Which of the following muscles is involved in flexing the leg?

The biceps femoris is a primary muscle involved in flexing the leg at the knee joint. It is one of the hamstring muscles located in the posterior compartment of the thigh, and it plays a critical role in both knee flexion and hip extension. When the biceps femoris contracts, it pulls the lower leg towards the back of the thigh, thereby flexing the knee.

Understanding the function of the other muscles in the choices can clarify why they are not involved in this specific action. For instance, the soleus primarily functions in plantarflexion of the ankle rather than leg flexion. The adductor magnus and adductor longus are mainly involved in adduction, flexion, and rotation of the thigh, but do not contribute directly to leg flexion at the knee. Thus, the biceps femoris distinctly stands out as the muscle responsible for flexing the leg.