The knee joint is a complex structure, often misunderstood regarding its muscular components. While no muscles are situated inside the knee joint capsule, the knee functions as a hinge joint, enabling primary movements like bending (flexion) and straightening (extension) of the leg. Its stability and wide range of motion are supported by surrounding muscles and other anatomical elements. These elements work together to facilitate movement, bear the body’s weight, and allow the knee to perform its role in standing, walking, running, and jumping.
Structures Within the Knee Joint
The knee joint’s internal architecture consists of several distinct components, none of which are muscles. Three bones converge to form this joint: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). The fibula, though running alongside the tibia, does not directly articulate with the knee joint. Articular cartilage, a smooth tissue, covers these bones, reducing friction and allowing effortless movement.
Within the joint, two C-shaped pads of fibrocartilage, known as menisci, sit between the femur and tibia. These menisci act as shock absorbers, distributing weight and increasing joint stability. The knee’s stability is reinforced by ligaments, including the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL), which prevent excessive movement and maintain proper alignment. The joint is encased in a capsule, lined by a synovial membrane that produces synovial fluid. This viscous liquid lubricates the joint and nourishes the cartilage, ensuring smooth, low-friction movement.
Muscles Acting on the Knee
Although muscles are not found inside the knee joint, several muscle groups surrounding it are important for its function, movement, and stability. The quadriceps femoris, located on the front of the thigh, is a primary group for knee extension, the action of straightening the leg. This group includes four muscles—the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis—all of which converge to insert near the patella. These muscles are important for daily activities such as standing up from a chair, climbing stairs, and various athletic movements.
Conversely, the hamstrings, situated on the back of the thigh, are the main muscles responsible for knee flexion, or bending the leg. This group comprises the biceps femoris, semitendinosus, and semimembranosus. The hamstrings also contribute to hip extension and stabilize the knee by resisting forward movement of the tibia. The gastrocnemius, a major calf muscle, also assists in knee flexion, particularly when the knee is extended.
Beyond these primary movers, other muscles, such as the gracilis and sartorius, contribute to knee flexion and stability, particularly on the medial side of the knee. The popliteus muscle, located behind the knee, plays a role in “unlocking” the knee to initiate flexion from a fully straightened position. The coordinated action of these muscle groups allows the knee to perform a wide range of movements while maintaining balance and support.