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The purpose of this study was to investigate the effect of stair ascending speed on lower extremity muscle activations during stance phase of healthy young males. [METHOD] Ten college male students participated in the experiment. The speed of stair ascending was controlled at four different conditions (slow: 75steps/min, normal: 108steps/min, fast: 118 steps/min, preferred) by using a metronome. Pressure insole was placed in the shoe and EMG electrodes were attached to eight muscles (retus femoris, vastus lateralis, vastus medialis, tibialis anterior, lateral gastrocnemius, medial gastrocnemius, semitendinosus, and gluteus maximus) of the right leg. Subject was asked to climb fifteen consecutive stairs for each speed. Six steps of the right leg were selected for the analysis of stance and swing time, vertical force, peak pressure and area as well as peak EMG and IEMG(the area of EMG signal or amount of muscle activations). The order of speed was randomized to eliminate their learning effect. [RESULT] Stance and swing time were decreased as the speed was increased. Vertical force and peak pressure was increased with an increased speed but impulse was decreased with an increased speed. Peak EMG was increased in flexor and extensor of the knee and plantar flexor of the ankle as the ascending speed was increased. However, there was a tendency to show decreased peak EMG in gluteus maximus and tibialis anterior. On the other hands, an IIEMG was decreased in knee extensors (retus femoris, vastus lateralis, and vastus medialis) while there was a tendency to show increased IEMG of ankle dorsi-flexor with an increased speed of stair ascending. Especially, The level of peak EMG during the preferred stair ascending was placed between slow and medium stair ascending speed but the integrated EMG of the preferred stair ascending was the lowest among the four conditions. [CONCLUSION] Peak EMG activity of majority lower extremity muscles except gluteus maximus was increased while IEMG was decreased with an increased ascending speed. Furthermore, a preferred stair walking speed seems to be more efficient during stair ascending in terms of muscle recruitment compared to the other three conditions. Based on the results, the understanding of lower extremity muscle recruitment and pressure patterns by stair ascending speed would be valuable for future development of effective exercise intervention using stairs.