Which muscle is the primary contributor to lateral rotation of a flexed knee when non-weight bearing?

Which muscle is the primary contributor to lateral rotation of a flexed knee when non-weight bearing?

• A. Sartorius
• B. Popliteus
• C. Biceps Femoris
• D. Gastrocnemius

Answer: (C)

External rotation of a flexed knee in an open-chain (non-weight bearing) movement is driven mainly by the biceps femoris. As the knee bends, the fibers of this muscle pull on the fibular head and the lateral side of the tibia, producing lateral (external) rotation of the tibia relative to the femur. The popliteus, in contrast, primarily medially rotates the tibia to unlock the knee from extension, so it isn’t the primary contributor to lateral rotation. The sartorius can assist with outward rotation when the hip is flexed, but it isn’t the main knee rotator. Gastrocnemius mainly plantarflexes the ankle and aids knee flexion without being a primary knee rotator. Thus, the biceps femoris is the key muscle for lateral rotation of a flexed knee in non-weight bearing.

External rotation of a flexed knee in an open-chain (non-weight bearing) movement is driven mainly by the biceps femoris. As the knee bends, the fibers of this muscle pull on the fibular head and the lateral side of the tibia, producing lateral (external) rotation of the tibia relative to the femur. The popliteus, in contrast, primarily medially rotates the tibia to unlock the knee from extension, so it isn’t the primary contributor to lateral rotation. The sartorius can assist with outward rotation when the hip is flexed, but it isn’t the main knee rotator. Gastrocnemius mainly plantarflexes the ankle and aids knee flexion without being a primary knee rotator. Thus, the biceps femoris is the key muscle for lateral rotation of a flexed knee in non-weight bearing.