precentral gyrus, it contracts, extending the knee and flexing the thigh at the hip. Function The rectus femoris, sartorius, and iliopsoas are the flexors of the thigh at the hip. The rectus femoris is a weaker hip flexor when the knee is extended because it is already shortened and thus suffers from active insufficiency; the action will recruit more iliacus, psoas major, tensor fasciae latae, and the remaining hip flexors than it will the rectus femoris. Similarly, the rectus femoris is not dominant in knee extension when the hip is flexed since it is already shortened and thus suffers from active insufficiency. In essence: the action of extending the knee from a seated position is primarily driven by the vastus lateralis, vastus medialis, and vastus intermedius, and less by the rectus femoris. In the other extreme, the muscle's ability to flex the hip and extend the knee can be compromised in a position of full hip extension and knee flexion, due to passive insufficiency. The rectus femoris is a direct antagonist to the hamstrings, at the hip and at the knee. Clinical significance Strain Rectus femoris strain, referred to as hip flexor strain, is an injury commonly at the tendon that attaches to the patella or in the muscle itself. The injury is usually a partial tear, but could be a full tear. The injury is caused by a forceful movement related to sprinting, jumping, or kicking and is common in sports like football or soccer. The rectus femoris is prone to injury, since it crosses both the knee and the hip. Symptoms include a sudden sharp pain at the front of the hip or in the groin, swelling and bruising, and an inability to contract the rectus femoris with a full tear. Avulsion fracture The Rectus femoris tendon can cause a fragment of anterior inferior ilac spine of the hip (AIIS) to avulse in what is known as an Avulsion fracture. This is due to forceful contraction of the muscle that generates a force greater than that which holds the bone together. This is a well recognized, but unusual sports injury that can affect young athletes. References External links PTCentral Hip flexors Knee extensors Muscles of the quadriceps Anterior compartment of thigh Muscles of the lower limb