The properties of isometric force developments during leg extensions performed unilaterally and bilaterally were investigated in the untrained and the trained group. The bilateral leg extension force (BL) was significantly lower than the summed unilateral leg extension force (UL), and BL/UL ratio was 88.4% in the untrained group. On the other hand, BL was significantly greater than UL and BL/UL ratio was 104.3% in the trained group. The left leg extension force, BL and BL/UL ratio in the trained group were significantly greater than those in the untrained group. There was a significant difference in time to 50% MVC between the unilateral and the bilateral extension in the trained group. The correlation between the force and the rate of force development in the untrained group was not significant, but in the trained group it was significant in every leg extension. The EMG power spectrum shifted to the high frequency band in the bilateral leg extension in every trained subject , and the peak power frequency which has the maximum power in the power spectrum was significantly greater in the bilateral extension than that in the unilateral extension. As the above results show, it was suggested that the properties of force development in the unilateral and the bilateralleg extension were changed by training.