Whole body vibration training reduces plantar foot sensitivity but improves balance control of healthy subjects

Abstract

The goal of this study was to investigate the effects of short-time whole body vibration (WBV) training on foot vibration sensitivity of healthy subjects. Furthermore, the effects of WBV on a balance task (one-leg stand) were also evaluated. 30 young healthy subjects participated in the study. Vibration perception thresholds and balance were measured prior and after a single session of a 4-min WBV training (27 Hz, 2 mm horizontal amplitude). Thresholds were measured at 200 Hz at three anatomical locations of the plantar foot area (first and fifth metatarsal heads and heel). Body balance was quantified using the length as well as the area described by the center of pressure (COP) at quiet, one-leg standing. Whereas vibration thresholds significantly increased after WBV training at all measured locations, there was a significant decrease in the balance related parameters after WBV exercise. The results indicate that the above-threshold, sinusoidal vibration used during WBV training is not an adequate strategy to stimulate/improve vibration sensitivity. The improvements seen in balance after WBV are likely to have neuromuscular mechanisms as their main component rather than increased foot sensitivity.

Introduction

The contribution of the feet's cutaneous mechanoreceptors for the coordination of lower leg movements has been increasingly accepted in the literature. Joint position sense [19], contact-control tasks of the leg [31] as well as balance control and gait [2], [23] have been shown to be influenced by the activity of the mechanoreceptors of the foot.

Due to its importance for movement performance, different strategies have been developed to “exercise” foot sensitivity, with the goal to enhance it in healthy subjects or to reduce sensory impairments seen in patients with polyneuropathic diseases. Among these strategies, the application of stochastic resonance on the foot sole has proven to be very successful [8], [21]. This procedure combines vibration stimuli that cannot be perceived by subjects (below-threshold) with low-level mechanical noise to stimulate the mechanoreceptors, e.g. with vibration insoles. Through the stochastic nature of noise, this type of stimulation causes below-threshold vibration to randomly exceed threshold and thus become detectable [9]. Several studies using this technique have shown enhancements in foot sensitivity and consequently improvements in balance control and gait performance [12], [18], [24], [25], [26].

Another vibration-based method used with the goal to improve movement control is the whole body vibration (WBV) training. This method is based on vibration stimuli that can be perceived by subjects (above-threshold) and is transmitted to the body through the feet, generally using a vibration plate. This type of stimulation is also characterized by a sinusoidal, linear pattern, in which the same vibration stimuli are repeated over time. The positive effects of WBV on different systems of the human body have been extensively reported in the literature over the past years. Increases in muscular strength [3], [20] and bone density [35] as well as movement-related improvements in gait performance and balance control [4], [17] have been found as a result of WBV training. The main neuromuscular mechanism believed to be active during WBV exercise is based on the tonic vibration reflex, which is mediated by Ia-afferents of the muscle spindles [6], [12]. Since the mechanoreceptors of the skin are sensitive to vibration and are believed to respond to similar frequencies and amplitudes as the muscle-spindle afferents [29], [33], an influence WVB on plantar foot sensitivity could be expected.

The effects of WBV on the sensitivity of the foot's cutaneous mechanoreceptors have not been investigated so far. To understand the influence of WBV on foot sensitivity may contribute to elucidate whether above-threshold stimulation also has positive effects on foot sensitivity as well as to clarify the mechanisms underlying improvements in movement control reported after WBV training.

Therefore, the aim of this study was to investigate the effects of short-time WBV training on foot vibration sensitivity of healthy subjects. Furthermore, due to the close relationship between foot sensitivity and movement control, the effects of WBV on a balance task (one-leg stand) were also evaluated, to assist in the interpretation of the practical relevance of the results. We hypothesized an increase in foot sensitivity as well as balance control of the subject after WBV exercises.