The human knee is a complex joint, often misunderstood regarding its internal composition. It is important to clarify that muscles are not located inside the knee joint itself. Instead, the knee functions as a hinge joint, facilitating movements essential for daily activities like walking, running, and jumping.
Anatomy of the Knee Joint
The knee joint is formed by the articulation of three main bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). While the fibula, a smaller bone in the lower leg, runs alongside the tibia, it does not directly participate in the knee joint’s articulation. These bones are covered by articular cartilage, a smooth, slippery tissue allowing them to glide against each other with minimal friction.
Supporting the knee are various connective tissues that provide stability and enable smooth function. Ligaments are tough, fibrous bands of tissue that connect bones to other bones. Within the knee, the cruciate ligaments (anterior cruciate ligament or ACL, and posterior cruciate ligament or PCL) are located inside the joint, crossing each other to control forward and backward movement of the tibia relative to the femur.
On the sides of the knee, the collateral ligaments (medial collateral ligament or MCL, and lateral collateral ligament or LCL) help prevent excessive side-to-side motion. Additionally, crescent-shaped pads of fibrocartilage called menisci sit between the femur and tibia. These structures act as shock absorbers, distributing weight across the joint and enhancing stability. The entire knee joint is enclosed by an articular capsule, which contains synovial fluid for lubrication, aiding in frictionless movement.
Muscles Responsible for Knee Movement
Powerful muscle groups surrounding the knee are responsible for its extensive range of motion. These muscles attach to the bones via strong, fibrous tissues called tendons. When a muscle contracts, it shortens, pulling on its attached tendon, which then exerts force on the bone to create movement at the joint.
The quadriceps femoris group, located on the front of the thigh, is a primary mover of the knee. This group consists of four muscles: the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. Their main action is to extend, or straighten, the knee joint. These muscles converge into the quadriceps tendon, which encases the patella and then continues as the patellar ligament to attach to the tibia.
The hamstring muscles, found on the back of the thigh, are responsible for bending, or flexing, the knee. This group includes the biceps femoris, semitendinosus, and semimembranosus. Beyond knee flexion, hamstrings also contribute to hip extension. The gastrocnemius, which forms a significant portion of the calf muscle, also assists in knee flexion. Although its primary role is to move the ankle, the gastrocnemius crosses the knee joint and contributes to its bending motion.