Which muscle causes knee flexion along with hip flexion and lateral rotation?

Which muscle causes knee flexion along with hip flexion and lateral rotation?

• A. Sartorius
• B. Biceps Femoris
• C. Semitendinosus
• D. Semimembranosus

Answer: (A)

When a movement requires flexing both the knee and the hip while also rotating the thigh laterally, you need a muscle that crosses both joints and has those actions. The sartorius fits perfectly: it runs from the hip to the knee and, at the hip, flexes the thigh and, when the hip is flexed, laterally rotates the thigh. At the knee, it assists knee flexion. So it can produce knee bending together with hip flexion and hip lateral rotation. The other muscles mainly either extend the hip or flex the knee without providing that combination of hip flexion with hip lateral rotation, so they don’t match all three actions.

When a movement requires flexing both the knee and the hip while also rotating the thigh laterally, you need a muscle that crosses both joints and has those actions. The sartorius fits perfectly: it runs from the hip to the knee and, at the hip, flexes the thigh and, when the hip is flexed, laterally rotates the thigh. At the knee, it assists knee flexion. So it can produce knee bending together with hip flexion and hip lateral rotation. The other muscles mainly either extend the hip or flex the knee without providing that combination of hip flexion with hip lateral rotation, so they don’t match all three actions.