FREQUENCY-DEPENDENT SOLEUS REFLEX MODULATION DURING WHOLE-BODY VIBRATION

Experimental Procedure At the beginning of the experiment, participants will perform three repetitions of maximal voluntary plantarflexion (heel-rise) to determine maximal electromyographic (EMG) activity of the soleus muscle. These recordings will be used for normalization and reference purposes. Subsequently, whole-body vibration will be applied at 13 different frequencies ranging from 25 to 49 Hz (25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, and 49 Hz). The order of vibration frequencies will be randomized for each participant to minimize order effects. Whole-Body Vibration Protocol Participants will stand upright in an anatomically neutral position on the vibration platform and will be allowed to lightly hold the device's handrail to maintain balance without providing mechanical support. Vibration amplitude will be set at 2 mm. Each frequency condition will be applied for 15 s, with a 10 s rest period between trials. All WBV applications will be delivered using a Power Plate Pro5 device (Power Plate International Ltd., UK). Surface Electromyography Recording Surface electromyography (sEMG) will be recorded from the left soleus muscle using bipolar Ag/AgCl electrodes (Redline®, Istanbul, Türkiye; 10-mm diameter, 4-cm interelectrode distance) aligned with the muscle fibers following appropriate skin preparation (impedance \<10 kΩ). The electrodes will be placed over the lateral belly of the soleus muscle, with the ground electrode positioned on the right lateral malleolus. The sEMG signals will be amplified (×1000), band-pass filtered between 10 and 1,000 Hz, and sampled at 5,000 Hz. EMG signals will be acquired using a CED 1902 Quad System MKIII amplifier and a CED Power 1401 MKII A/D converter and will be recorded with Spike2 software (version 7.20; Cambridge Electronic Design, UK). Signals will be stored for offline analysis. Frequency-domain analysis will be performed using fast Fourier transform (FFT) algorithms implemented in Spike2.