During human walking, plantar tactile sensation provides somatosensory inputs to the central nervous system as feedback from the plantar surface, thereby making it easier to control balance and walking speed. A temporary reduction in these inputs reduces the walking speed [1
] and increases the variability of temporal parameters and lumbar acceleration [2
]. In addition, joint kinematics, muscle activity [3
], and pressure distribution patterns [5
] change during walking. Therefore, older adults suffering from sensory deficits caused by diabetic neuropathy have a lower walking speed and greater gait variability than healthy older adults [6
Vibrotactile stimulation can augment somatosensory feedback during walking. Previous studies found that vibrotactile stimulation could change the stride-to-stride variability of the step width and stride length in the elderly [7
], increase the walking speed, single-limb support time, and step length, and significantly decrease the cadence in stroke subjects [8
]. In particular, the effect of vibrotactile stimulation on the plantar surface is affected by the values obtained without stimulation at the baseline [9
]. Thus, this stimulation method provides a baseline-dependent reduction in gait variability for subjects with a more unsteady gait. Vibrotactile stimulation-based interventions could improve walking instability in subjects with sensory deficits.
Previous studies used vibrators embedded at the bottom of insoles or shoes to stimulate the plantar surface [7
]. Because these devices are integrated in shoes, users have to wear them to be stimulated. However, the plantar skin is thick; therefore, a relatively large stimulation strength is required to transmit vibrations. Furthermore, vibrators cannot be used when walking barefoot and the sensory input from the ground is blocked by the shoe sole and vibrator.
To overcome these problems, Sakai et al. [11
] proposed a method to generate somatosensory inputs by stimulating the nail of the hallux. Toes have a particularly high distribution density of mechanoreceptors in the plantar region [14
]. In addition, the transmission efficiency of vibration is high because a nail is harder than the skin. This method supports daily walking without blocking information from the plantar surface, unlike the technique in which a vibrator is embedded at the bottom of insoles or shoes. In previous studies [11
], the vibrotactile stimulation of the nail of the hallux increased the stability of the center-of-mass (CoM) movement while walking on a spot by enhancing the tactile sensation experienced by the foot in healthy young males. On the other hand, one never moves forward while walking on the spot, and it is rarely practiced in daily living. Therefore, it is necessary to verify the effect of vibrotactile stimulation of the nail of the hallux during walking, which is the most frequently performed activity in daily living. However, there are no studies on the effect of the vibrotactile stimulation method on the CoM movement during walking.
The CoM is a theoretical point through which the force of gravity acts on an object [15
], and it controls the imbalance and the risk of falls in the elderly [16
]. Previous studies investigated the effect of the vibrotactile stimuli on the gait performance, that is, walking speed, cadence, and step length [7
]. However, the effect of vibrotactile stimulation on the CoM movement is still unclear. If the effect of intervention as a result of using this novel method on the CoM movement during walking is clarified, it can be applied to people with unstable CoM movement, such as older adults and patients with hemiplegia.
This study investigated whether the vibrotactile stimulation of the nail of the hallux affects the CoM movement during walking. A previous study found that vibrotactile stimulation of the plantar surface had a baseline-dependent effect on gait variability and resulted in a greater decrease in variability in subjects with greater baseline variability [9
]. We hypothesized that the previous findings of walking on the spot [11
] are also applicable to walking; in other words, among healthy young people, vibrotactile stimulation of the nail of the hallux is more effective in subjects with a large variability of the CoM movement during walking.
This study examined the effect of vibrotactile stimulation of the nail of the hallux 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 hallux 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 hallux 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 4
c). The CoM movement in the lateral direction is associated with walking stability [15
]. 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 hallux 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
]. In previous studies [6
], older adults were found to have greater walking variability during walking compared to young people. Thus, vibrotactile stimulation of the nail of the hallux 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 4
a,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 hallux 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. Therefore, to clarify the effect of this stimulation method on walking stability in the elderly, further studies should to be conducted using a randomized controlled trial with a larger sample size, including older adults. Second, a previous study reported that there are gender differences in biomechanical gait parameters [32
]. Therefore, only male participants were considered in this study to eliminate the effects of gender differences. To elucidate the gender-specific effect, further studies are necessary to examine the effect of this stimulation method on walking in female subjects during walking. Third, the stimulation amplitude was set to a suprathreshold level and was adjusted to be as weak as possible by tuning it for each subject. The differences in the magnitude and frequency of vibration, including supra- and subthreshold values, affected the stride-to-stride fluctuations in the stride length and interval [10
]. In addition, no consensus was reached on the effect of the differences between vibrating devices or the duration and magnitude of the stimulation on gait parameters [33
]. In particular, older adults have an age-related decline in the sensorimotor and/or cognitive function [6
]. In a future study, we intend to investigate the optimal setting of the vibration applied to the nail of the hallux considering the age of the subjects. Furthermore, in this study, the effectiveness of the proposed method could not be compared with that of the conventional method that stimulated the plantar surface because the subjects may feel uncomfortable while attaching the vibrator to the plantar skin and walking barefoot. In the future, we will consider fabricating shoes for the experiments to avoid discomfort and to compare the effects of both the proposed and the conventional methods. Finally, the lateral control of walking was achieved using hip abductor muscles [34
]. Analysis of the kinetics and muscle activity of this muscle may be useful to verify the effect of the vibration stimulation on the lateral displacement of the CoM.