Methodology to generate and analyze periodic postural oscillations induced by triceps surae tendon vibration

This study presents an experimental methodology to induce postural oscillations through triceps surae tendon vibration. The objectives are to (1) verify if it is indeed possible to induce periodic postural oscillations in a standing human through tendon vibrations and (2) in the positive case, to verify signal processing approaches that can be used to quantify the levels of the induced oscillations at different frequencies (f_m) of the modulating signal.

Utilizing a vibration system, both triceps surae tendons of a participant in a quiet standing posture were bilaterally stimulated with an amplitude-modulated (AM) signal. This signal comprised a high-frequency carrier signal (100 Hz) amplitude-modulated by a sinusoidal signal at frequency f_m (e.g., at 0.2 Hz). Body movement was assessed using a camera system capturing the position of an infrared light-emitting marker on the subject’s back, enabling the tracking of the center of mass signal in the antero-posterior direction. The analyses were done by the estimation of the power spectral density and the magnitude-squared coherence.

The results of this study showed that it is possible to generate a sinusoidal-like induced postural oscillation in healthy young subjects following the sinusoidal signal at frequency f_m of the AM vibratory tendon stimulation. These induced periodic oscillations coexisted with the stochastic postural oscillations, being relatively visible (in a qualitative sense) when viewing a figure of the center of mass signal as a function of time. As the sinusoidal-like oscillations were not amenable for quantification when the center of mass signal was viewed in the time domain, this work found that spectral signal-processing techniques (power spectral analysis and magnitude squared coherence) were useful to clearly detect and quantify the level of the periodic component of the center of mass movement induced at a given frequency f_m by the proposed tendon vibration methodology.

The methodology—AM tendon vibration and center of mass analyses in the frequency domain—permits the induction of quantifiable periodic sway in standing humans, providing a tool to analyze the influence of proprioceptive sensory input on postural control. It has the potential to serve as a tool for diagnosing postural control dysfunctions in patients with proprioceptive deficits.