A balance control model predicts how vestibular loss subjects benefit from a vibrotactile balance prosthesis

Vibratory cue training elicits anticipatory postural responses to an external perturbation

Anticipatory postural adjustments (APAs) represent the feedforward mechanism of neuromuscular control essential for maintaining balance under predictable perturbations. The importance of vision as a distal sensory modality in the generation of APAs is well established. However, the capabilities of external cues in generating APAs are less explored. In the present study, vibratory cue was investigated for its reliability among healthy individuals in generating anticipatory response under external perturbation in the absence of vision. Ten participants, in quiet stance, were provided with external perturbation in the form of pendulum impact in anterior-posterior (AP) direction under conditions of: both vision and vibratory cue absent; vision present but vibratory cue was absent; vision and vibratory cue both were present; only vibratory cue is present with vision being absent. EMG activities of the leg muscles and displacement of center of pressure (COP) in AP direction were recorded. The data were later analyzed and quantified in the time frame of anticipatory and compensatory phases. The results showed that with training, participants were able to generate significant APAs relying on the vibratory cue alone. Improvement in APAs was accompanied by minimizing the need for larger CPA and improved stability (COP displacement) under perturbation. The study outcome indicates the possibility of using vibratory cues for APA-based interventions.

Potential Mechanisms of Sensory Augmentation Systems on Human Balance Control

Numerous studies have demonstrated the real-time use of visual, vibrotactile, auditory, and multimodal sensory augmentation technologies for reducing postural sway during static tasks and improving balance during dynamic tasks. The mechanism by which sensory augmentation information is processed and used by the CNS is not well understood. The dominant hypothesis, which has not been supported by rigorous experimental evidence, posits that observed reductions in postural sway are due to sensory reweighting: feedback of body motion provides the CNS with a correlate to the inputs from its intact sensory channels (e.g., vision, proprioception), so individuals receiving sensory augmentation learn to increasingly depend on these intact systems. Other possible mechanisms for observed postural sway reductions include: cognition (processing of sensory augmentation information is solely cognitive with no selective adjustment of sensory weights by the CNS), “sixth” sense (CNS interprets sensory augmentation information as a new and distinct sensory channel), context-specific adaptation (new sensorimotor program is developed through repeated interaction with the device and accessible only when the device is used), and combined volitional and non-volitional responses. This critical review summarizes the reported sensory augmentation findings spanning postural control models, clinical rehabilitation, laboratory-based real-time usage, and neuroimaging to critically evaluate each of the aforementioned mechanistic theories. Cognition and sensory re-weighting are identified as two mechanisms supported by the existing literature.

Effects of long-term balance training with vibrotactile sensory augmentation among community-dwelling healthy older adults: a randomized preliminary study

Background

Sensory augmentation has been shown to improve postural stability during real-time balance applications. Limited long-term controlled studies have examined retention of balance improvements in healthy older adults after training with sensory augmentation has ceased. This pilot study aimed to assess the efficacy of long-term balance training with and without sensory augmentation among community-dwelling healthy older adults.

Methods

Twelve participants (four males, eight females; 75.6 ± 4.9 yrs) were randomly assigned to the experimental group (n = 6) or control group (n = 6). Participants trained in their homes for eight weeks, completing three 45-min exercise sessions per week using smart phone balance trainers that provided written, graphic, and video guidance, and monitored trunk sway. During each session, participants performed six repetitions of six exercises selected from five categories (static standing, compliant surface standing, weight shifting, modified center of gravity, and gait). The experimental group received vibrotactile sensory augmentation for four of the six repetitions per exercise via the smart phone balance trainers, while the control group performed exercises without sensory augmentation. The smart phone balance trainers sent exercise performance data to a physical therapist, who recommended exercises on a weekly basis. Balance performance was assessed using a battery of clinical balance tests (Activity Balance Confidence Scale, Sensory Organization Test, Mini Balance Evaluation Systems Test, Five Times Sit to Stand Test, Four Square Step Test, Functional Reach Test, Gait Speed Test, Timed Up and Go, and Timed Up and Go with Cognitive Task) before training, after four weeks of training, and after eight weeks of training.

Results

Participants in the experimental group were able to use vibrotactile sensory augmentation independently in their homes. After training, the experimental group had significantly greater improvements in Sensory Organization Test and Mini Balance Evaluation Systems Test scores than the control group. Significant improvement was also observed for Five Times Sit to Stand Test duration within the experimental group, but not in the control group. No significant improvements between the two groups were observed in the remaining clinical outcome measures.

Conclusion

The findings of this study support the use of sensory augmentation devices by community-dwelling healthy older adults as balance rehabilitation tools, and indicate feasibility of telerehabilitation therapy with reduced input from clinicians.

The effect of prosthetic feedback on the strategies and synergies used by vestibular loss subjects to control stance

Background

This study investigated changes in stance movement strategies and muscle synergies when bilateral peripheral vestibular loss (BVL) subjects are provided feedback of pelvis sway angle.

Methods

Six BVL (all male) and 7 age-matched male healthy control (HC) subjects performed 3 stance tasks: standing feet hip width apart, eyes closed, on a firm and foam surface, and eyes open on foam. Pelvis and upper trunk movements were recorded in the roll and pitch planes. Surface EMG was recorded from pairs of antagonistic muscles at the lower leg, trunk and upper arm. Subjects were first assessed without feedback. Then, they received training with vibrotactile, auditory, and fall-warning visual feedback during stance tasks before being reassessed with feedback.

Results

Feedback reduced pelvis sway angle displacements to values of HCs for all tasks. Movement strategies were reduced in amplitude but not otherwise changed by feedback. These strategies were not different from those of HCs before or after use of feedback. Low frequency motion was in-phase and high frequency motion anti-phasic. Feedback reduced amplitudes of EMG, activity ratios (synergies) of antagonistic muscle pairs and slightly reduced baseline muscle activity.

Conclusions

This is the first study demonstrating how vestibular loss subjects achieve a reduction of sway during stance with prosthetic feedback. Unchanged movement strategies with reduced amplitudes are achieved with improved antagonistic muscle synergies. This study suggests that both body movement and muscle measures could be explored when choosing feedback variables, feedback location, and patient groups for prosthetic devices which reduce sway of those with a tendency to fall.

Strategies and synergies underlying replacement of vestibular function with prosthetic feedback

This study investigated changes in movement strategies and muscle synergies when bilateral peripheral vestibular loss (BVL) subjects are provided prosthetic feedback of their pelvis sway during stance. Six BVL subjects performed 3, for them, difficult stance tasks: standing eyes closed, on a firm surface, on a foam surface, and standing eyes open on foam. Movement strategies were recorded as roll and pitch ratios of upper and lower body velocities with body-worn gyroscopes. Surface EMG recordings were taken from two pairs of antagonistic, lower leg and trunk muscles in order to note synergy changes with feedback. Subjects were first assessed without feedback. Then they were provided stance training with vibro-tactile and auditory feedback of pelvis angle sway, and finally reassessed with the same feedback active. For analysis of movement strategies, angle values integrated from angular velocity samples, were split into 3 frequency bands (<0.7, 0.7-3, and >3 Hz). Feedback caused a reduction in pelvis sway angle displacements to values of age-matched healthy controls (HC) for all tasks. Pelvis sway velocity was only reduced for the task with largest angle displacements, standing eyes closed on foam. Movement strategies in each frequency band examined were unaltered by feedback, except for amplitude, and were not different from those of HCs before or after use of feedback. Low frequency motion was in-phase as if the upper and lower body moved as an inverted pendulum, high frequency motion anti-phasic. Amplitudes of EMG were reduced with feedback. Synergies recorded in the form of activity ratios of antagonistic muscle pairs were reduced with feedback. This is the first study that demonstrates how vestibular loss subjects achieve a reduction of sway during stance with prosthetic feedback. Unchanged movement strategies with reduced amplitudes are achieved with reduced antagonistic muscle synergies. This study has implications for the choice of feedback parameters (angle or velocity) and patient groups when using prosthetic devices to reduce sway of those with a tendency to fall.