It is suggested that the hamstrings function to prevent the anterior tibial displacement that is produced by contraction of the quadriceps in the terminal phase of extension of the knee in various studies. The objective of this study is to quantify the coactivation patterns of the knee flexor and extensor muscles in mainataining knee joint stability. Ten healthy adult men were selected in our study. Extension of the knee from 90 to 0 degrees was performed with the leg unimpeded and with weights of 7.0kg attached to the ankle. We used a machine with biofeedback mechanism, which converted muscle power into microvoltage during extension of the knee and measured muslce powers of the hamstrings and the quadriceps and EMG of 2 men were recorded. The results were as follows;1. During terminal phase of extension of the knee (from 10 to 0 degrees), the hamstrings functioned as antagonist of the quadriceps and the powers of the hamstrings were about 1/9-1/8 of those of quadriceps. 2. It is suggested that the strengthening of the hamstrings muscle power would be important after repair and reconstruction of the anterior cruciate ligament.