Which muscle is active during lateral rotation of the flexed leg in non-weight-bearing conditions?

Which muscle is active during lateral rotation of the flexed leg in non-weight-bearing conditions?

• A. Rectus Femoris
• B. Tibialis Posterior
• C. Gastrocnemius
• D. Biceps Femoris

Answer: (D)

Lateral rotation of a flexed leg in a non-weight-bearing setting is produced mainly by the biceps femoris. The short head crosses only the knee and attaches to the fibular head, so when the knee is flexed, its contraction pulls the tibia laterally, rotating the leg outward. The long head also crosses the knee and hip, contributing to knee flexion, and can assist with rotation, but its effect on lateral rotation is not as direct as the short head. Rectus femoris mainly extends the knee and flexes the hip, so it doesn’t drive lateral rotation of a flexed knee. Tibialis posterior acts on the foot, not the knee. Gastrocnemius crosses the knee and ankle but is best described as a knee flexor and plantarflexor; it’s not a primary driver of knee rotation.

Lateral rotation of a flexed leg in a non-weight-bearing setting is produced mainly by the biceps femoris. The short head crosses only the knee and attaches to the fibular head, so when the knee is flexed, its contraction pulls the tibia laterally, rotating the leg outward. The long head also crosses the knee and hip, contributing to knee flexion, and can assist with rotation, but its effect on lateral rotation is not as direct as the short head.

Rectus femoris mainly extends the knee and flexes the hip, so it doesn’t drive lateral rotation of a flexed knee. Tibialis posterior acts on the foot, not the knee. Gastrocnemius crosses the knee and ankle but is best described as a knee flexor and plantarflexor; it’s not a primary driver of knee rotation.