In an open-chain knee exercise, at what knee flexion angle is the knee weakest?

In an open-chain knee exercise, at what knee flexion angle is the knee weakest?

• A. 0 Degrees of Flexion
• B. 90 Degrees of Flexion
• C. Full Extension
• D. 45 Degrees of Flexion

Answer: (D)

The key idea is that knee extension strength in an open-chain movement varies with knee angle because both the quadriceps length (length-tension relationship) and the lever arm (moment arm of the patellar tendon) change with flexion. At mid-range, around 45 degrees of flexion, the quadriceps are not at their most favorable length for producing force, and the patellar tendon moment arm is not yet at its maximum. This combination yields the smallest external knee extension torque, making the knee weakest at this angle during open-chain exercises. As flexion increases toward about 60–70 degrees, the moment arm and the muscle length align more favorably for force production, increasing strength; beyond that range, the moment arm decreases again and the muscle length can become less optimal, reducing torque. So, around 45 degrees of knee flexion is the period with the lowest available knee extension torque in open-chain movements.

The key idea is that knee extension strength in an open-chain movement varies with knee angle because both the quadriceps length (length-tension relationship) and the lever arm (moment arm of the patellar tendon) change with flexion.

At mid-range, around 45 degrees of flexion, the quadriceps are not at their most favorable length for producing force, and the patellar tendon moment arm is not yet at its maximum. This combination yields the smallest external knee extension torque, making the knee weakest at this angle during open-chain exercises. As flexion increases toward about 60–70 degrees, the moment arm and the muscle length align more favorably for force production, increasing strength; beyond that range, the moment arm decreases again and the muscle length can become less optimal, reducing torque.

So, around 45 degrees of knee flexion is the period with the lowest available knee extension torque in open-chain movements.