What action does the biceps femoris long head perform?

What action does the biceps femoris long head perform?

• A. Flexes leg
• B. Extends thigh at hip
• C. Medially rotates tibia
• D. Abducts thigh

Answer: (B)

The biceps femoris long head crosses both the hip and the knee, so it can act at two joints. Its origin on the ischial tuberosity and its insertion on the fibular head let it extend the thigh at the hip. This hip-extending action is most evident when the knee is flexed or you rise from a seated position, making extending the thigh at the hip its defining role. It also helps flex the knee, but that action is shared with other hamstrings and isn’t the distinguishing hip-joint action. It does not medially rotate the tibia (it tends to laterally rotate the leg when the knee is flexed) and it does not abduct the thigh. So extending the thigh at the hip best captures what this muscle does.

The biceps femoris long head crosses both the hip and the knee, so it can act at two joints. Its origin on the ischial tuberosity and its insertion on the fibular head let it extend the thigh at the hip. This hip-extending action is most evident when the knee is flexed or you rise from a seated position, making extending the thigh at the hip its defining role. It also helps flex the knee, but that action is shared with other hamstrings and isn’t the distinguishing hip-joint action. It does not medially rotate the tibia (it tends to laterally rotate the leg when the knee is flexed) and it does not abduct the thigh. So extending the thigh at the hip best captures what this muscle does.