The knee connects the thigh bone (femur) to the shin bone (tibia). While often described as a hinge joint, its complex mechanics allow for movement in multiple planes. Muscles provide stability, manage rotation, control alignment, and absorb forces necessary for activities like walking and running. Maintaining the strength and balance of these muscles is directly related to the health and function of the knee joint.
The Primary Extensors of the Knee
The primary action of straightening the leg at the knee joint is performed by the Quadriceps Femoris group, a large muscle complex located on the front of the thigh. This group consists of four muscles. All four converge into a single tendon that encloses the kneecap (patella) before inserting onto the tibia.
The Rectus Femoris is unique among the group because it originates high on the pelvis, meaning it crosses both the hip and knee joints. This bi-articular nature allows it to assist with hip flexion in addition to its main role in knee extension. The other three muscles, the Vastus Lateralis, Vastus Medialis, and Vastus Intermedius, all originate directly from the femur and are focused solely on extending the knee.
The Vastus Lateralis is the largest muscle of the group and sits on the outer side of the thigh, while the Vastus Medialis is located on the inner side. The Vastus Intermedius lies beneath the Rectus Femoris, nestled between the other two Vastus muscles. The Vastus Medialis, particularly its lower fibers, plays an important role in tracking the patella, ensuring it glides correctly during movement.
The Primary Flexors of the Knee
The opposite action of bending the knee is primarily handled by the Hamstring muscle group, located in the posterior compartment of the thigh. This group is composed of three muscles: the Biceps Femoris, the Semitendinosus, and the Semimembranosus. All three muscles originate from the ischial tuberosity, a bony prominence on the pelvis, which means they cross both the hip and knee joints.
Because they cross the hip joint, the hamstrings also serve the secondary function of extending the hip. Their primary action on the knee is to pull the lower leg backward, initiating flexion. The Biceps Femoris is the most lateral of the three, inserting onto the head of the fibula and the lateral side of the tibia.
The Biceps Femoris is further divided into a long head and a short head, with only the long head originating from the pelvis and crossing the hip. The Semitendinosus and Semimembranosus are the medial hamstrings, inserting on the inner side of the tibia. These two muscles also contribute to the internal rotation of the lower leg when the knee is bent, working in opposition to the external rotation provided by the Biceps Femoris.
Deep and Stabilizing Structures
Beyond the large muscle groups responsible for main movements, several smaller muscles and connective tissues provide the fine control and stability the knee requires. The Popliteus muscle is a small, flat muscle located deep behind the knee joint. Its unique function is to “unlock” the knee from a fully extended position by initiating the internal rotation of the tibia.
When the knee is fully straightened, a slight rotation occurs, effectively locking the joint in place, a mechanism known as the screw-home mechanism. The Popliteus reverses this rotation, allowing the knee to bend and preventing the crushing of the lateral meniscus between the femur and tibia as flexion begins.
The Gastrocnemius, the large muscle that forms the bulk of the calf, also assists in knee movement because it crosses both the knee and ankle joints. While its main role is pointing the foot downward (plantarflexion), it acts as a secondary knee flexor, particularly when the ankle is pulled upward.
Lateral stability is supported by the Iliotibial Band (IT Band), a thick, fibrous sheath of connective tissue that runs from the hip down the outside of the thigh to the knee. This band is not a muscle itself, but it is a continuation of connective tissue from the hip muscles, including the Tensor Fascia Latae and the Gluteus Maximus. These muscles pull on the IT Band, allowing it to act like a dynamic cable that helps maintain proper alignment during activities like running and walking.