Cortical potentials time-locked to discrete postural events during quiet standing are facilitated during postural threat exposure.

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

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.

Does spinal excitability scale to the difficulty of the dual-task?

PURPOSE: This study examined whether spinal excitability, as measured by the soleus Hoffmann reflex (H-reflex), is scaled to the difficulty level of the dual-task being performed. METHODS: Twenty-two participants completed a combination of three balance task and three secondary cognitive (visuo-motor) task difficulty levels for a total of nine dual-task conditions. An additional eight participants were tested while performing the same three balance task difficulty levels on its own (i.e., single-tasking). The balance task required participants to maintain their balance on a fixed or rotating stabilometer while the visuo-motor task required participants to respond to moving targets presented on a monitor. Throughout each single- and dual-task trial, H-reflexes were elicited from the soleus. RESULTS: Although dual-task performance, as quantified by visuo-motor task accuracy as well as the root mean square of the stabilometer position and velocity, decreased by 10-34% with increasing dual-task difficulty (p < 0.05), no changes in the soleus H-reflex amplitude were observed between dual-task conditions (p = 0.483-0.758). This contrasts to when participants performed the balance task as a single-task, where the H-reflex amplitude decreased by ~25% from the easy to the hard balance task difficulty level (p = 0.037). CONCLUSIONS: In contrast to the commonly reported finding of a reduced soleus H-reflex amplitude when individuals perform a less posturally stable task by itself, the results indicate that spinal excitability is not modulated as a function of dual-task difficulty. It is possible that when an individual's attentional resource capacity is exceeded during dual-tasking, they become ineffective in regulating spinal excitability for balance control.

What will they think? The relationship between self-presentational concerns and balance and mobility outcomes in older women.

UNLABELLED: BACKGROUND/STUDY CONTEXT: Indirect evidence suggests that concerns over the impressions made on others (self-presentational concerns) may be associated with balance-related outcomes in older adults, but no empirical evidence exists to support this speculation. The present study examined the relationship between self-presentational concerns (social anxiety, social physique anxiety, and self-presentational efficacy) and functional mobility, accounting for age, balance confidence, falls, and muscle strength. METHODS: Healthy women (60 years or older; N = 187) completed measures of self-presentational concerns, balance confidence, and fall history, and performed the timed up and go (TUG) test and a test of leg strength. Bivariate correlations were conducted. A hierarchical regression predicted TUG duration from the three self-presentational concerns, controlling for age, balance confidence, falls, and muscle strength to examine the unique variance in TUG duration explained by self-presentational concerns. RESULTS: Self-presentational efficacy was a significant predictor of TUG duration over and above that of age, balance confidence, falls, and muscle strength. The results also showed significant correlations between social anxiety and self-presentational efficacy and TUG duration, between all three self-presentational concerns and balance confidence, and between social physique anxiety and self-presentational efficacy and falls. CONCLUSIONS: Research is needed to examine the causal relationship between these outcomes. Investigating self-presentational concerns in older women may provide novel ways to impact balance-related outcomes in this population. Practical implications for clinicians are discussed, as the social and physical environment may influence self-presentational concerns in this population and subsequently impact assessment and treatment outcomes.

Effects of postural threat on the scaling of anticipatory postural adjustments in young and older adults.

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

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.

Facilitation and Habituation of Cortical and Subcortical Control of Standing Balance Following Repeated Exposure to a Height-related Postural Threat.

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

Initial experience of balance assessment introduces 'first trial' effects on emotional state and postural control.

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.

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 distraction on threat-related changes in standing balance control.

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.