Which muscle extends the thigh during leg curls?

The semimembranosus is a muscle located in the posterior compartment of the thigh and is part of the hamstring group. It plays a crucial role in various movements of the leg. During leg curls, the primary action involved is knee flexion, but the semimembranosus assists in extending the thigh at the hip joint when the knees bend. This muscle not only flexes the knee but also helps in stabilizing the pelvis, making it essential during movements where the thigh needs to be extended.

The other muscles listed do not primarily contribute to thigh extension during leg curls. The adductor brevis functions mainly in thigh adduction and does not have a significant role in knee flexion. The sartorius is involved more in hip flexion and external rotation and does not extend the thigh. The pectineus primarily assists in hip flexion and adduction, rather than extension. Thus, the semimembranosus is the correct choice as it directly contributes to the thigh extension while engaging in leg curls.