Adaptation of anticipatory postural adjustments during gait initiation induced by short-term whole-body vibration: A randomized sham intervention study in young adults and elderly

Abstract

Whole-body vibration (WBV) is a training method that exposes the entire body to mechanical oscillations while standing on a vibrating platform. WBV are used by clinicians to improve balance and gait. However, WBV may also induce an increase in leg stiffness through a “tonic reflex vibration”, which may potentially hinder balance control during locomotor tasks. Gait initiation (GI) is a functional task that is classically used to investigate balance control. It includes two phases, the “Anticipatory Postural Adjustments” (APAs) and step-execution phase. During GI, stability is known to depend on the capacity of the central nervous system (CNS) to generate adequate APAs along the mediolateral direction. This study tested the hypothesis that, in young and elderly adults, the CNS programs mediolateral APAs in function of the changes in stance leg stiffness induced by WBV so as to maintain (or even improve) stability. Participants (i.e. 23 young adults and 20 elderly) were randomly divided in two subgroups and exposed to a single WBV session or a sham intervention. Series of ten GI trials were performed on a force-plate before and after WBV or sham intervention. In young adults, the amplitude of mediolateral APAs (peak center-of-pressure shift and center-of-mass velocity at heel-off) and stance leg stiffness significantly increased following WBV. Mechanical modelling of the body showed that the increased leg stiffness was detrimental to stability. However, experimental data showed that stability remained unchanged following WBV, and even tended to be improved. Results further showed that center-of-mass velocity at heel-off and stance leg stiffness were positively correlated. In elderly, there was no significant effect of WBV on GI parameters. Young adults compensate the negative effect of increased leg stiffness on stability by larger mediolateral APAs. In elderly, the absence of WBV effects may possibly be due to proprioceptive and/or plantar inputs alterations.