Brian C. Horslen

Does increased postural threat lead to more conscious control of posture

Although it is well established that postural threat modifies postural control, little is known regarding the underlying mechanism(s) responsible for these changes. It is possible that changes in postural control under conditions of elevated postural threat result from a shift to a more conscious control of posture. The purpose of this study was to determine the influence of elevated postural threat on conscious control of posture and to determine the relationship between conscious control and postural control measures. Forty-eight healthy young adults stood on a force plate at two different surface heights: ground level (LOW) and 3.2-m above ground level (HIGH). Centre of pressure measures calculated in the anterior-posterior (AP) direction were mean position (AP-MP), root mean square (AP-RMS) and mean power frequency (AP-MPF). A modified state-specific version of the Movement Specific Reinvestment Scale was used to measure conscious motor processing (CMP) and movement self-consciousness (MSC). Balance confidence, fear of falling, perceived stability, and perceived and actual anxiety indicators were also collected. A significant effect of postural threat was found for movement reinvestment as participants reported more conscious control and a greater concern about their posture at the HIGH height. Significant correlations between CMP and MSC with AP-MP were observed as participants who consciously controlled and were more concerned for their posture leaned further away from the platform edge. It is possible that changes in movement reinvestment can influence specific aspects of posture (leaning) but other aspects may be immune to these changes (amplitude and frequency).

Modulation of human vestibular reflexes with increased postural threat

The vestibular system is an important sensory contributor to the control of standing balance. Fear, anxiety and arousal are thought to influence the excitability of the vestibular system, but it is not clear if these changes lead to altered vestibular‐evoked balance reflexes. Low and high standing surface heights were used to manipulate fear and anxiety in this study, while stochastic vestibular stimulation was used to evoke balance reflexes. High surface heights lead to greater coupling between vestibular inputs and balance reflexes, as well as larger responses. These results support the idea that the manner in which vestibular information is processed is altered when people are exposed to a threat to their balance, and this altered processing may explain why normal balance behaviour is different in threatening scenarios.

Effects of postural threat on spinal stretch reflexes: evidence for increased muscle spindle sensitivity?

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.

Influence of real and virtual heights on standing balance

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.

Human proprioceptive adaptations during states of height-induced fear and anxiety

Clinical and experimental research has demonstrated that the emotional experience of fear and anxiety impairs postural stability in humans. The current study investigated whether changes in fear and anxiety can also modulate spinal stretch reflexes and the gain of afferent inputs to the primary somatosensory cortex. To do so, two separate experiments were performed on two separate groups of participants while they stood under conditions of low and high postural threat. In experiment 1, the proprioceptive system was probed using phasic mechanical stimulation of the Achilles tendon while simultaneously recording the ensuing tendon reflexes in the soleus muscle and cortical-evoked potentials over the somatosensory cortex during low and high threat conditions. In experiment 2, phasic electrical stimulation of the tibial nerve was used to examine the effect of postural threat on somatosensory evoked potentials. Results from experiment 1 demonstrated that soleus tendon reflex excitability was facilitated during states of height-induced fear and anxiety while the magnitude of the tendon-tap-evoked cortical potential was not significantly different between threat conditions. Results from experiment 2 demonstrated that the amplitudes of somatosensory-evoked potentials were also unchanged between threat conditions. The results support the hypothesis that muscle spindle sensitivity in the triceps surae muscles may be facilitated when humans stand under conditions of elevated postural threat, although the presumed increase in spindle sensitivity does not result in higher afferent feedback gain at the level of the somatosensory cortex.

Improving impaired balance function: Real-time versus carry-over effects of prosthetic feedback

This study investigated whether training with realtime prosthetic biofeedback (BF) of trunk sway induces a carry-over improvement in balance control once BF is removed. 12 healthy older adults and 7 uncompensated unilateral vestibular loss patients were tested. All participants performed a battery of 14 balance and gait tasks (pre-test) upon their initial lab visit during which trunk angular sway was measured at L1–3. They then received balance BF training on a subset of 7 tasks, three times per week, for two consecutive weeks. BF was provided using a multi-modal biofeedback system with graded vibrotactile, auditory, and visual cues in relation to subject-specific angular displacement thresholds. Performance on the battery of the 14 balance and gait tasks (without BF) was re-assessed immediately after the 2 week training period, as well as 1 week later to examine BF carry-over effects. Significant reductions in trunk angular displacement were observed with the real-time BF, compared to the pre-test trials. The effects of BF persisted when BF was removed immediately after the final training session. BF carry-over effects were less evident at one week post-training. This evidence supports the potential short-term effects of BF training in a limited number of tasks after the BF is removed in healthy elderly subjects and those with vestibular loss. However, the prospect for longer term (>1 week) effects of prosthetic training on balance control remains currently unknown.

Both standing and postural threat decrease Achilles’ tendon reflex inhibition from tendon electrical stimulation

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.

Benefits of multi-session balance and gait training with multi-modal biofeedback in healthy older adults

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.

Increased human stretch reflex dynamic sensitivity with height‐induced postural threat

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.

Influence of emotional stimuli on lower limb cutaneous reflexes during human gait

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.