Pain behind the knee typically comes from one of several soft tissue structures packed into the small space at the back of your knee, called the popliteal fossa. The most common causes are Baker’s cysts, hamstring tendon problems, meniscus tears, and muscle strains, though vascular issues like blood clots can also produce pain in this area. Understanding where exactly your pain is and what triggers it helps narrow down the cause.
Why the Back of the Knee Is Vulnerable
The hollow behind your knee contains a dense network of structures in a surprisingly small space. The popliteal artery and vein run through it, along with the tibial nerve and common peroneal nerve. The hamstring muscles converge from above while the calf muscles (the two heads of the gastrocnemius) form the lower border. A small, deep muscle called the popliteus sits right against the back of the knee joint itself.
Because so many structures overlap here, pain behind the knee can come from muscles, tendons, fluid-filled cysts, cartilage tears inside the joint, compressed nerves, or blood vessel problems. The location of the pain, whether it’s more toward the inner or outer side, and what makes it worse are all useful clues.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most recognizable causes of pain and fullness behind the knee. It forms when excess joint fluid pushes out of the knee and collects in a pouch between the calf muscle and a hamstring tendon on the inner side of the back of the knee. The fluid inside is thick and gel-like.
In adults, Baker’s cysts almost always develop because of an underlying knee problem. Degenerative meniscus tears are one of the most common triggers, followed by osteoarthritis, rheumatoid arthritis, and cartilage injuries. In children, these cysts can form on their own without any joint disease.
You’ll typically notice a sensation of tightness, discomfort, or aching behind the knee that worsens with activity. Swelling is often more visible when you stand with your leg fully straight. As the cyst grows, it can limit how far you bend or straighten the knee. A large cyst sometimes compresses nearby veins, causing swelling in the lower leg.
If a Baker’s cyst ruptures, the experience is hard to miss: sharp pain in the knee and calf, calf swelling or redness, and a sensation that feels like water running down the back of your leg. A ruptured cyst can closely mimic a blood clot, so imaging is often needed to tell them apart. In rare cases, a rupture can trap the tibial nerve (causing numbness in the sole of the foot) or block the popliteal artery. When treated with ultrasound-guided drainage and injection, the recurrence rate is around 13%, which compares favorably to surgical removal, where recurrence ranges from 5% to 70% depending on the technique and whether the underlying joint problem is also addressed.
Meniscus Tears
Each knee has two C-shaped cartilage pads called menisci. Tears in the back portion (the posterior horn) of either meniscus are a common source of pain that localizes behind or deep within the knee. Posterior horn tears of the medial meniscus are especially frequent, often occurring alongside early arthritis in middle-aged and older adults or from twisting injuries in younger people.
The hallmark of a meniscus tear is mechanical symptoms: catching, clicking, locking, or a sensation that something is stuck when you bend or straighten the knee. Pain tends to be sharp with specific movements rather than constant. You may also notice tenderness along the joint line and intermittent swelling. During an exam, a clinician may perform a McMurray test, rotating and extending the knee while feeling for a painful click along the joint line.
Hamstring Tendon Problems
Your three hamstring muscles narrow into tendons that attach around the back of the knee. The biceps femoris tendon inserts on the outer side (near the top of the smaller lower leg bone, the fibula), while the semimembranosus and semitendinosus attach on the inner side. Injuries to these tendons cause pain that’s specifically posterolateral (outer back) or posteromedial (inner back), depending on which tendon is involved.
A distal biceps femoris injury typically causes pain around the posterolateral knee, sometimes with bruising and noticeable weakness when bending the knee against resistance. With a complete tear or avulsion, you or a clinician may feel a gap or defect just above the fibular head. Some people develop a “snapping” sensation on the outer side of the knee from the biceps femoris tendon slipping over bony structures during movement.
Injuries to the semimembranosus tendon on the inner side are considerably rarer but produce a similar pattern of pain with resisted knee bending. Both types of hamstring tendon injuries are typically confirmed with MRI or ultrasound.
Popliteus Muscle Strain
The popliteus is a small but important muscle deep in the back of the knee. Its job is to “unlock” the knee from a fully straight position by rotating the thigh bone slightly outward on the shinbone. Without it, you couldn’t smoothly initiate bending from a locked-out knee.
Popliteus tendinopathy causes posterolateral knee pain that’s often hard to distinguish from other problems in the same area. A useful clue is that symptoms tend to worsen with downhill running, particularly with a long stride. A clinician can test for it by having you lie on your back with hips and knees bent to 90 degrees, then resisting internal rotation or applying external rotation at the knee. Pain with either maneuver points toward the popliteus.
Nerve Compression
The tibial nerve runs through the center of the popliteal fossa and can become trapped where it passes under a tendinous arch at the top of the calf muscle (the soleus). When this happens, you’ll typically feel severe pain and tenderness directly in the hollow behind the knee. Tapping on the spot may produce tingling or electric sensations shooting down into the calf or foot, a finding known as a positive Tinel sign.
This condition is distinct from nerve compression at the ankle (tarsal tunnel syndrome) or a pinched nerve in the lower back, though it can mimic both. Electrodiagnostic testing can pinpoint the exact site of compression. In a case series of nine patients, six required surgical release of the tendinous arch and recovered fully, while three improved on their own.
Blood Clots in the Popliteal Vein
Deep vein thrombosis is the most serious potential cause of pain behind the knee and in the calf. A blood clot in the popliteal vein produces leg swelling, cramping or soreness (often starting in the calf), warmth in the affected area, and sometimes a color change in the skin to red or purple. The critical difference from a muscle strain is that DVT swelling tends to involve the entire lower leg, the skin may feel warm to the touch, and the pain doesn’t clearly correspond to a specific movement or position.
DVT can also occur without noticeable symptoms, which is part of what makes it dangerous. The concern is that a clot can break loose and travel to the lungs. If you have calf or posterior knee pain with unexplained leg swelling, warmth, or skin color changes, particularly after a period of immobility, surgery, or travel, seek medical evaluation promptly.
Popliteal Artery Entrapment
This is an uncommon but underdiagnosed condition where the popliteal artery gets compressed by surrounding muscle, typically during exercise. It predominantly affects young, active males in their 20s and 30s who have no cardiovascular risk factors. About 85% of cases occur in men, and nearly 60% are in young athletes.
Symptoms develop gradually and include cramping or aching in the calf during exercise that resolves with rest, similar to what older adults experience from artery disease. The key physical finding is that pulses in the foot disappear when you actively point your toes down or when the foot is passively pulled upward. Calf muscles are often visibly well-developed. Diagnosis usually involves ultrasound with these provocative foot positions, sometimes followed by MRI with angiography to confirm the anatomy.
How the Cause Is Identified
A thorough physical exam narrows the possibilities significantly. The location of tenderness, whether pain occurs with specific movements, and the presence or absence of swelling all guide the evaluation. Clinicians use targeted tests: the McMurray test for meniscus tears (rotating and extending the knee while feeling for clicks), resisted knee bending for hamstring tendon injuries, and internal rotation testing for popliteus problems.
Imaging comes next when the diagnosis isn’t clear. MRI is the standard for evaluating soft tissue, including meniscus tears, tendon injuries, Baker’s cysts, and nerve or artery abnormalities. Ultrasound is particularly useful for Baker’s cysts, blood clots, and dynamic problems like a snapping biceps femoris tendon, because the examiner can watch structures move in real time. For suspected arterial entrapment, duplex ultrasound with foot positioning maneuvers provides a quick initial screening test.
In most cases, posterior knee pain turns out to be a treatable musculoskeletal problem. The pattern of your symptoms, what makes the pain better or worse, and how it started will usually point your clinician in the right direction quickly.