## 4 Discussion  
This study examined the effect of vibrotactile stimulation of the nail of the hallmark on the CoM movement during the stance phase. The proposed method had a baseline-dependent effect on the variability of the CoM movement and was more effective for subjects with a large variability of the CoM movement in the lateral direction during walking. Stimulation did not affect the amplitude of the CoM movement. These results support our hypothesis. This study highlighted the effect of the novel method in which vibrotactile stimulation was applied to the nail of the hallmark to the variability of the CoM movement during walking in healthy young people. As shown in Figure 3, the effect of vibrotactile stimulation of the nail of the hallmark depends on the variability of the walking speed, stance time, and the CoM movement in the lateral direction at the baseline. Stimulation decreased the fluctuations in these variables for subjects exhibiting a large intrasubject variability when walking without the stimulation. This result is in good agreement with that of a previous study that examined the effect of stimulation of the plantar surface [9]. In particular, stimulation was remarkably effective for subjects with a large variability of the CoM movement in the lateral direction (Figure 4c). The CoM movement in the lateral direction is associated with walking stability [15; 25]. In addition, a large walking variability is considered indicative of poor walking stability [26]. The lateral movement of walking was actively controlled using the feedback control [27]. We speculated that vibrotactile stimulation of the nail of the hallmark could enhance the tactile sensation perceived by the foot. In addition, the stimulation was likely to put the attention of the participant on the foot by suprathreshold stimulation. Attention has an impact on the control of non-automated motion [28]. Therefore, by using this stimulation method, the CoM movement in the lateral direction during walking was stabilized for subjects with unsteady movement, in the same manner as in previous studies that achieved stabilization while walking on the spot [11; 12; 13]. In previous studies [6; 29; 30], older adults were found to have greater walking variability during walking compared to young people. Thus, vibrotactile stimulation of the nail of the hallmark might be useful in reducing the variability of the CoM movement in the elderly. In contrast, stimulation increased he variabilities of the walking speed and stance time in the steady group, as shown in Figure 4a,b, respectively. The sensory feedback from the foot contributes to the cyclical gait patterns produced by neural rhythm generators in the central nervous system [31]. We speculated that the feedback system of the subject in the steady group was already working when walking without stimulation. However, by applying the stimulation to such a subject using vibration, the cyclical gait patterns may be disrupted. Therefore, the variabilities of these variables, including the temporal aspect of gait, increased in the steady group as a result of stimulation. The walking speed decreased with the CoM movement in the progression direction because of the stimulation (Table 1). However, the values of these variables decreased by only a few percent and were smaller than the MDC values in this study. The MDC is the minimal amount of change that is not likely to be considered a change due to a measurement error [22]. Thus, these differences were caused by a measurement error and were not sufficient to show the effect of stimulation. Consequently, vibrotactile stimulation did not affect the amount of the CoM movement during walking in healthy young people. These results suggest that the vibrotactile stimulation of the nail of the hallmark is more likely to affect the variability of the CoM movement than with the amount of the CoM movement. The present study also has some limitations. First, a relatively small sample size of young people was considered, based on the before-after design.