The patellar tendon is the thick band of tissue that connects your kneecap (patella) to your shinbone (tibia). It sits just below the kneecap and is essential for straightening your leg. Every time you stand up from a chair, climb stairs, kick a ball, or jump, the patellar tendon is doing the work of transferring force from your thigh muscles to your lower leg.
Where the Patellar Tendon Sits
The patellar tendon starts at the bottom of the kneecap and runs downward to attach at a bony bump near the top of the shinbone called the tibial tuberosity. You can feel this bump if you run your finger a few inches below your kneecap. In an average adult, the tendon is roughly 42 mm long (about 1.7 inches) and 33 mm wide, though individual measurements vary.
Technically, because it connects two bones (the patella to the tibia), some anatomists argue it should be called the “infrapatellar ligament” rather than a tendon. Tendons normally connect muscle to bone, while ligaments connect bone to bone. The patellar tendon gets its common name because it functions as a continuation of the quadriceps tendon above the kneecap, making it part of one continuous chain from muscle to bone.
How It Helps You Straighten Your Knee
Your quadriceps, the large muscle group on the front of your thigh, pulls on the kneecap through the quadriceps tendon. The kneecap then transfers that force through the patellar tendon down to the shinbone, which straightens the leg. This entire system is called the extensor mechanism of the knee, and a break in any link of the chain makes it impossible to fully extend the knee or resist the leg from buckling.
The kneecap plays a clever role in this system. It acts as a pulley, increasing the distance between the tendon and the knee’s center of rotation. This creates leverage that makes the quadriceps significantly more effective. Without a functioning kneecap, the effective strength of knee extension drops by at least 30%. The patellar tendon is the final link that delivers all of that amplified force to the leg bone.
What the Tendon Is Made Of
Under a microscope, the patellar tendon is made of tightly packed bundles of collagen fibers sitting in a gel-like matrix. About 72% of the collagen is type I, the same strong, rope-like protein that makes up most tendons, bones, and skin. Another 16% is type III collagen, which is thinner and more flexible. Scattered between the collagen bundles are rows of fibroblasts, the cells responsible for producing and maintaining the collagen.
This composition makes the patellar tendon remarkably strong. Its ultimate tensile strength averages around 59 megapascals, with reported values ranging from about 27 to 96 megapascals depending on the individual. In practical terms, the tendon can handle forces many times your body weight during activities like jumping or sprinting before it would tear.
Blood reaches the tendon through a network of small arteries that form a ring around the kneecap. Six arteries supply the knee joint overall, and their branches feed the surrounding tendons, ligaments, and soft tissue. However, blood flow within tendons is generally modest compared to muscle, which is one reason tendon injuries can be slow to heal.
Patellar Tendinopathy (Jumper’s Knee)
The most common problem affecting the patellar tendon is patellar tendinopathy, often called jumper’s knee. Despite the nickname, it doesn’t only affect athletes who jump. Any activity that repeatedly loads the tendon, such as running, squatting, or cycling at high intensity, can trigger it. The condition involves small-scale damage and degeneration within the tendon’s collagen fibers, typically at the point where the tendon attaches to the bottom of the kneecap.
You’ll notice pain just below the kneecap that worsens with activity, especially jumping, lunging, or going downstairs. It often starts as a dull ache after exercise and progresses to pain during exercise if left unaddressed. The terms “patellar tendonitis” and “patellar tendinopathy” refer to the same condition in clinical practice, though “tendinopathy” is more accurate because the problem is usually degenerative rather than purely inflammatory.
Diagnosis is straightforward. A clinician will press on the tendon, compare it to your other knee, and ask about your activity history. Imaging with ultrasound or MRI can confirm the diagnosis and show the extent of tendon changes, though it’s not always necessary. Treatment centers on progressive loading exercises that gradually strengthen the tendon, along with activity modification. Recovery often takes weeks to months because tendon tissue remodels slowly.
Osgood-Schlatter Disease in Young Athletes
In children and adolescents who are still growing, the spot where the patellar tendon attaches to the shinbone contains a growth plate made of cartilage rather than solid bone. Cartilage is weaker than bone, so repeated pulling from the patellar tendon during running, jumping, or kicking can irritate this growth plate. The result is Osgood-Schlatter disease: pain, swelling, and tenderness at the bony bump below the knee.
It’s most common during growth spurts, when bones are lengthening faster than muscles and tendons can adapt, which increases tension at the attachment site. The condition is self-limiting and resolves once the growth plate closes and hardens into bone, typically by the mid-to-late teenage years. In the meantime, reducing high-impact activity and using ice can keep symptoms manageable.
The Patellar Tendon in ACL Surgery
The patellar tendon’s strength and accessibility make it one of the most commonly used grafts for reconstructing a torn anterior cruciate ligament (ACL) inside the knee. During this procedure, a surgeon removes the middle third of the patellar tendon along with a small plug of bone from each end, one from the kneecap and one from the shinbone. The remaining two-thirds of the tendon on either side are sutured closed.
This “bone-patellar tendon-bone” graft is threaded through tunnels drilled in the thighbone and shinbone to replace the torn ACL. The bone plugs on each end help the graft heal solidly into the surrounding bone. The donor site on the front of the knee does recover: the gap fills in with scar tissue over time, though some people experience temporary soreness or sensitivity when kneeling during the healing period.
Patellar Tendon Tears
A complete or partial tear of the patellar tendon is less common than tendinopathy but far more serious. It typically happens during a sudden, forceful contraction of the quadriceps, such as landing awkwardly from a jump or stumbling. You’ll feel a sudden pop or tearing sensation, followed by immediate pain, swelling, and an inability to straighten the knee or bear weight.
A complete tear is a medical emergency in the sense that it requires surgical repair to restore function. Without the tendon connecting the kneecap to the shinbone, the extensor mechanism is broken, and you simply cannot straighten the leg on your own. After surgical repair, recovery involves several months of progressive rehabilitation to restore strength and range of motion. Partial tears may sometimes be managed without surgery depending on how much of the tendon is intact and how well you can still extend the knee.