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The primary action of the quadriceps at the knee joint is extension. The quadriceps muscle group, located at the front of the thigh, consists of four muscles: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. When the quadriceps contracts, it pulls on the patellar tendon, which in turn pulls on the tibia, leading to straightening the knee joint.

This mechanism is essential for various functional movements such as standing up, walking, running, and jumping, all of which require the knee to extend. Flexion, on the other hand, involves bending the knee, which is primarily facilitated by muscles such as the hamstrings. Hyperextension is a movement that occurs beyond the normal range of extension, which is not the primary action of the quadriceps and can often lead to injury if excessive. Rotation at the knee is another action, but it is not the primary function of the quadriceps, which primarily serves to extend the knee. Therefore, recognizing that the main action of the quadriceps is knee extension highlights its crucial role in lower body mobility and stability.