Different Types of Mastoid Process Vibrations Affect Dynamic Margin of Stability Differently

Abstract

The vestibular system is critical for human locomotion. Any deteriorated vestibular system leads to gait instability. In the past decades, these alternations in gait patterns have been majorly measured by the spatial-temporal gait parameters and respective variabilities. However, measuring gait characteristics cannot capture the full aspect of motor controls. Thus, to further understand the effects of deteriorated vestibular system on gait performance, additional measurement needs to be taken into consideration. This study proposed using the margin of stability (MOS) to identify the patterns of dynamic control under different types of mastoid vibrations in walking. This study hypothesized that (1) using the MOS method could facilitate the understanding of another aspect of motor control induced by different types of mastoid vibrations, and (2) applying the mastoid vibrations could induce the asymmetric MOS. Twenty healthy young adults were recruited. Two electromechanical vibrotactile transducers were placed on the bilateral mastoid process to apply different types of vestibular vibrations (bilateral, unilateral, and no vibration). A motion capture system with eight cameras was used to measure the MOSap (margin of stability in the anterior-posterior direction), MOSml (margin of stability in the medial-lateral direction), and respective variabilities. The results were in line with the hypotheses that both bilateral and unilateral mastoid vibrations significantly increased MOSap (p = 0.036, p < 0.001), MOSml (p = 0.012, p < 0.001), and respective variabilities p = 0.001, p < 0.001; p = 0.001, p < 0.01 when compared to the no vibration condition. Also, significantly larger MOSml (p = 0.001), MOSml variability (p < 0.023), MOSap (p < 0.001), and MOSap variability (p = 0.002) were observed under the unilateral vibration condition than that observed under the bilateral vibration condition. The above-mentioned result found that different types of mastoid vibrations affected the MOS differently, suggesting different patterns of control mechanisms under different sensory-conflicted situations. Besides, a significant difference between the dominant and non-dominant legs was observed in MOSml. Moreover, applying the unilateral mastoid vibrations induced a greater symmetric index of MOSml, suggesting that more active control in balance was needed in the medial-lateral than in the anterior-posterior direction.

Keywords: gait; stability; treadmill; vestibular system; vibration.

Figure 1

The mastoid vibration device.

Figure 2

Margin of stability in the anterior-posterior direction (MOSap) was defined as the horizontal distance between the anterior boundary of the base of support (BOS) and the extrapolated center of mass (XCoM). The anterior boundary of the BOS was defined as the position of the heel marker in the anterior-posterior direction. Margin of stability in the medial-lateral direction (MOSml) was defined as the horizontal distance between the lateral boundary of the BOS and the XCoM. The lateral boundary of the BOS was defined as the position of the heel marker in the medial-lateral direction. CoM, center of mass; XCoM, extrapolated center of mass; BOS, base of support; VBOS, velocity of BOS, defined as treadmill belt speed; VCoM, velocity of CoM; MOS, margin of stability; L, the distance from CoM to the heel marker; g, gravity.

Figure 3

The effects of mastoid vibration on the margin of stability and its variability. (A) Mean margin of stability in the anterior-posterior direction (MOSap), (B) mean margin of stability in the medial-lateral direction (MOSml), (C) variability of MOSap, and (D) variability of MOSml. Error bars indicate between-subject standard deviations. *p < 0.05, **p < 0.01, ***p < 0.001, Bi, bilateral mastoid vibration; No, no mastoid vibration; Uni, unilateral mastoid vibration. Symbol x meant the mean value.

Figure 4

The symmetric index of margin of stability and its variability. (A) Symmetric index of margin of stability in the anterior-posterior direction (MOSap), (B) symmetric index of margin of stability in the medial-lateral direction (MOSml), (C) symmetric index of MOSap variability, and (D) symmetric index of MOSml variability, *p < 0.05, Bi, bilateral mastoid vibration; No, no mastoid vibration; Uni, unilateral mastoid vibration. Symbol x meant the mean value.