This study examines how different standing postures and whole-body vibration conditions affect spinal reflex activity. Participants will stand in various positions-with and without vibration of two amplitudes-and H-reflex responses from the soleus muscle will be recorded. Additionally, vibration-induced reflex latency will be assessed across multiple vibration frequencies.

Participants will engage in a structured set of experimental tasks designed to evaluate the effects of postural mechanical loading, and whole-body vibration.

The protocol includes four primary standing conditions:

Quiet bipedal stance,

Single-leg stance on the left side,

Bipedal stance during whole-body vibration (WBV), and

Single-leg stance on the left side during WBV.

During all WBV conditions, two vibration amplitudes-2.2 mm and 1.2 mm-will be applied sequentially to assess amplitude-dependent modulation of reflex responses.

Throughout each condition, H-reflex recordings from the soleus muscle will be collected to quantify changes in spinal reflex excitability under varying postural and vibratory loads. In addition to these measurements, vibration-induced reflex latency will be evaluated by delivering vibratory stimuli at multiple frequencies (30, 32, 34, and 36 Hz) within the same experimental protocol. This approach allows for a comprehensive analysis of how both posture and vibratory parameters influence neuromuscular reflex pathways.

The investigators conducted an a priori power analysis for a repeated-measures ANOVA (within-subjects design) to determine the required sample size. The analysis was based on a medium expected effect size, specified as partial eta squared (ηₚ²) = 0.06, which corresponds to an effect size of f = 0.25. The investigators set the alpha error probability at 0.05 and the desired statistical power (1-β) at 0.80. The design included one group with five repeated measurements, assuming a correlation of 0.70 among repeated measures. Under these conditions, the critical F value was 2.5652405, with 4 numerator degrees of freedom and 48 denominator degrees of freedom. The power analysis indicated that a total sample size of 13 participants is required. The required sample size was calculated using G*Power version 3.1.9.4 (Franz Faul, University of Kiel, Germany).

Inclusion Criteria:

• Healthy adults between 25 and 50 years of age
• Individuals willing to participate and able to provide informed consent
• Individuals who can stand independently for at least 10 minutes
• Participants with no known neurological, orthopedic, or balance impairments

Exclusion Criteria:

• Individuals who can not tolerate WBV
• Individuals who can not tolerate electrical stimulation
• Pregnancy