Background: Postural stability is maintained by the central integration of the multisensory inputs to produce motor outputs. When visual and somatosensory input is available and reliable, this reduces the postural control system’s reliance on the vestibular system. Despite this, vestibular loss can still cause severe postural dysfunction. Training one or more of the three sensory systems can alter sensory weighting and change postural behavior. Exercises to activate the vestibular system is one form of training which resolves symptoms of vestibular impairments. Vestibular activation exercises, including horizontal and vertical headshaking, influences vestibular-ocular and -motor responses and have been showed to be effective in vestibular rehabilitation. However, no study has employed a concurrent vestibular activation and weight shift postural training to realize a more effective rehabilitation method by positively influencing sensory reweighting mechanisms and vestibular reflexes. Our pilot study (n=33) has demonstrated significant postural stability improvement in the center of pressure (COP) medio-lateral standard deviation sway (ML Std) and multiscale entropy (MSE) sway velocity among the horizontal headshake group. This improvement was found in the vestibular and visual-vestibular conditions of the Sensory Organization Test (SOT) assessment when compared to a non-headshake training group and no training group (control). Aims: The main aim of this study was to assess sensory reweighting of postural control processing and vestibular-ocular and -motor responses after combined vestibular activation with postural training in healthy young adults. It was hypothesized that the effect of this training would significantly alter the pattern of sensory weighting by changing the ratio of visual, somatosensory and vestibular dependence needed to maintain postural stability, and significantly decrease vestibular responses. Methods: Forty-two young healthy individuals (22 females; 23.0+3.9 years [18-35 years]; 1.6+0.1 meters) were randomly assigned into four groups: 1) visual feedback weight shift training (WST) coupled with an active horizontal headshake (HHS), 2) same WST with vertical headshake (VHS), 3) WST with no headshake (NHS) and 4) no training/headshake control (CTL) groups. The headshake groups performed an intensive body weight shift training (WST) together with horizontal or vertical rhythmic headshake 30° in both directions in accordance to the beat of a metronome ranging from 80 to 120 beats per minute. The NHS group performed the WST with no headshake while the control did not perform any training. Five 15-minute training sessions were performed on consecutive days for one week with the weight shift exercises involving upright limits of stability activities on a flat surface, foam or rocker board. All groups performed baseline- and post-assessments including SOT and force plate platform up and down unpredictable ramp perturbations, coupled with electromyographic (EMG) and electro-oculographic (EOG) recordings. The video head impulse test (vHIT) system was also used to record horizontal VOR gain. Statistical analysis: A between- and within-group repeated measures ANOVA of 6 (3 visuals x 2 surfaces) conditions x 4 groups x 2 sessions was used to analyze five COP sway variables, the equilibrium and composite scores and sensory ratios of the SOT as well as EMG (onset, duration, peak amplitude, peak time and power spectral densities) signals and horizontal VOR gain. The five COP variables were: sway area, sway velocity, antero-posterior (AP) standard deviation, ML Std and MSE sway velocity. Similarly, COP variables, EMG, as well as EOG (angle in degrees) and vestibular reflex (vertical VOR, VCR and VSR gain) data during ramp perturbation trials were analyzed. Pearson product-moment correlation was used to evaluate the relationships between outcome measures. Alpha level will be set at p<.05. Results: The concurrent vestibular and WST showed a significant somatosensory downweighting (p = .050) in the headshake groups compared to the other groups. The training also showed a significant decreased horizontal VOR gain (p = .040), faster automatic postural response (p = .003) with improved flexibility (p = .010) in the headshake groups. Muscle activation pattern in medial gastrocnemius (p = .033) and eye movement variability (p = .024) were significantly decreased in the headshake groups following training. Pearson correlations showed moderate associations between postural sway, eye movement variables and vestibular reflex gains. Specifically, there were negative associations between VOR gain versus postural sway (r = -0.460 – -0.553; p = < .008), and eye movement variability versus postural sway (r = -0.404 – -0.521; p = < .015), and positive associations between EMG peak amplitude versus postural sway (r = 0.435 – 0.498; p = < .004) and eye movement variability (r = 0.467; p = .007). Conclusion: The concurrent vestibular activation and weight shift training modifies vestibular-dependent responses after the training intervention as evidenced in somatosensory downweighting, decreased VOR gain, decreased eye movement variability and better postural flexibility and faster automatic postural response. The findings suggest this is predominantly due to vestibular habituation and adaptation of VOR, VCR and VSR which induced sensory reweighting. The study also found moderate associations between postural measures and vestibular responses in vestibular reflex gains, eye movement variability and muscle activations. These findings may help predict postural changes through vestibular habituation and also provide insight into the behavior of eye movements and muscle activations following vestibular training. In addition, the findings may be used to guide development of a vestibular-postural rehabilitation intervention in impaired neurological populations, such as with vestibular disorders or sensory integration problems present in traumatic brain injuries. / Physical Therapy