Abstract
BACKGROUND: Unilateral neck muscle vibration (NMV) activates the primary endings of muscle spindles and modulates both subjective visual vertical and subjective straight-ahead perception. However, its effects on subjective postural vertical (SPV), crucial for postural balance, remain poorly understood. We aimed to investigate the effects of unilateral NMV-induced proprioceptive stimulation on SPV tilt direction and intraindividual variability in the frontal plane in healthy participants. METHODS: We included 48 healthy adults (29 males, 19 females; age 22.5 ± 1.1 years; height 167.7 ± 7.4 cm; weight 58.7 ± 8.4 kg), randomly divided into four groups: vibrations to the left (L-Vib) and right sides (R-Vib), as well as sham stimulations to the left (L-Sham) and right (R-Sham). Vibration was applied for 10 min at 80 Hz with an amplitude of 0.8 mm. SPV was measured using a motorized vertical-tilting chair equipped with a backrest and lateral supports. Participants were seated without ground contact, with their trunk fixed and arms crossed; the ir head and legs re mained unrestrained. The experimenter tilted the chair from an initial position of 15° or 20° in the frontal plane toward the vertical at a speed of 1.5°/s. A digital inclinometer recorded the tilt angle when participants reported their body felt upright. Each session comprised eight trials with pseudorandom starting directions and angles. The mean tilt direction and standard deviation across trials were calculated. SPV was assessed before, during, and after stimulation. A two-way analysis of variance was conducted to analyze the effect s of unilateral NMV on SPV outcomes. RESULTS: There were no significant demographic differences across groups. For SPV tilt direction, there was no statistically significant interaction between group and time. However, for SPV variability, significant effects were observed for time (F1,44 = 9.591, p = 0.003, partial η(2) = 0.179) and the interaction between group and time (F6,44 = 2.325, p = 0.039, partial η(2) = 0.137). Participants in the L-Vib group exhibited significantly reduced variability both during and after stimulation compared with those in the L-Sham (p = 0.004) and R-Sham (p < 0.001) groups. Similarly, participants in the R-Vib group showed significantly lower variability than those in the R-Sham group (p = 0.02). DISCUSSION: These findings highlight the role of sensorimotor integration in body orientation and suggest that unilateral NMV may enhance the precision of verticality estimation. Based on this preliminary study, NMV could be a promising intervention for individuals with SPV abnormalities.