## 4 Discussion
### Changes in the Co-Activation by Localized Vibratory Stimuli during Stair Climbing  
To investigate the effect of the localized vibratory stimulation on the stability and function of stair climbing, co-activation was analyzed across stair-climbing phases, muscle groups and stimulation intensities. At the threshold intensity condition, a reduced co-activation appeared predominantly in the phases WA, PU and FCN. During the WA phase, the body weight is transferred to the leading leg, and the joint flexion of the hip and knee begins to decrease [33; 35]; then, the joint extension of the hip and knee develops in the PU phase, and subsequently the transition of the ankle joint from dorsiflexion to plantarflexion occurs in the FCN phase. An increase of co-activation translates into improved joint protection and stability through an increase in joint stiffness [4]. However, excessive co-activation has been reported to reduce the degree of joint freedom resulting from stiff posture [36]. In other words, a decrease of co-activation means a decrease in joint stiffness, resulting in an increase in the degree of joint freedom. All three joints are extending during the phases WA, PU and FCN. Thus, a reduced co-activation means that an extension of the lower limb segments will be achieved harmoniously and that the local vibratory stimulation contributes positively to joint extension. Even if a reduced co-activation decreases the stability of stair climbing, the stability of stair ascending will not be harmed because the extension of the femur muscles in the leading leg and the plantarflexion of the calf muscles in the trailing leg perform the body lift. At the sub-threshold intensity conditions, an increased co-activation appeared predominantly in the phases of WA and PU, except for the PU phase of the TA-SOL group. During the PU phase, the upright alignment of the lower limb segments is achieved while decreasing the dorsiflexion. The localized vibratory stimulation in the initial PU phase facilitates the shank upright alignment by reducing TA activity, and in the later PU phase, it facilitates a rapid increase in activity of the gastrocnemius muscle (Figure 3), which contributes to providing joint stiffness for the vertical movement of the trunk. In contrast, the co-activation of the TA-SOL group decreased. This is presumed to occur due to the difference in the type of contraction in the muscle during movement. [PERSON] et al. [37] have reported that the GM fascicle exhibits a near-isometric behavior when the GM muscles are activated during stair climbing. [PERSON] et al. [38] have shown that the GM fascicle was stretched during early single-limb support during level walking and remained isometrical in a later stance phase. By contrast, [PERSON]'s fascicle reportedly continued to lengthen until the end of the single-limb support. [PERSON] et al. [39] have reported that the GL's tendinous tissue, the muscle-tendon complex, and fascicle were passively shortened by the TA movement immediately after initial contact and extended to a later stance. Thus, it can be expected that there will be a difference in the contraction method or state of the muscles according to the movement performed by a person and that the change in co-activation due to vibratory stimulation may be different for each muscle group. In the swing phase (i.e., the FC phase), co-activation was decreased by the sub-max. This decrease in co-activation can be interpreted as an increase in the degree of joint freedom, along with a decrease in the dorsiflexion of the ankle. The common area of co-activation refers to the area where two muscles, which are in an agonist-antagonist relation, are activated simultaneously. During the FC phase, TA takes on the role of the agonist muscle, peaks are observed, and the EMG increases under vibratory conditions. The GL and GM act as antagonists during this phase, and do not show changes in the EMG due to vibration.