Climbing stairs is the one of the activities of daily living (ADL) and is more biomechanically demanding than level walking. Recently, local muscle vibration (LMV) has been applied to movement and rehabilitation, and it was revealed that LMV significantly affects muscle activation. However, there have been no investigations of mechanical muscle behavior in real time during the application of LMV. The purpose of this study was to investigate the effects of LMV on muscle behavior and postural stability during stair ascent by the elderly using ultrasound. Fifteen young adults and ten elderly participants were asked to walk and climb stairs. LMV was applied during stair climbing. Center of pressure (COP), electromyography (EMG), and ultrasound data were recorded for each condition. When LMV was applied, a greater root mean square (RMS)-EMG was observed during forward continuance (FCN) along with a shorter fascicle length and larger pennation angle (PA). The COP of elderly participants was greater than that of young adults. When LMV was applied, the COP decreased. These results suggest that LMV affects muscle behavior, leading to more activation during locomotion. In particular, in elderly people with reduced ADL due to aging, LMV may contribute to muscle activation and increase muscle efficiency, improving ADL ability.