Pain in the back of your knee can come from several different structures packed into a small space: tendons, ligaments, cartilage, fluid-filled sacs, and the muscles that cross behind the joint. The cause usually depends on how the pain started, whether it came on suddenly or gradually, and what makes it worse. Most causes are manageable without surgery, but one rare possibility, a blood clot, needs urgent attention.
Baker’s Cyst
A Baker’s cyst is one of the most recognizable causes of posterior knee pain. It shows up as a visible or palpable bulge behind the knee, along with a feeling of tightness that gets worse when you straighten or fully bend the leg. The cyst itself isn’t the root problem. It forms because something else, usually arthritis or a cartilage tear, causes your knee to overproduce synovial fluid, the lubricant that normally reduces friction inside the joint. When too much fluid builds up, it migrates into a small sac called a bursa behind the knee, inflating it like a water balloon.
Baker’s cysts sometimes cause no symptoms at all and are discovered incidentally on imaging. When they do cause trouble, you’ll typically notice swelling behind the knee (sometimes extending into the calf), stiffness, and a dull ache that worsens with activity. If a cyst ruptures, it can mimic the sudden calf pain and swelling of a blood clot, which is why imaging is often used to tell the two apart. Treating the underlying knee problem, whether that’s arthritis or a torn meniscus, usually resolves the cyst over time.
Hamstring and Calf Muscle Strains
The hamstrings run down the back of your thigh and attach just below the knee, while the calf muscle (gastrocnemius) originates just above the back of the knee. Either one can send pain into the area behind the joint when strained, but they feel different and happen in different ways.
A hamstring strain typically happens during running, kicking, or jumping, activities that force the muscle to contract while it’s being stretched. You may feel or hear a pop at the back of the thigh, and the pain tends to radiate down toward the knee. A calf strain, by contrast, usually occurs when your foot suddenly bends upward, overstretching the muscle. Think of stepping into a hole or having your heel slip off a curb. The pain is generally centered in the calf itself rather than right behind the knee, but it can extend upward.
Mild strains respond well to rest, ice, gentle stretching, and a gradual return to activity over a few weeks. More severe tears, where you can’t bear weight or the bruising is significant, take longer and may need guided rehabilitation.
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 either meniscus can produce pain specifically behind the knee. These tears happen when the knee twists suddenly, whether during sports, an awkward step, or even just pivoting on a planted foot.
The hallmark signs go beyond pain. You may notice a popping sensation at the time of injury, followed by swelling and stiffness over the next day or two. Many people describe a catching or locking feeling, as if the knee gets stuck partway through bending or straightening. Some feel the knee “give way” unexpectedly. Smaller tears can heal with rest, physical therapy, and activity modification. Larger or mechanically problematic tears, the kind that cause persistent locking, sometimes require surgical repair.
PCL Injuries
The posterior cruciate ligament (PCL) sits at the very back of your knee, connecting the thighbone to the shinbone and preventing the shin from sliding too far backward. PCL tears account for less than 20% of all knee ligament injuries, making them far less common than ACL tears. They usually result from a direct blow to the front of the knee, like hitting a dashboard in a car accident or falling hard onto a bent knee.
A PCL injury often causes deep, aching pain behind the knee, swelling, and a sense of instability, especially when walking downhill or on stairs. Unlike ACL tears, many PCL injuries can be managed without surgery through strengthening exercises focused on the quadriceps, which help compensate for the loosened ligament. Diagnosis involves a physical exam where a clinician checks whether the shinbone shifts backward relative to the thigh. This test is highly accurate, catching about 90% of PCL tears.
Arthritis
Osteoarthritis is the most common form of arthritis affecting the knee, and it can absolutely cause pain in the back of the joint. The pain tends to build gradually over months or years, feels worse after prolonged activity or at the end of the day, and improves somewhat with rest. Stiffness after sitting for a while is typical. Rheumatoid arthritis can also affect the knee, though it usually involves other joints simultaneously and comes with more pronounced morning stiffness.
Current clinical guidelines recommend non-surgical management at every stage of knee osteoarthritis, combining exercise, weight management, and pain relief as needed. In the UK, nine out of ten people with knee osteoarthritis manage their condition without ever needing joint replacement. Arthroscopic “clean-up” procedures have been shown to provide little meaningful benefit for uncomplicated osteoarthritis and are no longer recommended as a primary treatment. Surgery is reserved for people with severe functional impairment that persists despite sustained non-surgical care.
Tendinitis
Several tendons cross behind the knee, and inflammation in any of them can cause localized pain. The hamstring tendons and the popliteus tendon (a small stabilizer deep in the back of the knee) are common culprits. Tendinitis pain is typically activity-related: it flares during or after exercise and eases with rest. You may feel tenderness when pressing directly on the affected tendon.
Tendinitis is almost always an overuse injury. Sudden increases in training volume, running on hills, or repetitive deep squatting are frequent triggers. It responds well to relative rest (reducing the aggravating activity without stopping all movement), ice, and a progressive strengthening program.
When It Could Be a Blood Clot
Deep vein thrombosis (DVT) is uncommon but worth knowing about because it requires immediate medical care. A blood clot in the popliteal vein, which runs directly behind the knee, can produce pain, swelling, warmth, and a change in skin color (reddish or purplish) in the affected leg. The pain often starts in the calf and feels like a deep cramp or soreness that doesn’t go away with typical measures.
Risk factors include recent surgery, prolonged immobility (like a long flight or bed rest), pregnancy, smoking, and use of hormonal birth control. If you have unexplained leg swelling along with warmth and skin color changes, especially on just one side, seek medical evaluation promptly. The concern with DVT is that a clot can break loose and travel to the lungs.
Simple Exercises That Help
For most non-traumatic causes of posterior knee pain, gentle exercises can reduce stiffness and support recovery. Two straightforward options to start with:
- Heel slides: Lie on your back with both knees bent. Slowly slide the heel of your affected leg back, bending the knee as far as comfortable. You can hook your other foot around the ankle to gently pull it a bit farther. Hold for about 6 seconds, return to the start, and repeat 8 to 12 times.
- Quad sets: Sit or lie with your affected leg straight and a small rolled towel under the knee. Press the back of your knee down into the towel, tightening the front thigh muscles. Hold for 6 seconds, relax, and repeat 8 to 12 times.
These exercises work by maintaining range of motion and strengthening the quadriceps, which play a key role in stabilizing the knee from the front and reducing load on the structures behind it. They’re appropriate for mild ligament sprains, early arthritis, and general stiffness, but if your pain is sharp, involves locking or giving way, or came from a significant injury, get it assessed before starting a rehab program.