Which muscle group is the primary extensor of the knee?

Which muscle group is the primary extensor of the knee?

• A. Quadriceps femoris
• B. Rectus abdominus
• C. Biceps brachii
• D. Gastrocnemius

Answer: (A)

The knee is extended mainly by the quadriceps femoris group, the large muscle group on the front of the thigh. When this group contracts, it pulls on the patellar tendon, which routes the force to the tibia and straightens the leg at the knee joint. This action is the primary way the knee moves from a bent to a straight position, such as when you stand up straight or kick a ball. The other options don’t produce knee extension. The abdominal rectus muscle flexes the trunk, not the knee. The biceps brachii acts on the arm, mainly bending the elbow and rotating the forearm. The gastrocnemius crosses the knee and the ankle, but its main roles are knee flexion (when the leg is not bearing weight) and plantarflexion of the foot, not extending the knee.

The knee is extended mainly by the quadriceps femoris group, the large muscle group on the front of the thigh. When this group contracts, it pulls on the patellar tendon, which routes the force to the tibia and straightens the leg at the knee joint. This action is the primary way the knee moves from a bent to a straight position, such as when you stand up straight or kick a ball.

The other options don’t produce knee extension. The abdominal rectus muscle flexes the trunk, not the knee. The biceps brachii acts on the arm, mainly bending the elbow and rotating the forearm. The gastrocnemius crosses the knee and the ankle, but its main roles are knee flexion (when the leg is not bearing weight) and plantarflexion of the foot, not extending the knee.