Pain behind the knee usually comes from one of a handful of common causes: a fluid-filled cyst, a muscle or tendon strain, a meniscus tear, or arthritis. Less often, it signals something vascular like a blood clot. The location, timing, and quality of your pain can help narrow down what’s going on.
Baker’s Cyst: The Most Recognizable Cause
A Baker’s cyst is a fluid-filled sac that forms in the hollow behind your knee. It develops when something damages the knee joint or the tissue around it, causing extra fluid to drain out the back of the joint, where it pools and creates a visible or palpable bump. The most obvious sign is that bump itself, but you may also notice stiffness, swelling that extends into your calf or thigh, and difficulty bending your knee as far as you normally can.
Baker’s cysts are especially common in people with osteoarthritis. About 23% of people with symptomatic knee osteoarthritis have a moderate-to-large Baker’s cyst, compared to roughly 9% of people without arthritis. The cyst itself isn’t always the primary problem. It’s often a sign that something else in the knee is producing excess fluid, whether that’s arthritis, a cartilage tear, or another injury. Treating the underlying cause usually resolves the cyst over time.
Meniscus Tears
Each knee has two C-shaped pieces of cartilage called menisci that act as shock absorbers between the thighbone and shinbone. A tear in the back portion of the meniscus (the posterior horn) is a common source of pain specifically behind the knee. These tears can happen during a sudden twist, an awkward step, or a sports injury, but they also occur gradually with age and wear.
The hallmark sign is deep knee pain that gets worse with full bending, squatting, or any movement that compresses the back of the joint. As you bend deeper past about 90 degrees, the torn portion of the cartilage gets pushed further backward and squeezed under increasing pressure. That’s why squatting to pick something up or sitting cross-legged can trigger a sharp ache that standing and walking don’t. Some people also feel a catching or locking sensation when the torn flap shifts out of position.
Muscle and Tendon Strains
Several muscles and tendons cross behind the knee, and irritation in any of them can produce posterior knee pain. The two most common culprits are the hamstring tendons and a small, deep muscle called the popliteus.
Your hamstrings attach just below the knee joint on the inner side. When the tendon there becomes inflamed, you’ll typically feel an aching pain at the inner back of the knee that’s tender to the touch. It tends to worsen with activities that load the hamstrings: running, climbing stairs, or bending the knee against resistance.
The popliteus sits deeper, on the outer back of the knee. Its job is to “unlock” the knee from a fully straight position so you can start bending it. When this muscle or its tendon is strained, the pain is more toward the outer back of the knee and often flares with walking downhill or decelerating from a run. You might also feel pain when you try to rotate your lower leg inward.
The calf muscles (gastrocnemius) also originate behind the knee. A strain here causes pain during calf raises or when you push off while walking. It can feel similar to deeper knee problems, but the tenderness is usually lower and more superficial.
Arthritis
Osteoarthritis and rheumatoid arthritis both cause posterior knee pain, though they do it differently. Osteoarthritis wears down the cartilage inside the joint over years, leading to stiffness and aching that’s worst in the morning or after sitting for a while and improves somewhat with gentle movement. Rheumatoid arthritis is an autoimmune condition that inflames the joint lining itself, producing swelling, warmth, and pain that can come and go in flares.
In both cases, excess fluid produced by the irritated joint often migrates to the back of the knee, which is why arthritis and Baker’s cysts so frequently overlap. If you’re over 50 and the pain has been building gradually over months without a specific injury, arthritis is one of the more likely explanations.
Ligament Injuries
The posterior cruciate ligament (PCL) runs along the back of the knee and prevents the shinbone from sliding too far backward. PCL injuries are less common than ACL tears, but they happen during falls onto a bent knee, car accidents where the dashboard pushes the shin backward, or contact sports like football and soccer. The pain is usually immediate and centered deep behind the knee, often with swelling and a feeling of instability, as though the knee might give way.
Unlike some other causes on this list, a PCL tear rarely happens without a clear moment of injury. If your pain came on gradually or without trauma, a ligament tear is unlikely.
Referred Pain From Other Areas
Not all pain behind the knee originates in the knee. The lower back and the kneecap joint can both refer pain to the back of the knee. If your posterior knee pain comes with lower back stiffness or aching around the front of the kneecap, the source may be elsewhere. This is worth keeping in mind if knee-focused treatments haven’t helped.
Blood Clots and Vascular Causes
A deep vein thrombosis (DVT), or blood clot in a leg vein, can cause pain, swelling, and warmth behind the knee and into the calf. The skin over the area may turn red or purple, and the affected leg often feels noticeably warmer than the other. DVT pain typically starts in the calf and feels like a persistent cramp or soreness that doesn’t improve with rest or stretching.
Popliteal artery aneurysms are rare but worth knowing about. They can cause a pulsing sensation behind the knee, along with pain and swelling. Complications include blood clots that severely reduce blood flow to the lower leg, which can cause the skin to feel cold, turn pale, and go numb.
If you have leg swelling on one side only, skin color changes, warmth, or any sign of reduced circulation (cold foot, numbness, absent pulse), these are not wait-and-see symptoms. They need same-day medical evaluation.
What Helps in the First Few Days
For pain that started after a strain, twist, or minor injury, the general approach in the first one to three days is to protect the knee by reducing movement enough to prevent further damage, while avoiding complete immobilization. Prolonged rest actually weakens the tissue and slows healing. Let pain be your guide: if an activity hurts, back off, but don’t stay off the leg entirely.
Elevating the leg above heart level helps reduce swelling by encouraging fluid to drain away from the knee. This is simple and low-risk, even though the evidence for it is modest.
One shift in sports medicine thinking is that anti-inflammatory medications and ice, while effective for short-term pain relief, may actually slow tissue healing if used heavily. The inflammatory process is part of how your body repairs damage, so suppressing it aggressively in the early days isn’t always beneficial. If you do use ice, think of it as a pain management tool rather than a healing one.
After the first few days, gradually reloading the knee with gentle movement helps the tissue rebuild with proper strength and quality. Walking, light cycling, or simple range-of-motion exercises are good starting points.
When Imaging Is Needed
Not every case of posterior knee pain needs an X-ray or MRI. Standardized guidelines (known as the Ottawa Knee Rules) suggest imaging after an injury if you’re 55 or older, if you have point tenderness at the small bone on the outer side of the knee (the fibular head), if you can’t bend the knee to 90 degrees, or if you can’t take four steps on it. Meeting any one of these criteria is enough reason to get imaging.
For pain that developed without a specific injury, imaging is most useful when symptoms haven’t improved after several weeks of conservative care, or when the pattern of pain suggests something structural like a meniscus tear or a cyst that needs confirmation. An ultrasound can quickly identify a Baker’s cyst, while an MRI gives a detailed look at cartilage, ligaments, and soft tissue.