College of Massage Therapists of British Columbia (CMTBC) Practice Exam

Which muscle is primarily responsible for knee flexion?

1. rectus femoris

2. hamstrings

3. quadriceps

4. gastrocnemius

The correct answer is: hamstrings

The muscle primarily responsible for knee flexion is the hamstrings. The hamstrings group, consisting of three main muscles—the biceps femoris, semitendinosus, and semimembranosus—plays a crucial role in bending the knee. These muscles are located at the back of the thigh and work to pull the lower leg towards the buttocks during activities such as walking, running, and cycling. While the rectus femoris, part of the quadriceps group, also crosses the knee joint, its primary function is knee extension and it works in opposition to the hamstrings. The quadriceps, as a whole, are the major knee extensor muscles. The gastrocnemius, although it assists in knee flexion due to its origin above the knee, primarily functions to perform plantarflexion at the ankle joint when the knee is bent. Understanding the roles of these muscles helps clarify why the hamstrings are the key player in knee flexion. They are most effective at flexing the knee because of their anatomical positioning and their muscle fibers, designed for this specific movement.