## 1 Introduction  
Stair climbing is a motion that combines a forward movement and an ascent performed simultaneously; it requires a stable body support and joint stiffness. Co-activation describes the simultaneous activation or co-contraction in the agonist and antagonist muscles around the joint [1; 2; 3]. The role of muscle co-activation is known to be that of increasing joint stiffness of the upper or lower limbs, thereby increasing joint stability [3; 4]. For example, [PERSON] et al. [5] have confirmed that muscle co-activation increases when standing on an unstable board. In addition, [PERSON] et al. [6] have reported that subjects use co-contraction control to offset unstable forces in a static state [6]. [PERSON] et al. [7] have found that increased co-activation during strong contractions can protect joints and maintain stability. Other studies have reported that co-activation helps in the functional aspects of joints [1; 8]. According to [PERSON] et al. [8], ankle co-activation during level walking can contribute positively to physiological tasks such as foot reversal, balance improvement,ankle stability control, and knee flexion. In addition, high co-activation can strengthen joints without time delay during sequential movements, helping in counteracting external forces and performing sequential movements [1]. Many previous studies have shown that co-activation contributes to the stability and functionality of movement. Therefore, understanding and controlling co-activation can help improve or assist movement. Localized vibratory stimulation can be used as a method to affect the stability and functionality of movement by controlling co-activation. Vibrations applied to the limbs affect the sensorimotor cortex through the afferent sensory nerves, resulting in a decrease in mu rhythm [9]. The mu rhythm is an electroencephalogram (EEG) signal of a frequency of 8-13 Hz, generated by the sensorimotor cortex, and is attenuated by vibration, tactile stimulation, and exercise execution [10; 11]. Because of this characteristic, it can be used as an index of excitability of the sensorimotor cortex [9; 12]. [PERSON] et al. [13] have used the mu rhythm to evaluate the effects of vibratory stimulation-induced kinesthetic illusions. [PERSON] et al. [9] reported that a local vibration applied during the isometric contraction of limb muscles can improve the excitability of the sensorimotor cortex. The Ia afferent fiber-induced by vibration activates the \(\alpha\)-motoneurons, resulting in an increase in the electromyogram (EMG), which is indicative of an increase in the muscle contraction force [14; 15]. Another previous study has shown that there is an inverse linear correlation between the mu rhythm of the sensorimotor cortex and the force used during isometric contraction [16], and it was reported that the motor-evoked potential (MEP) was changed by local vibrations [17]. Others have shown improved muscle performance through vibratory stimulation applied to the limbs [14; 18; 19]. [PERSON] et al. [19] have reported an increase in joint flexibility due to repeated vibration applied to an Achilles tendon. [PERSON] et al. [18] have applied repeated vibrations of 8 Hz and 26 Hz to two groups, respectively, and they confirmed that the performance of the lower limb muscles was subsequently improved. Finally, [PERSON] et al. [14] have confirmed an increase in the EMG amplitude due to the vibration applied to the elbow joint. Through these studies, it can be inferred that local vibrations can be used to control the stability and functionality of movements. However, the vibration frequencies and intensities used in previous studies were varied, which made it difficult to objectively compare the effects of vibrational characteristics with each other. Therefore, applying vibrations according to a specific standard is important. The purpose of this study was to investigate whether the vibratory stimulation selected according to a specific standard affects the stability and functionality of stair climbing.