This study evaluated the effects of saddle height on the muscle activity and oxygen uptake during bicycling. The subjects were 20 young adult volunteers. Muscle activity and oxygen uptake were measured with the two saddle heights (maximum knee extension of 180˚ and 120˚) and at two power outputs (70 and 100 watts, respectively.) The pedaling rate was 40 rpm. The exercise time was 1 minute and the resting time between each condition was 3 minutes. The raw electromyogram activity was measured for 1 minute and was converted to a root mean square value. Oxygen uptake was measured during exercise using the mixing chamber mode. The activities of two flexors (the medial hamstring and medial head of gastrocnemius) increased at the high saddle height and the activities of four extensors (rectus femoris, vastus medialis, vastus lateralis, and tibialis anterior) increased at the low saddle height. The oxygen uptake at the low saddle height was significantly higher than that at the high saddle height. The oxygen uptake positively correlated with the muscle activities of the knee extensors. The muscle activity and oxygen uptake were significantly affected by the postures (saddle heights) in cycle ergometer. The postures should be considered in the exercise test and prescription.