Effects of age and pathology on stance modifications in response to increased postural threat

Proactive modifications to walking stability under the threat of large, anterior or posterior perturbations

Biomechanically, falling after a walking perturbation may be influenced by: (1) the pre-perturbation state of mechanical stability (e.g., stability margins) and (2) the response to a perturbation (i.e., recovery skill). Walking stability margins must be modifiable to serve as a target for fall-prevention interventions. We investigated if neurotypical adults could proactively modulate pre-perturbation anteroposterior stability margins while walking. Eleven adults walked on a treadmill at three speeds with and without anterior and posterior perturbations. We measured stability margins anteriorly at mid-swing and posteriorly at foot strike for pre-perturbation steps. A repeated-measures factorial ANOVA evaluated main effects and interactions of walking speed (0.6, 0.8, 1.0 stats/s) and perturbation type (anterior, none, posterior). With posterior perturbation threats, the posterior stability margins were more positive at foot strike (p < 0.01) compared to trials without perturbations. With anterior perturbation threats, the anterior stability margins were not different at mid-swing compared to trials without perturbations (p > 0.05). With any perturbation threat, step lengths shortened (p < 0.01) and step rates increased (p < 0.01). Step width was not different (p > 0.11). At slow speeds with posterior perturbation threats, double-support time decreased (p = 0.04). Proactive modifications to stability margins are indeed possible in a neurotypical population. Consequently, anteroposterior stability margins may be a feasible target for fall-prevention interventions by targeting decreased step lengths or increased step rates within a given walking speed. We do not know the extent to which the observed effects have a meaningful effect on perturbation recovery.

Habituation of vestibular-evoked balance responses after repeated exposure to a postural threat

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.

The effect of anxiety on gait: a threat-of-scream study

It is known that fear responses to clearly identified threats can inhibit motion, slowing down gait and inducing postural freezing. Nonetheless, it is less clear how anxiety, which emerges during threat anticipation, affects gait parameters. In the present work, we used a threat-of-scream paradigm to study the effects of anxiety on gait. Twenty-five participants (15 female, aged 23.4 ± 1.8) were instructed to walk on a 5-m walking track, while motion was recorded in 3D, via a VICON system. Four alternating blocks, two "threat" and two "safe" blocks of 10 trials each, were signaled by colored stripes on a screen in front of the walking path. Participants were informed that they could hear a human scream in their headphones at any time during threat blocks, which were in fact always delivered during walking. On the contrary, no screams were delivered in the safe blocks. Results indicated that participants reported higher subjective anxiety during threat vs. safe blocks. Furthermore, increases in self-reported anxiety from safe to threat showed significant moderate correlations with increased stride speed and length, decreased stride time and decreased stance phase duration. Increases in anxiety were also moderately correlated with increased arm/leg swing amplitude, an effect that was fully mediated by increased stride speed. Overall, these results indicate that anxiety invigorates motion in healthy subjects, by increasing speed. These results are discussed in terms of the recent advancements in the understanding of defensive behavior, its neural correlates and on clinical implications relative to pathological anxiety.

Isolated medial patellofemoral ligament reconstruction improves static bipedal balance control in young patients with recurrent lateral patellar instability

BACKGROUND

Knee stability can be safely and reliably restored using medial patellofemoral ligament (MPFL) reconstruction, which is widely recognized in patients with recurrent lateral patellar instability. However, the literature regarding its influence on static balance control is limited. Thus, this study aimed to assess the impact of MPFL reconstruction on balance control and determine its functional significance.

METHODS

The study comprised 26 patients with recurrent lateral patellar instability, scheduled for MPFL reconstruction, and 26 matched healthy controls who underwent double-leg stance static posturographic tests pre- and postoperatively on a vertical force platform. Four test conditions were performed with their eyes open and closed, without and with foam support to evaluate the balance control of all participants. The International Knee Documentation Committee subjective knee form, Lysholm knee scoring scale, Tampa scale for kinesiophobia, and active range of motion of the affected knee were synchronously obtained and assessed.

RESULTS

More postural sway was observed in patients compared to the healthy controls, 11 ± 5 days preoperatively (p < 0.01). However, 374 ± 23 days postoperatively, postural sway between the patients and control subjects was comparable (p > 0.05). Patients following MPFL reconstruction demonstrated better postural stability (p < 0.01). Significant ameliorations were found in all clinical assessments in the study patients postoperatively (p < 0.01).

CONCLUSIONS

Patients with recurrent lateral patellar instability have inefficient balance control. Static bipedal balance control can be improved under surface perturbation in these patients one year after isolated MPFL reconstruction that enhances the possibility of normal restoration of postural stability. Structural recovery of the ligament could help restore the sensorimotor efficiency and generate the compensatory and anticipatory balance regulation strategies, thereby improving joint function.

High tibial osteotomy improves balance control in patients with knee osteoarthritis and a varus deformity

BACKGROUND

Impaired knee stability is observed in patients with medial compartment knee osteoarthritis (OA) and varus malalignment. Although high tibial osteotomy (HTO) is widely used to treat OA-related knee varus deformity, its long-term influence on balance control in OA patients is poorly reported. This study aimed to evaluate the impact of HTO on balance control and assess its biological and functional significance.

METHODS

Thirty-two patients with medial compartment knee OA as well as varus deformity who were scheduled for HTO underwent static posturographic tests one month pre- and three months as well as one year postoperatively, respectively, along with forty matched control subjects. Radiographic and clinical evaluations were synchronously carried out on patients pre- and postoperatively.

RESULTS

Decreased postural sway was observed in patients one year after HTO. When compared to the control subjects, more postural sway was found in patients one month pre- and three months postoperatively. No difference was observed between the patients and control subjects one year postoperatively. The alignment and joint function of the affected knees significantly improved after HTO.

CONCLUSIONS

This study revealed that HTO improves balance control in patients with knee OA and varus deformity. Correct alignment and improved joint function enhance the likelihood of normal postural stability. Hence, this intervention allows the knee joint to recover its corrective compensatory role in postural regulation and should be taken into account for managing knee OA patients.

Postural threat increases sample entropy of postural control

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.

Effects of limited previously acquired information about falling height on lower limb biomechanics when individuals are landing with limited visual input

BACKGROUND

Inhibitions in the acquisition of accurate information about the environment can affect control of the lower extremities and lead to anterior cruciate ligament injury. This study aimed to clarify the effects of limited prior knowledge of the height of the fall, as well as limited visual input, on lower limb and trunk motion and ground reaction force during landing.

METHODS

Twenty healthy university students were recruited. Drop landings from a 30-cm platform were measured under three conditions: (1) unknown, without prior knowledge of the height of the fall and without visual input; (2) known, with prior knowledge of the height of the fall and without visual input; and (3) control, with prior knowledge of the height of the fall and visual input.

FINDINGS

In the unknown condition, the peak ground reaction force for the vertical and posterior directions was significantly higher than that in the known and control conditions; leg and knee stiffness, ankle joint work, and joint flexion motion of the knee, ankle, and trunk after landing were decreased as well. In the known condition, there were no significant differences in leg and knee stiffness and vertical ground reaction force compared to the control condition.

INTERPRETATION

The results of this study indicate that the risk of anterior cruciate ligament injury during landing increases when individuals have limited visual input and prior knowledge of the height of the fall. This finding suggests that an accurate perception of the surrounding environment may help prevent anterior cruciate ligament injuries.

Does increased gait variability improve stability when faced with an expected balance perturbation during treadmill walking?

BACKGROUND

Currently, there is uncertainty as to whether movement variability is errorful or exploratory.

RESEARCH QUESTION

This study aimed to determine if gait variability represents exploration to improve stability. We hypothesized that 1) spatiotemporal gait features will be more variable prior to an expected perturbation than during unperturbed walking, and 2) increased spatiotemporal gait variability pre-perturbation will correlate with improved stability post-perturbation.

METHODS

Sixteen healthy young adults completed 15 treadmill walking trials within a motion simulator under two conditions: unperturbed and expecting a perturbation. Participants were instructed not to expect a perturbation for unperturbed trials, and to expect a single transient medio-lateral balance perturbation for perturbed trials. Kinematic data were collected during the trials. Twenty steps were recorded post-perturbation. Unperturbed and pre-perturbation gait variabilities were defined by the short- and long-term variabilities of step length, width, and time, using 100 steps from pre-perturbation and unperturbed trials. Paired t-tests identified between-condition differences in variabilities. Stability was defined as the number of steps to centre of mass restabilization post-perturbation. Multiple regression analyses determined the effect of pre-perturbation variability on stability.

RESULTS

Long-term step width variability was significantly higher pre-perturbation compared to unperturbed walking (mean difference = 0.28 cm, p = 0.0073), with no significant differences between conditions for step length or time variabilities. There was no significant relationship between pre-perturbation variability and post-perturbation restabilization.

SIGNIFICANCE

Increased pre-perturbation step width variability was neither beneficial nor detrimental to stability. However, the increased variability in medio-lateral foot placement suggests that participants adopted an exploratory strategy in anticipation of a perturbation.

Selective preservation of changes to standing balance control despite psychological and autonomic habituation to a postural threat

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 effects of perturbation type and direction on threat-related changes in anticipatory postural control

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.

The Effect of Unilateral Vestibular Loss on Standing Balance During Postural Threat

OBJECTIVE

Vestibular deficit patients have an increased fall risk and fear of falling. Postural threat, known to increase balance-related fear and anxiety, influences vestibular gains during quiet standing in young healthy adults. The current study examined whether there is a similar relationship for peripheral unilateral vestibular loss (UVL) patients in comparison to age-matched healthy controls (HC).

SETTING

University laboratory.

STUDY DESIGN

Prospective laboratory study.

PATIENTS AND CONTROLS

Eleven UVL patients, nine with vestibular neurectomy. Eleven aged-matched HCs.

MAIN OUTCOME MEASURES

Subjects stood on a hydraulic lift placed at two heights: low (0.8 m, away from the edge) and high (3.2 m, at the edge). Amplitude (root mean square), mean power frequency (MPF), and mean position were analyzed for center of foot pressure (COP) and 90% ranges for angle amplitude and velocity for trunk sway.

RESULTS

Group interactions were strongest for anterior-posterior (AP) COP and trunk pitch angle. AP lean away from the edge was greater in HCs than UVLs. HCs, but not UVLs had a decrease in root mean square AP COP with height. Trunk pitch sway was changed similarly. Both groups had increased trunk pitch velocity at height. Changes with height were less for roll: MPF of lateral COP increased with height for UVLs with no changes for HCs, and trunk roll amplitude decreased for both groups.

CONCLUSIONS

This report provides evidence for a differential effect of height induced postural threat on balance control between UVLs and HCs presumably due to the reduced vestibular-spinal gain in UVL subjects.

Adaptation of emotional state and standing balance parameters following repeated exposure to height-induced postural threat

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.

Postural instability and position of the center of pressure into the base of support in postmenopausal osteoporotic and nonosteoporotic women with and without hyperkyphosis

In postmenopausal women, thoracic hyperkyphosis affects postural instability in the sagittal plane, whereas osteoporosis affects it in the frontal plane. Decrease of hip muscle strength can be changed the center of pressure distance to the center of base of support. These results may be important to design the therapeutic exercise for decreasing the postural instability.

PURPOSE

In this study, we investigated the effect of bone mineral density (BMD) and thoracic kyphosis on the center of pressure (CoP) sway and its location related to the base of support (BoS).

METHODS

Ten young and 39 postmenopausal women voluntarily participated in this study. Postmenopausal women were divided into four groups according to the thoracic kyphosis angle (normal kyphotic < 50° ≤ hyperkyphotic) and T-score values. The isometric strength of the trunk and lower limb muscles were measured. The CoP postural sway was measured in a comfortable double stance position, and the location of the CoP was then determined related to the BoS.

RESULTS

In both hyperkyphotic groups (osteoporotic and normal BMD), the strength of back extension and hip adduction showed a significant decrease compared to the normal kyphotic groups. In the osteoporotic groups (hyper- and normal kyphotic), hip abduction and ankle plantar flexion were significantly weaker than those in the nonosteoporotic groups. In both hyperkyphotic groups, velocity of the CoP displacement in the anterior-posterior (AP) direction was significantly higher than that in the young group, while, in both of the osteoporotic groups, velocity of the CoP displacement in the medio-lateral (ML) direction was significantly higher than that in the young group. In postmenopausal women, hip extensor strength negatively and significantly correlated with the CoP distance to the center of the BoS.

CONCLUSION

It appears that thoracic hyperkyphosis affects postural instability in the AP direction and that a decrease of BMD affects postural instability in the ML direction.

New Insights on Emotional Contributions to Human Postural Control

It has been just over 20 years since the effects of height-induced threat on human postural control were first investigated. Raising the height of the support surface on which individuals stood increased the perceived consequences of instability and generated postural control changes. Since this initial work, converging evidence has accumulated supporting the efficacy of using height-induced threat to study the effects of emotions on postural control and confirming a direct influence of threat-related changes in arousal, anxiety, and fear of falling on all aspects of postural control, including standing, anticipatory, and reactive balance. In general, threat-related postural changes promote a greater physical safety margin while maintaining upright stance. We use the static balance literature to critically examine the current state of knowledge regarding: (1) the extent to which threat-related changes in postural control are sensitive to threat-related changes in emotions; (2) the underlying neurophysiological and cognitive mechanisms that may contribute to explaining the relationship between emotions and postural control; and (3) the generalizability of threat-related changes across different populations and types of threat. These findings have important implications for understanding the neuromechanisms that control healthy balance, and highlight the need to recognize the potential contributions of psychological and physiological factors to balance deficits associated with age or pathology. We conclude with a discussion of the practical significance of this research, its impact on improving diagnosis and treatment of postural control deficits, and potential directions for future research.

Exploring the relationship between threat-related changes in anxiety, attention focus, and postural control

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.

Introducing a psychological postural threat alters gait and balance parameters among young participants but not among most older participants

The fear of falling can be manipulated by introducing a postural threat (e.g., an elevated support surface) during stance and gait. Under these conditions, balance parameters are altered in both young and elderly individuals. This study aimed to dissociate the physical and psychological aspects of the threat and show the impact of a verbal warning cue of imminent perturbation during gait among young and elderly healthy participants. Ten young subjects (29.4 ± 3.9 years) and ten subjects aged over 65 years (72.9 ± 3.5) participated in the study. Spatiotemporal and balance parameters were quantified during eight consecutive gait cycles using a motion analysis system and an instrumented treadmill. These parameters were compared twice in the control trial and before/after a verbal warning cue of imminent perturbation during gait ("postural threat") in perturbation trials and between groups using repeated measure ANOVAs.

RESULTS

The verbal cue yielded reduced step length (p = 0.008), increased step width (p = 0.049), advanced relative position of the center of mass (p = 0.016), increased stabilizing force (p = 0.003), and decreased destabilizing force (p = 0.002). This warning effect was not observed in the older participant group analyses but was found for three participants based on individual data analyses. The warning effect in younger participants was not specific to impending perturbation conditions. Most gait and balance parameters were altered in the older group (p < 0.05) versus the younger group in each condition, regardless of the warning cue. A psychological threat affects gait and balance similarly to a physical threat among young participants but not among most older participants.

Postural threat influences vestibular-evoked muscular responses

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.

The threat of a support surface translation affects anticipatory postural control

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.

Virtual reality applications in assessing the effect of anxiety on sensorimotor integration in human postural control

Falls are a leading cause of injury and mortality among adults over the age of 65 years. Given the strong relation between fear of falling and fall risk, identification of the mechanisms that underlie anxiety-related changes in postural control may pave the way to the development of novel therapeutic strategies aimed at reducing fall risk in older adults. First, we review potential mechanisms underlying anxiety-mediated changes in postural control in older adults with and without neurological conditions. We then present a system that allows for the simultaneous recording of neural, physiological, and behavioral data in an immersive virtual reality (VR) environment while implementing sensory and mechanical perturbations to evaluate alterations in sensorimotor integration under conditions with high postural threat. We also discuss applications of VR in minimizing falls in older adults and potential future studies.

Postural threat influences conscious perception of postural sway

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.

Personality traits and individual differences predict threat-induced changes in postural control

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.