Purpose] To clarify the effect of changes in flexion torque and angle in knee flexion exercises on the muscle activity of the quadriceps femoris, an antagonist of flexion. [Subjects and Methods] The subjects were 12 healthy adults. An isometric knee flexion exercise was performed with the knee flexed at 30 and 90˚. The muscle activities of the rectus femoris (RF), vastus medialis (VM), and vastus lateralis (VL) were measured using electromyography. [Results] The muscle activity of RF at knee

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... xion of 30˚ was significantly lower than that of 90˚. In contrast, the muscle activity of VL was significantly greater at knee flexion of 30˚ than that of 90˚. [Conclusion] The results suggest that in open kinetic chain knee flexion exercises, the posterior cruciate ligament experiences stress at knee flexion of 90˚, suppressing the muscle activity of RF.