The muscles that affect the knee’s movement run along the thigh and calf. They are attached to the femur (thighbone), tibia (shinbone), and fibula (calf bone) by fibrous tissues called ligaments. Tendons attach the muscles to each other.

There are many important tendons and ligaments in the knee as it is the largest joint in the body that is under weight-bearing strain and regular use.

The functions of the large exterior muscles that affect the knee are easier to understand when viewed in their collective groups.

The hamstring muscles are three muscles at the back of the thigh that affect hip and knee movement. They begin under the gluteus maximus behind the hip bone and attach to the tibia at the knee. They are:

  • Biceps femoris: This long muscle flexes the knee. It begins in the thigh area and extends to the head of the fibula in the knee.
  • Semimembranosus: This long muscle extends from the pelvis to the tibia. It extends the thigh, flexes the knee, and helps rotate the tibia.
  • Semitendinosus: This muscle also extends the thigh and flexes the knee, but the tendons connecting it to the bone are much narrower than those of the semimembranosus.

The quadriceps comprise a four-muscle group at the front of the thigh that performs the majority of the work to extend the knee. These muscles are the strongest and leanest in the entire body. They are:

  • Rectus femoris: This muscle attaches to the kneecap. It has the least effect on flexing the knee.
  • Vastus medialis: This teardrop-shaped muscle of the inner thigh attaches along the femur and down to the inner border of the kneecap. It aids in knee extension.
  • Vastus intermedius: Between the vastus medialis and the vastus lateralis at the front of the femur, it is the deepest of the four quadriceps muscles.
  • Vastus lateralis: On the outside of the thigh, this is the largest of the four quadriceps muscles. It extends from the top of the femur at the hip and to the kneecap.

A large muscle in the calf, the gastrocnemius, is also responsible for flexing the knee.