Two hundred and thirty-seven patients who had paresis or paralysis of the quadriceps muscles underwent a transfer of the biceps femoris by Caldwell's method or medial hamstring to the patella. Seventy-six legs of seventy-two patients who had followed-up more than one year were reviewed and analysed. The results showed there was an overall improvement measured by decreased extension lag in all age groups and relatively uniform good results in young age group. Again we reviewed the forty-six patients chronologically to eliminate the outer factors who were transfered normal biceps femoris or medial hamstring power to absent quadriceps. By that we found the extension power of the knee joint after muscle transfer had been decreased by age.