Quantifying the Dynamics That Link Leg Tendon Vibration to Induced Periodic Postural Oscillations in Young Subjects: Differential Effects of Light Touch on the the Induced Sway.

Periodic sensory and mechanical perturbations applied to standing humans have provided key insights into postural control in health and disease. However, the contribution of sinusoidal proprioceptive drive and its interaction with haptic cues has not yet been systematically explored. Here, a bilateral Achilles tendon-vibration paradigm using a 100 Hz sinusoidal carrier, which was amplitude-modulated (AM) by 0.1-0.5 Hz sinusoids, was applied to 16 healthy young adults, and anteroposterior center-of-mass (COM) displacement was analyzed. The vibration protocol resulted in entrainment between the envelope signal and the COM, visible as narrow peaks in the power spectrum superimposed on the broadband sway background. Magnitude-squared coherence was significant at the modulation frequencies, and spectral gain was higher at lower modulation frequencies, indicating more efficient transmission of low-frequency proprioceptive drive to the COM. A consistent phase lag was observed between the envelope of stimulation and the COM at all tested modulation frequencies. When fingertip light touch on a rigid surface provided a minimal haptic cue, sway power at the modulation frequencies decreased by 8-13 dB, coherence was reduced, spectral gain decreased by 7-14 dB, and the phase-frequency relation deviated from the approximately linear pattern observed without light touch. Together, these findings show that sinusoidally amplitude-modulated Achilles tendon vibration provides a controlled, frequency-specific proprioceptive drive that can be quantified from whole-body sway. The attenuation and phase reshaping produced by light touch indicate that minimal haptic information can weaken the transmission of this proprioceptive perturbation to postural motion. This paradigm, therefore, provides a quantitative framework for probing how proprioceptive and haptic inputs are integrated into human postural control.