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The rectus femoris is one of the four muscles that make up the quadriceps group. Its primary functions are to flex the hip and extend the knee, making this an important muscle for a number of activities that require movement of the lower body. The muscle is located in the center of the thigh and runs from the bottom of the spine down to the kneecap. Only the sartorius muscle is also included in the quadriceps group and is involved in both hip and knee movement.
The rectus femoris has two origins. The first is the anterior origin and occurs at the bottom of the spine. There is also the posterior origin at the acetabulum. Both of these start with a tendon and come together before running down to the thigh to where the muscle fibers begin. The insertion of the muscle is via a tendon known as the quadriceps tendon, which connects into the patella.
Extending the knee is one of the main jobs of the rectus femoris muscle. This muscle, however, becomes weaker in relation to hip flexion when the knee is extended because it becomes shortened, meaning it cannot work as well. For this reason when the rectus femoris is shortened other muscles are more involved in hip flexion.
In a similar way the muscle is also weaker in knee extension when there is already hip flexion. Again, this is because the muscle is placed in a shortened state and cannot provide the same level of force. In this situation, knee extension is achieved through other muscles in the quadriceps groups.
The three other main muscles in the quadriceps muscle group are the vastus lateralis, vastus intermedius and vastus medialis. All of these muscles assist in keeping the patella stable while extending the knee. For this reason they are all essential to basic activities such as walking or running. The rectus femoris is the most central of the muscles and is named due the way it reaches straight down the front of the thigh.
If this muscle becomes ruptured, it becomes a serious and painful injury. Often a rectus femoris tendon rupture occurs through repeated overuse of the muscle during actions such as kicking or sprinting. Symptoms include a sharp pain in the hip or groin along with bruising and sometimes swelling. The area will also usually be painful to touch.
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