A tendon is a strong, flexible band of connective tissue that connects muscle to bone. The knee is the largest and most complex joint in the body, serving as a meeting point for powerful muscle groups in the thigh and lower leg. These muscles transfer force across the knee via major tendons to produce movement. Understanding the location and function of these tendons is fundamental to comprehending how the joint moves and maintains stability.
The Primary Extensor Unit
The primary extensor unit is a powerful, continuous structure that runs along the front of the thigh and knee, enabling the leg to straighten, or extend. This mechanism begins with the Quadriceps Tendon, which is formed by the convergence of the four quadriceps muscles in the thigh. This broad, thick tendon attaches the entire quadriceps muscle group directly to the upper pole of the kneecap, which is anatomically known as the patella.
This mechanism continues below the patella with the Patellar Tendon, which connects the lower part of the kneecap to the shin bone, or tibia. When the quadriceps muscles contract, the force is transmitted through the quadriceps tendon, across the patella, and finally through the patellar tendon to pull on the tibia, resulting in knee extension. The kneecap acts as a fulcrum to enhance the leverage of this entire extensor system.
The Patellar Tendon connects the patella and the tibia. Although it technically fits the definition of a ligament (connecting bone to bone), it is functionally part of the muscle-to-bone extensor chain. The structure is essentially a continuation of the quadriceps tendon, with the patella housed within the complex.
The Flexor and Posterior Group
Movement in the opposite direction, known as flexion or bending the leg, is handled by a separate set of tendons located on the back of the knee. The major tendons in this group belong to the three hamstring muscles, which originate at the pelvis and run down the back of the thigh before crossing the knee joint. These tendons are responsible for knee flexion and also provide important posterior stability to the joint.
The Biceps Femoris tendon is located on the outer side of the knee, known as the lateral side, and inserts primarily onto the fibula bone. Its function includes flexing the knee and rotating the lower leg outward, and it is the most common hamstring tendon involved in acute injuries. The remaining two hamstring tendons are located on the inner side of the knee.
The Semitendinosus and the Semimembranosus tendons are situated on the medial (inside) aspect of the knee. The semitendinosus is more superficial and contributes to the Pes Anserine complex, inserting onto the tibia. The semimembranosus is deeper, attaching to the back and inside of the tibia, and both work to flex the knee and rotate the lower leg inward.
Common Tendon Injuries of the Knee
The dynamic forces placed on the knee during activities like running, jumping, and landing make its tendons susceptible to specific types of injury. The most common pathology is tendinopathy, an overuse injury resulting from repetitive micro-trauma that causes breakdown within the tendon fibers. Patellar tendinopathy, widely known as “Jumper’s Knee,” causes pain just below the kneecap and is common in athletes due to repeated powerful extension movements.
Acute, high-force events can lead to partial or complete ruptures of the extensor unit tendons. A complete rupture of either the quadriceps or patellar tendon is a serious injury that typically results in the inability to straighten the knee. This type of injury often occurs when the quadriceps muscle contracts suddenly and powerfully while the knee is flexed, such as when missing a step or landing from a jump.
Hamstring tendons are frequently affected by strains, which are tears occurring near the musculotendinous junction. These strains are common in sports involving sudden acceleration or high-speed running. Hamstring tendons can also develop tendinopathy near their attachment points, causing pain in the back of the knee or upper buttock.