What action does the semimembranosus perform?

What action does the semimembranosus perform?

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

Answer: (C)

The semimembranosus is a hamstring muscle crossing both the hip and knee joints, so it can act at either joint. Its main action at the knee is to flex the leg. When the knee flexes, it also helps medially rotate the tibia, providing coordinated rotation during knee bending. At the hip, it assists in extending the thigh, but the most directly tested action for this muscle is knee flexion. Abducting the thigh isn’t a function of this muscle, and while extending the thigh is possible, the primary, most characteristic action emphasized in many tests is flexing the leg.

The semimembranosus is a hamstring muscle crossing both the hip and knee joints, so it can act at either joint. Its main action at the knee is to flex the leg. When the knee flexes, it also helps medially rotate the tibia, providing coordinated rotation during knee bending. At the hip, it assists in extending the thigh, but the most directly tested action for this muscle is knee flexion. Abducting the thigh isn’t a function of this muscle, and while extending the thigh is possible, the primary, most characteristic action emphasized in many tests is flexing the leg.