Seven high school boys (16.4 +/- 0.5 y, mean +/- SD) and 7 girls (16.4 +/- 0.5 y), who specialized in track and field events, performed ten 5-s maximal sprint runs with an interval of 10s between each sprint on a non-motorized running ergometer. In each sprint, the mean mechanical power (MP) from the start until the belt velocity of the ergometer (i. e., running velocity) peaked was calculated. The boys showed significantly higher MP than the girls in all sprints. However, when MP was expressed as the ratio to the total volume of muscles located in the right lower limb (MP x MV(-1)), estimated using a bioelectrical impedance analysis, significant gender effect was limited to the values at the 1 st and 2 nd sprints. The decline of MP over the ten sprints, expressed as a relative value to that at the 1 st sprint, was greater in boys (46.2 +/- 7.6 %) than in girls (33.9 +/- 8.6 %), and significantly correlated with MP x MV(-1) at the 1st sprint (r = 0.568, p < 0.05). However, no significant difference between the boys and girls was found in the relative difference between MP values at the 3rd and 10th sprints, where the gender difference in MP x MV(-1) at every sprint was insignificant. The findings here indicate that, for trained teenage boys and girls, (1) significant gender difference in mechanical power developed during repeated bouts of maximal running exists only in the initial phase of the task, when the difference in the volume of the lower limb muscles is normalized, and (2) it may be a reason for a greater decline of mechanical power developed during the bout in boys compared to girls.