## 1 Introduction  
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, 4], 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 vibrotactilestimulation 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; 8; 9; 10]. 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, [PERSON] et al. [11; 12; 13] proposed a method to generate somatosensory inputs by stimulating the nail of the hallmark. 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; 12; 13], the vibrotactile stimulation of the nail of the hallmark 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 hallmark 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; 8; 9; 10]. 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 hallmark affects the CoM movement during walking.