasing task difficulty, it seems unlikely that alterations in perceived threat are primarily responsible for the neurophysiological changes that are observed with increasing postural task difficulty. NEW & NOTEWORTHY We examined how task difficulty and postural threat influence the cortical control of posture. Results indicated that the motor corticospinal pathway and intracortical inhibition were modulated more by task difficulty than postural threat. Furthermore, because the presence of postural threat during the performance of various postural tasks did not lead to summative changes in motor-evoked potentials, alterations in perceived threat are not responsible for the neurophysiological changes that occur with increasing postural task difficulty.

Previous research has shown that cutaneous reflexes are modulated when walking with a threat to stability. It is unclear if this reflex modulation is purely related to the context of the imposed threat or if emotional changes associated with the threat exert an independent influence on reflex excitability. This study investigated the influence of emotional stimuli on lower limb cutaneous reflexes during treadmill walking. Twenty-eight healthy young adults walked at a self-selected pace while viewing pictures that manipulated emotional arousal and valence (confirmed with electrodermal and self-report measures). Throughout each trial, cutaneous reflexes were evoked by electrically stimulating the sural nerve at heel contact, mid-stance, or toe off. Surface electromyography of the ipsilateral soleus (SOL), medial gastrocnemius (MG), tibialis anterior (TA), biceps femoris (BF), and vastus lateralis (VL) was recorded to assess reflexes. Highly arousing pictures, independent of valence, significantly facilitated TA, and trended toward facilitating SOL and BF reflexes during mid-stance. Unpleasant pictures, independent of arousal, significantly reduced reflex amplitudes in BF during mid-stance and TA during toe off. While changes in background muscle activity and step cadence were observed, they did not correlate with reflex changes. This study provides the first evidence that emotional stimuli exert an independent influence on cutaneous reflex excitability during gait. As cutaneous reflexes contribute to stability during gait, these findings support the notion that emotional state influences important sensorimotor processes underlying balance control., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Study Design: Cross-sectional study., Objectives: To investigate the attentional requirements for maintaining standing balance in people with spinal cord injury (SCI) using a dual-task paradigm and to compare standing balance performance between SCI and able-bodied (AB) controls., Setting: LaboratoryMethods:Nine adults with incomplete SCI, who were able to stand unassisted were recruited, along with eight AB controls. Subjects performed a dual task involving counting backwards by 3 s out loud while standing with eyes open or closed. The primary outcome measures were the differences between SCI and control groups for movement reinvestment and the change in performance between single task and dual task for: (i) maximum standing time (STime); (ii) error ratio and total number of words uttered; and (iii) center of pressure measures. Perceptual measures included perceived mental workload, fear and confidence., Results: SCI subjects stood for shorter duration during dual task (stand and count) than single task (stand) compared with controls during eyes closed. Significant differences between groups were observed for movement reinvestment, center of pressure, perceived mental effort, fear and confidence. No significant effects were observed for math-task performance., Conclusions: Total STime during eyes closed is adversely affected by the addition of a math task for SCI subjects. Perceptual measures appear to correspond to increases in postural sway and conscious control of standing in subjects with SCI. Individuals who can stand for >60 s with eyes closed do not appear to be significantly affected by the addition of a concurrent secondary task of minimal mental workload.

Objective: To examine the effect of a targeted balance training program on dynamic balance and self-reported physical function in people with medial tibiofemoral osteoarthritis (OA)., Design: Single-blind randomized controlled trial., Setting: Exercise gymnasium and community dwellings., Participants: Individuals with medial compartment knee OA (N=40)., Interventions: Ten weeks of partially supervised exercises targeting dynamic balance and strength performed 4 times per week or no intervention (nonintervention group)., Main Outcome Measures: Dynamic balance was measured using the Community Balance and Mobility Scale (CB&M), and self-reported physical function was measured using the Western Ontario and McMaster Universities Arthritis Index physical function subscale. Secondary outcomes included knee pain, fear of movement, knee joint proprioception, and muscle strength., Results: Forty individuals underwent baseline testing, with 36 participants completing follow-up testing. Adherence to exercise in the training group was high, with 82.2% of all home-based exercise sessions completed. No significant changes were observed in any outcome in the nonintervention group at follow-up. Significant improvements in self-reported pain, physical function, and fear of movement were observed in the training group when compared with the nonintervention group. No other within- or between-group differences were observed., Conclusions: A 10-week dynamic balance training program for people with knee OA significantly improved self-reported knee pain, physical function, and fear of movement; however, there was no change in dynamic balance as quantified by the CB&M. Further research is needed to investigate how exercise may result in improvement on objective measures of dynamic balance., (Copyright © 2017 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Key Points: Golgi tendon organs (GTOs) and associated Ib reflexes contribute to standing balance, but the potential impacts of threats to standing balance on Ib reflexes are unknown. Tendon electrical stimulation to the Achilles' tendon was used to probe changes in Ib inhibition in medial gastrocnemius with postural orientation (lying prone vs. upright standing; experiment 1) and height-induced postural threat (standing at low and high surface heights; experiment 2). Ib inhibition was reduced while participants stood upright, compared to lying prone (42.2%); and further reduced when standing in the high, compared to low, threat condition (32.4%). These experiments will impact future research because they demonstrate that tendon electrical stimulation can be used to probe Ib reflexes in muscles engaged in standing balance. These results provide novel evidence that human short-latency GTO-Ib reflexes are dependent upon both task, as evidenced by changes with postural orientation, and context, such as height-induced postural threat during standing., Abstract: Golgi tendon organ Ib reflexes are thought to contribute to standing balance control, but it is unknown if they are modulated when people are exposed to a postural threat. We used a novel application of tendon electrical stimulation (TStim) to elicit Ib inhibitory reflexes in the medial gastrocnemius, while actively engaged in upright standing balance, to examine (a) how Ib reflexes to TStim are influenced by upright stance, and (b) the effects of height-induced postural threat on Ib reflexes during standing. TStim evoked short-latency (<47 ms) inhibition apparent in trigger-averaged rectified EMG, which was quantified in terms of area, duration and mean amplitude of inhibition. In order to validate the use of TStim in a standing model, TStim-Ib inhibition was compared from conditions where participants were lying prone vs. standing upright. TStim evoked Ib inhibition in both conditions; however, significant reductions in Ib inhibition area (42.2%) and duration (32.9%) were observed during stance. Postural threat, manipulated by having participants stand at LOW (0.8 m high, 0.6 m from edge) and HIGH (3.2 m, at edge) elevated surfaces, significantly reduced Ib inhibition area (32.4%), duration (16.4%) and amplitude (24.8%) in the HIGH, compared to LOW, threat condition. These results demonstrate TStim is a viable technique for investigating Ib reflexes in standing, and confirm Ib reflexes are modulated with postural orientation. The novel observation of reduced Ib inhibition with elevated postural threat reveals that human Ib reflexes are context dependent, and the human Ib reflex pathways are modulated by threat or emotional processing centres of the CNS., (© 2017 The Authors. The Journal of Physiology © 2017 The Physiological Society.)

Objective: The purpose of this study was to examine simultaneously the level of physiological arousal and the postural response to external perturbations in people post-stroke compared to age-matched controls to build a more comprehensive understanding of the effect of stroke on postural control and balance self-efficacy., Methods: Participants stood with each foot on separate force platforms. Ten applications of loads of 2% body weight at the hips perturbed the participant anteriorly under two conditions: investigator-triggered or self-triggered (total 20). Electrodermal activity (EDA; measurement of physiological arousal), electromyography (EMG) of the ankle plantarflexor muscles and anterior-posterior center of pressure measurements were taken pre-perturbation (anticipatory) and post-perturbation (response) and compared between the initial (first two) and final (last two) perturbations., Results: Participants post-stroke demonstrated significantly higher levels of anticipatory EDA and anticipatory paretic plantarflexor EMG during both self- and investigator-triggered conditions compared to controls. Anticipatory EDA levels were higher in the final perturbations in participants post-stroke in both conditions, but not in controls. Habituation of the EDA responses post-perturbation was exhibited in the self-triggered perturbations in controls, but not in participants post-stroke., Conclusions: Physiological arousal and postural control strategies of controls revealed habituation in response to self-triggered perturbations, whereas this was not seen in participants post-stroke., Significance: Understanding the physiological arousal response to challenges to standing balance post-stroke furthers our understanding of postural control mechanisms post-stroke., (Copyright © 2017. Published by Elsevier B.V.)

Standing balance is significantly influenced by postural threat. While this effect has been well established, the underlying mechanisms of the effect are less understood. The involvement of the vestibular system is under current debate, and recent studies that investigated the effects of height-induced postural threat on vestibular-evoked responses provide conflicting results based on kinetic (Horslen BC, Dakin CJ, Inglis JT, Blouin JS, Carpenter MG. J Physiol 592: 3671-3685, 2014) and kinematic (Osler CJ, Tersteeg MC, Reynolds RF, Loram ID. Eur J Neurosci 38: 3239-3247, 2013) data. We examined the effect of threat of perturbation, a different form of postural threat, on coupling (cross-correlation, coherence, and gain) of the vestibulo-muscular relationship in 25 participants who maintained standing balance. In the "No-Threat" conditions, participants stood quietly on a stable surface. In the "Threat" condition, participants' balance was threatened with unpredictable mediolateral support surface tilts. Quiet standing immediately before the surface tilts was compared to an equivalent time from the No-Threat conditions. Surface EMG was recorded from bilateral trunk, hip, and leg muscles. Hip and leg muscles exhibited significant increases in peak cross-correlation amplitudes, coherence, and gain (1.23-2.66×) in the Threat condition compared with No-Threat conditions, and significant correlations were observed between threat-related changes in physiological arousal and medium-latency peak cross-correlation amplitude in medial gastrocnemius (r = 0.408) muscles. These findings show a clear threat effect on vestibular-evoked responses in muscles in the lower body, with less robust effects of threat on trunk muscles. Combined with previous work, the present results can provide insight into observed changes during balance control in threatening situations., New & Noteworthy: This is the first study to show increases in vestibular-evoked responses of the lower body muscles under conditions of increased threat of postural perturbation. While robust findings were observed in hip and leg muscles, less consistent results were found in muscles of the trunk. The present findings provide further support in the ongoing debate for arguments that vestibular-evoked balance responses are influenced by fear and anxiety and explain previous threat-related changes in balance., (Copyright © 2017 the American Physiological Society.)

Small-amplitude, higher frequency oscillations of the body or limb are typically observed when humans attempt to maintain the position of a body or limb in space. Recent investigations have suggested that these involuntary movements of the body during stance could be used as an exploratory means of acquiring sensory information. In the present study, we wanted to determine whether a similar phenomenon would be observed in an upper limb postural task that does not involve whole body postural control. Participants were placed in a supine position with the arm pointing vertically and were asked to maintain the position of the limb in space with and without visual feedback. The wrist was attached to an apparatus that allowed the experimenter to stabilize or "lock" movements of the arm without the participants' awareness. When participants were "locked," the forces recorded predicted greater accelerations than those observed when the arm was freely moving with and without visual feedback. From unlocked to locked, angular accelerations increased in the eyes-closed condition and when participants were provided visual feedback of arm angular displacements. Irrespective of their origin, small displacements of the limb may be used as an exploratory means of acquiring sensory information from the surrounding environment. NEW & NOTEWORTHY The role of movement variability during a static limb position task is currently unknown. We tested whether variability remains in the absence of sensory-based error with an apparatus that stabilized the limb without the participant's knowledge during a static postural task. Increased forces observed during arm stabilization predicted movements greater than those observed when not externally stabilized. These results suggest movement variability during static postures could facilitate the gathering of sensory information from the surrounding environment., (Copyright © 2017 the American Physiological Society.)

Age-related changes in the density, morphology, and physiology of plantar cutaneous receptors negatively impact the quality and quantity of balance-relevant information arising from the foot soles. Plantar perceptual sensitivity declines with age and may predict postural instability; however, alteration in lower limb cutaneous reflex strength may also explain greater instability in older adults and has yet to be investigated. We replicated the age-related decline in sensitivity by assessing monofilament and vibrotactile (30 and 250 Hz) detection thresholds near the first metatarsal head bilaterally in healthy young and older adults. We additionally applied continuous 30- and 250-Hz vibration to drive mechanically evoked reflex responses in the tibialis anterior muscle, measured via surface electromyography. To investigate potential relationships between plantar sensitivity, cutaneous reflex strength, and postural stability, we performed posturography in subjects during quiet standing without vision. Anteroposterior and mediolateral postural stability decreased with age, and increases in postural sway amplitude and frequency were significantly correlated with increases in plantar detection thresholds. With 30-Hz vibration, cutaneous reflexes were observed in 95% of young adults but in only 53% of older adults, and reflex gain, coherence, and cumulant density at 30 Hz were lower in older adults. Reflexes were not observed with 250-Hz vibration, suggesting this high-frequency cutaneous input is filtered out by motoneurons innervating tibialis anterior. Our findings have important implications for assessing the risk of balance impairment in older adults., (Copyright © 2016 the American Physiological Society.)

This study examined how postural threat in the form of a potential perturbation affects an individual's ability to perform a heel raise. Seventeen adults completed three conditions: i) low threat, where participants performed a heel raise in response to a "go" tone, ii) high threat, where participants either heard the same "go" tone, for which they performed a heel raise, or experienced a support surface translation in the medio-lateral direction that disturbed their balance, and iii) choice reaction time task, where participants either completed a heel raise in response to the same "go" tone or a toe raise in response to a lower pitched tone. For all heel raise trials, anticipatory postural adjustments (APAs) were quantified from center of pressure (COP) recordings and electromyographic (EMG) activity from the tibialis anterior (TA) and soleus (SOL). Results indicated that participants exhibited larger APAs, as reflected by the greater backward COP displacement (p=0.038) and velocity (p=0.022) as well as a larger TA EMG amplitude (p=0.045), during the high threat condition. During the execution phase of the heel raise, an earlier (p=0.014) and larger (p=0.041) SOL EMG activation were observed during the high threat condition. These results contrast with previous findings of reduced APAs when the postural threat was evoked through changes in surface height. Therefore, the characteristics of the postural threat must be considered to isolate the effects of threat on anticipatory movement control., (Copyright © 2016 Elsevier B.V. All rights reserved.)

As fear of falling is related to the increased likelihood of falls, it is important to understand the effects of threat-related factors (fear, anxiety and confidence) on dynamic postural reactions. Previous studies designed to examine threat effects on dynamic postural reactions have methodological limitations and lack a comprehensive analysis of simultaneous kinetic, kinematic and electromyographical recordings. The current study addressed these limitations by examining postural reactions of 26 healthy young adults to unpredictable anterior-posterior support-surface translations (acceleration=0.6m/s(2), constant velocity=0.25m/s, total displacement=0.75m) while standing on a narrow virtual surface at Low (0.4cm) and High (3.2m) virtual heights. Standing at virtual height increased fear and anxiety, and decreased confidence. Prior to perturbations, threat led to increased tonic muscle activity in tibialis anterior, resulting in a higher co-contraction index between lower leg muscles. For backward perturbations, muscle activity in the lower leg and arm, and center of pressure peak displacements, were earlier and larger when standing at virtual height. In addition, arm flexion significantly increased while leg, trunk and center of mass displacements remained unchanged across heights. When controlling for leaning, threat-related factors can influence the neuro-mechanical responses to an unpredictable perturbation, causing specific characteristics of postural reactions to be facilitated in young adults when their balance is threatened., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Real-time balance-relevant biofeedback from a wearable sensor can improve balance in many patient populations, however, it is unknown if balance training with biofeedback has lasting benefits for healthy older adults once training is completed and biofeedback removed. This study was designed to determine if multi-session balance training with and without biofeedback leads to changes in balance performance in healthy older adults; and if changes persist after training. 36 participants (age 60-88) were randomly divided into two groups. Both groups trained on seven stance and gait tasks for 2 consecutive weeks (3×/week) while trunk angular sway and task duration were monitored. One group received real-time multi-modal biofeedback of trunk sway and a control group trained without biofeedback. Training effects were assessed at the last training session, with biofeedback available to the feedback group. Post-training effects (without biofeedback) were assessed immediately after, 1-week, and 1-month post-training. Both groups demonstrated training effects; participants swayed less when standing on foam with eyes closed (EC), maintained tandem-stance EC longer, and completed 8 tandem-steps EC faster and with less sway at the last training session. Changes in sway and duration, indicative of faster walking, were also observed after training for other gait tasks. While changes in walking speed persisted post-training, few other post-training effects were observed. These data suggest there is little added benefit to balance training with biofeedback, beyond training without, in healthy older adults. However, transient use of wearable balance biofeedback systems as balance aides remains beneficial for challenging balance situations and some clinical populations., (Copyright © 2016 Elsevier B.V. All rights reserved.)

This study examined how changes in threat influenced conscious perceptions of postural sway. Young healthy adults stood on a forceplate mounted to a hydraulic lift placed at two heights (0.8m and 3.2m). At each height, subjects stood quietly with eyes open and eyes closed for 60s. Subjects were instructed to either stand normal, or stand normal and track their perceived sway in the antero-posterior plane by rotating a hand-held potentiometer. Participants reported an increased level of fear, anxiety, arousal and a decreased level of balance confidence when standing at height. In addition, postural sway amplitude decreased and frequency increased at height. However, there were no effects of height on perceived sway. When standing under conditions of increased postural threat, sway amplitude is reduced, while sway perception appears to remain unchanged. Therefore, when threat is increased, sensory gain may be increased to compensate for postural strategies that reduce sway (i.e. stiffening strategy), thereby ensuring sufficient afferent information is available to maintain, or even increase the conscious perception of postural sway., (Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.)

Postural threat, manipulated through changes in surface height, influences postural control. Evidence suggests changes in attention may contribute to this relationship. However, limited research has explored where and how attention is reallocated when threatened. The primary aim of this study was to describe changes in attention when presented with a postural threat, while a secondary aim was to explore associations between changes in attention and postural control. Eighty-two healthy young adults completed tests of static (quiet standing) and anticipatory (rise to toes) postural control under threatening and non-threatening conditions. Participants completed an open-ended questionnaire after each postural task which asked them to list what they thought about or directed their attention toward. Each item listed was assigned a percentage value reflecting how much attention it occupied. Exit interviews were completed to help confirm where attention was directed. Five attention categories were identified: movement processes, threat-relevant stimuli, self-regulatory strategies, task objectives, and task-irrelevant information. For both postural tasks, the percentage values and number of items listed for movement processes, threat-relevant stimuli, and self-regulatory strategies increased under threatening compared to non-threatening conditions, while the percentage values and number of items listed for task objectives and task-irrelevant information decreased. Changes in attention related to movement processes and self-regulatory strategies were associated with changes in static postural control, while changes in attention related to threat-relevant stimuli were associated with changes in anticipatory postural control. These results suggest that threat-induced changes in attention are multidimensional and contribute to changes in postural control., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Objective: To identify potential neuromuscular factors associated with dynamic balance in individuals with knee osteoarthritis (OA)., Design: Cross-sectional observational study; backward stepwise multiple linear regression was used to identify factors associated with dynamic balance in 2 statistical models., Setting: University clinical research laboratory., Participants: Individuals aged ≥50 years (N=52) with osteoarthritic changes on radiograph participated., Interventions: Not applicable., Main Outcome Measures: Dynamic balance was assessed using the Community Balance and Mobility Scale (CB&M). Potentially modifiable neuromuscular factors associated with dynamic balance were measured, including the sum of concentric and eccentric lower-extremity muscle strength, 2 quadriceps-hamstrings muscle strength ratios, knee joint proprioception (joint position sense), anticipatory postural control velocity, and knee joint range of motion., Results: The first model for explaining variance in CB&M scores consisted of eccentric lower-extremity muscle strength and knee joint range of motion as factors. The model containing these 2 variables explained 50% of the variance in CB&M scores. The second model adjusted for descriptive variables, including age, body mass index, and knee pain, contained only the neuromuscular variables eccentric lower-extremity muscle strength, and explained 68% of the variance in CB&M scores., Conclusions: These results suggest that muscle strength and, to a lesser extent, knee joint range of motion are important factors associated with dynamic balance as measured by the CB&M and should be considered in dynamic balance interventions., (Copyright © 2015 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.)

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Objective: To measure changes in amplitudes of vestibular evoked myogenic potentials (VEMPs) elicited from neck, upper and lower limb muscles during a quiet standing task with increased postural threat achieved by manipulating surface height., Methods: Twenty eight subjects were tested while standing on a platform raised to 0.8 m and 3.2 m from the ground. Surface electromyography was recorded from the ipsilateral sternocleidomastoid (SCM), biceps brachii (BB), flexor carpi radialis (FCR), soleus (SOL) and medial gastrocnemius (MG) muscles. Stimulation was with air-conducted short tone bursts (4 ms). After controlling for background muscle activity, VEMP amplitudes were compared between heights and correlated with changes in state anxiety, fear and arousal., Results: VEMP amplitude significantly increased in SCM (9%) and SOL (12.7%) with increased surface height (p<0.05). These modest increases in SCM VEMP amplitude were significantly correlated with anxiety (Rho=0.57, p=0.004) and confidence (Rho=-0.38, p=0.047) and those for SOL were significantly correlated with anxiety (Rho=0.33, p=0.049) and fear (Rho=0.36, p=0.037)., Conclusion: Postural threat significantly increased vestibulospinal reflex (VSR) gains. Results demonstrate that VEMPs can be used to test different VSR pathways simultaneously during stance. Since fear and anxiety are prevalent with vestibular disorders, they should be considered as potential contributing factors for clinical vestibular outcome measures., (Copyright © 2015 IBRO. Published by Elsevier Ltd. All rights reserved.)

This study explored whether specific personality traits and individual differences could predict changes in postural control when presented with a height-induced postural threat. Eighty-two healthy young adults completed questionnaires to assess trait anxiety, trait movement reinvestment (conscious motor processing, movement self-consciousness), physical risk-taking, and previous experience with height-related activities. Tests of static (quiet standing) and anticipatory (rise to toes) postural control were completed under low and high postural threat conditions. Personality traits and individual differences significantly predicted height-induced changes in static, but not anticipatory postural control. Individuals less prone to taking physical risks were more likely to lean further away from the platform edge and sway at higher frequencies and smaller amplitudes. Individuals more prone to conscious motor processing were more likely to lean further away from the platform edge and sway at larger amplitudes. Individuals more self-conscious about their movement appearance were more likely to sway at smaller amplitudes. Evidence is also provided that relationships between physical risk-taking and changes in static postural control are mediated through changes in fear of falling and physiological arousal. Results from this study may have indirect implications for balance assessment and treatment; however, further work exploring these factors in patient populations is necessary., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Anxiety and arousal have been shown to facilitate human vestibulo-ocular reflexes, presumably through direct neural connections between the vestibular nuclei and emotional processing areas of the brain. However, the effects of anxiety, fear and arousal on balance-relevant vestibular reflexes are currently unknown. The purpose of this study was to manipulate standing height to determine whether anxiety and fear can modulate the direct relationship between vestibular signals and balance reflexes during stance. Stochastic vestibular stimulation (SVS; 2-25 Hz) was used to evoke ground reaction forces (GRF) while subjects stood in both LOW and HIGH surface height conditions. Two separate experiments were conducted to investigate the SVS-GRF relationship, in terms of coupling (coherence and cumulant density) and gain, in the medio-lateral (ML) and antero-posterior (AP) directions. The short- and medium-latency cumulant density peaks were both significantly increased in the ML and AP directions when standing in HIGH, compared to LOW, conditions. Likewise, coherence was statistically greater between 4.3 Hz and 6.7 Hz in the ML, and between 5.5 and 17.7 Hz in the AP direction. When standing in the HIGH condition, the gain of the SVS-GRF relationship was increased 81% in the ML direction, and 231% in the AP direction. The significant increases in coupling and gain observed in both experiments demonstrate that vestibular-evoked balance responses are augmented in states of height-induced postural threat. These data support the possibility that fear or anxiety-mediated changes to balance control are affected by altered central processing of vestibular information., (© 2014 The Authors. The Journal of Physiology © 2014 The Physiological Society.)

Background: There is a high incidence of falls in older adults with knee osteoarthritis (OA). Adequate dynamic balance and mobility reduce the risk of falls; however, there are currently no validated, advanced tests of dynamic balance and mobility for individuals with knee OA., Objective: The purpose of this study was to determine the convergent validity, known-groups validity, and test-retest reliability of a dynamic test of balance and mobility, the Community Balance and Mobility Scale (CB&M), in a knee OA population., Design: A cross-sectional design was used., Methods: Twenty-five individuals aged 50 years and older with medial knee OA and an equal number of healthy controls completed the CB&M and other tests of balance and mobility, including the Berg Balance Scale, the Timed "Up & Go" Test, a test of maximal single-leg stance time, and the 10-Meter Walk Test (self-selected and fast walking speed). Convergent validity of balance tests with the CB&M was assessed using Pearson product moment correlation coefficients, and known-groups validity was assessed using independent t tests. Test-retest reliability of the CB&M was assessed using intraclass correlation coefficients (ICCs) and standard error of measurement (SEM)., Results: Scores on the CB&M were significantly correlated with all measures of balance and mobility for those with knee OA. There were significant differences in CB&M scores between groups. Scores on the CB&M were highly reliable in people with knee OA (ICC=.95, 95% confidence interval [95% CI]=0.70 to 0.99; SEM=3, 95% CI=2.68 to 4.67)., Limitations: Few participants had severe knee OA., Conclusions: The CB&M displayed moderate convergent validity, excellent known-groups validity, and high test-retest reliability. The CB&M can be used as a valid and reliable tool to assess dynamic balance and mobility deficits in people with knee OA., (© 2014 American Physical Therapy Association.)

There is very little consensus regarding the mechanisms underlying postural control. Whereas some theories suggest that posture is controlled at lower levels (i.e., brain stem and spinal cord), other theories have proposed that upright stance is controlled using higher centers, including the motor cortex. In the current investigation, we used corticomuscular coherence (CMC) to investigate the relationship between cortical and shank muscle activity during conditions of unrestricted and restricted postural sway. Participants were instructed to stand as still as possible in an apparatus that allowed the center of mass to move freely ("Unlocked") or to be stabilized ("Locked") without subject awareness. EEG (Cz) and electromyography (soleus and lateral/medial gastrocnemii) were collected and used to estimate CMC over the Unlocked and Locked periods. Confirming our previous results, increases in center of pressure (COP) displacements were observed in 9 of 12 participants in the Locked compared with Unlocked condition. Across these 9 participants, CMC was low or absent in both the Unlocked and Locked conditions. The results from the current study suggest that this increase is not associated with an increase in the relationship between cortical and shank muscle activities. Rather, it may be that increases in COP displacement with locking are mediated by subcortical structures as a means of increasing sway to provide the central nervous system with a critical level of sensory information.

Assessment of changes in standing balance following an intervention requires accurate measurement of balance parameters. The reliability of centre of pressure measures of balance during single-leg standing has not been reported in individuals with knee osteoarthritis. The purpose of this study was to assess the test re-test reliability of force platform centre of pressure measures during single-leg standing in older adults with knee osteoarthritis. Twenty-five adults with radiographic evidence of knee osteoarthritis performed single-leg standing balance trials on a laboratory-grade force platform on two occasions, no more than 14 days apart. Participants were asked to stand on their more symptomatic limb for three, ten second trials. Centre of pressure measures collected included: standard deviation in the mediolateral and anteroposterior directions, mean path length, velocity, and area. The mean of the three trials was calculated. Intraclass correlation coefficients, standard error of measurement, Bland and Altman plots and the minimum detectable change were calculated. Intraclass correlation coefficients ranged from 0.54 to 0.87, suggesting mixed reliability of measures. Reliability was lowest for the centre of pressure area (intraclass correlation coefficient=0.54), and highest for centre of pressure velocity and path length (intraclass correlation coefficient=0.87 for both). Standard error of measurement values were low for standard deviation in the mediolateral direction and high for centre of pressure area. These results suggest that centre of pressure values, in particular path length and velocity, are appropriate for assessment of standing balance in people with medial knee osteoarthritis., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Postural responses (PR) to a balance perturbation differ between the first and subsequent perturbations. One explanation for this first trial effect is that perturbations act as startling stimuli that initiate a generalized startle response (GSR) as well as the PR. Startling stimuli, such as startling acoustic stimuli (SAS), are known to elicit GSRs, as well as a StartReact effect, in which prepared movements are initiated earlier by a startling stimulus. In this study, a StartReact effect paradigm was used to determine if balance perturbations can also act as startle stimuli. Subjects completed two blocks of simple reaction time trials involving wrist extension to a visual imperative stimulus (IS). Each block included 15 CONTROL trials that involved a warning cue and subsequent IS, followed by 10 repeated TEST trials, where either a SAS (TESTSAS) or a toes-up support-surface rotation (TESTPERT) was presented coincident with the IS. StartReact effects were observed during the first trial in both TESTSAS and TESTPERT conditions as evidenced by significantly earlier wrist movement and muscle onsets compared with CONTROL. Likewise, StartReact effects were observed in all repeated TESTSAS and TESTPERT trials. In contrast, GSRs in sternocleidomastoid and PRs were large in the first trial, but significantly attenuated over repeated presentation of the TESTPERT trials. Results suggest that balance perturbations can act as startling stimuli. Thus first trial effects are likely PRs which are superimposed with a GSR that is initially large, but habituates over time with repeated exposure to the startling influence of the balance perturbation.

Previous research has shown that when the COM is stabilized without participant awareness, COP displacements increase. This finding suggests that postural sway under normal conditions may be exploratory and used as a means of acquiring sensory information. However, based on the theory that posture is controlled using internal models, it could be argued that increases in COP displacement reflect errors that arise as the central nervous system attempts to adapt the internal model used to control posture to the new conditions. The current study provided an explicit verbal cue to the participants indicating how and when COM stabilization would occur. Based on evidence suggesting that explicit verbal cues can reduce errors when the dynamics of the task are altered, we hypothesized that when participants were aware of COM stabilization, COP displacements would be reduced. However, we found that anterior-posterior COP displacements increased independent of cueing, suggesting that increases in COP displacements with locking were not the result of an attempt to adapt the internal model of postural control. The results provide further support for an exploratory role of postural sway., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Standing balance is often threatened in everyday life. These threats typically involve scenarios in which either the likelihood or the consequence of falling is higher than normal. When cats are placed in these scenarios they respond by increasing the sensitivity of muscle spindles imbedded in the leg muscles, presumably to increase balance-relevant afferent information available to the nervous system. At present, it is unknown whether humans also respond to such postural threats by altering muscle spindle sensitivity. Here we present two studies that probed the effects of postural threat on spinal stretch reflexes. In study 1 we manipulated the threat associated with an increased consequence of a fall by having subjects stand at the edge of an elevated surface (3.2 m). In study 2 we manipulated the threat by increasing the likelihood of a fall by occasionally tilting the support surface on which subjects stood. In both scenarios we used Hoffmann (H) and tendon stretch (T) reflexes to probe the spinal stretch reflex circuit of the soleus muscle. We observed increased T-reflex amplitudes and unchanged H-reflex amplitudes in both threat scenarios. These results suggest that the synaptic state of the spinal stretch reflex is unaffected by postural threat and that therefore the muscle spindles activated in the T-reflexes must be more sensitive in the threatening conditions. We propose that this increase in sensitivity may function to satisfy the conflicting needs to restrict movement with threat, while maintaining a certain amount of sensory information related to postural control.

Fear and anxiety induced by threatening scenarios, such as standing on elevated surfaces, have been shown to influence postural control in young adults. There is also a need to understand how postural threat influences postural control in populations with balance deficits and risk of falls. However, safety and feasibility issues limit opportunities to place such populations in physically threatening scenarios. Virtual reality (VR) has successfully been used to simulate threatening environments, although it is unclear whether the same postural changes can be elicited by changes in virtual and real threat conditions. Therefore, the purpose of this study was to compare the effects of real and virtual heights on changes to standing postural control, electrodermal activity (EDA) and psycho-social state. Seventeen subjects stood at low and high heights in both real and virtual environments matched in scale and visual detail. A repeated measures ANOVA revealed increases with height, independent of visual environment, in EDA, anxiety, fear, and center of pressure (COP) frequency, and decreases with height in perceived stability, balance confidence and COP amplitude. Interaction effects were seen for fear and COP mean position; where real elicited larger changes with height than VR. This study demonstrates the utility of VR, as simulated heights resulted in changes to postural, autonomic and psycho-social measures similar to those seen at real heights. As a result, VR may be a useful tool for studying threat related changes in postural control in populations at risk of falls, and to screen and rehabilitate balance deficits associated with fear and anxiety., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Recent evidence showing center of pressure (COP) displacements increase following an external stabilization of the center of mass (COM) supports the theory that postural sway may be exploratory and serve as a means of acquiring sensory information. The aim of the current study was to further test this theory and rule out potential confounding effects of sensory illusions or motor drift on prior observations. Participants stood as still as possible in an apparatus which allowed movements of the COM to be stabilized ("locked") without subject awareness, and they were provided real-time visual feedback of their COM or COP throughout the trial. If there was an influence of sensory illusions or motor drift, we hypothesized that the change in COP displacement with locking would be reduced when participants were provided visual confirmation of COM stabilization (COM feedback), or when they were aware of the position of the COP throughout the trial (COP feedback). Confirming our previous results, increases in COP displacement were observed when movements of the COM were stabilized. In addition, our results showed that increases in COP displacement could not be explained by the presence of sensory illusions or motor drift, since increases in COP were observed despite being provided convincing evidence that the COM had been stabilized, and when participants were aware of their COP position throughout the trial. These results provide further support for an exploratory role of postural sway. The theoretical basis of current clinical practices designed to deal with balance control deficits due to age or disease is typically based on the opinion that increases in sway are a consequence of a failing balance control system. Our results suggest that this may not be the case, and if sway is in fact exploratory, a serious re-evaluation of current clinical practices may be warranted., (Copyright © 2011 IBRO. Published by Elsevier Ltd. All rights reserved.)

There is mounting evidence to suggest that emotional state can influence postural control. Emotions are often qualified using dimensions such as valence (pleasantness) and arousal. While affective pictures have been used to detail the effects of valence on postural control, the influence of arousal independently, or in combination with valence, has yet to be investigated. This is an important oversight because there are multiple sensory and neuromuscular mechanisms that are known to be sensitive to arousal and to contribute to postural control. As such, the current study is the first to independently manipulate valence and arousal through affective pictures and to examine their independent effects on postural control. Subjects stood quietly for 90 s long blocks while watching affective pictures, grouped by normative ratings of arousal (high and low) and valence (pleasant and unpleasant), and during which centre of pressure (COP) and electrodermal activity (EDA) were collected. EDA and anterior-posterior COP frequency were both increased with arousal, but not by valence. The postural effects observed in this study parallel those typically seen in other highly arousing situations, such as standing at the edge of an elevated platform or during performance evaluation. Therefore, we argue that arousal is a mediator of postural control and should be considered as a potential confound when testing or diagnosing subjects in clinical or experimental settings.