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.