Pain behind the knee is common and usually comes from one of a handful of causes: a fluid-filled cyst, a cartilage tear, tight or inflamed hamstring tendons, arthritis, or even a pinched nerve in the lower back. The location and timing of your pain narrows down the list quickly. Here’s what each cause feels like and what you can do about it.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most frequent reasons for a noticeable lump or fullness behind the knee. It forms when the knee joint produces too much synovial fluid, the liquid that normally lubricates the joint for smooth, pain-free movement. When something irritates the knee, like arthritis or a cartilage tear, fluid production ramps up. That extra fluid migrates into a small sac called a bursa behind the knee, causing it to swell into a visible, sometimes egg-sized bulge.
A Baker’s cyst typically feels like tightness or pressure behind the knee that gets worse when you fully straighten or bend the leg. Smaller cysts may cause no pain at all and only show up on imaging. Larger ones can ache throughout the day and limit your range of motion. In rare cases, the cyst ruptures, sending fluid down into the calf. This causes sudden sharp pain, swelling, and redness in the lower leg that can mimic a blood clot, so it’s worth getting evaluated quickly if that happens.
Because a Baker’s cyst is almost always a symptom of another knee problem, treating the underlying cause (arthritis, a meniscus tear) is more effective than draining the cyst alone. Left untreated, the cyst tends to refill.
Meniscus Tears
Each knee has two crescent-shaped pads of cartilage called menisci that sit between the thighbone and shinbone. Tears in the back portion of the meniscus are a classic source of posterior knee pain, especially during deep bending, squatting, or twisting movements.
The hallmark symptoms are mechanical: a catching or locking sensation in the knee, as though something is stuck inside the joint. Certain tear patterns can actually flip like a bucket handle, getting pinched between the bones when the knee bends. This causes sharp, intermittent pain that may come and go depending on how the torn flap moves. You might also notice swelling that develops gradually over a day or two after the injury, stiffness that makes it hard to fully straighten the knee, and a sense that the knee could give out.
Small tears sometimes settle down on their own with rest and physical therapy. Larger tears that cause persistent clicking, catching, or locking are more likely to need surgical repair.
Hamstring Tendon Problems
Your hamstring muscles run down the back of your thigh and connect to bones in the pelvis, knee, and lower leg via tendons. When those tendons become inflamed or develop tiny areas of damage (tendinopathy), the pain often concentrates right behind the knee where the tendons attach.
Hamstring tendinopathy typically produces a dull ache behind the knee or lower thigh that worsens with activity, especially running, lunging, or bending the knee under load. You may notice stiffness when walking or bending the knee first thing in the morning, along with weakness in the back of the thigh. A sudden overstretch can cause sharper pain, sometimes with noticeable swelling right at the injury site. This is different from a full hamstring tear, which usually produces a sudden, dramatic “pop” sensation higher up the thigh.
Tight hamstrings alone, without any tendon damage, can also pull on the back of the knee and create a vague aching that improves with gentle stretching and worsens after long periods of sitting.
PCL Injuries
The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shinbone from sliding too far backward. PCL injuries are less common than ACL tears, but they do cause pain in the back of the knee.
This ligament usually requires a powerful force to tear. The classic scenario is a direct blow to the front of a bent knee: hitting the dashboard in a car accident, or falling hard onto a bent knee during sports. Twisting and hyperextension injuries can also damage the PCL. After the injury, the knee may feel unstable, especially going downstairs or decelerating during a run. One telltale sign is that the injured knee appears to sag backward compared to the uninjured side when the leg is bent.
Partial PCL tears often heal well with structured rehabilitation. Complete tears sometimes require surgical reconstruction, particularly in athletes or people who remain unstable despite physical therapy.
Arthritis
Both osteoarthritis (wear-and-tear degeneration) and rheumatoid arthritis (an autoimmune condition) can produce pain behind the knee. With osteoarthritis, the cartilage covering the joint surfaces gradually thins, and the resulting inflammation can cause stiffness and aching that worsens with activity and improves with rest. Rheumatoid arthritis tends to affect both knees and is often accompanied by morning stiffness lasting 30 minutes or more.
Arthritis-related posterior knee pain is usually more of a deep, diffuse ache rather than a sharp, pinpoint sensation. It often coexists with a Baker’s cyst, since the chronic inflammation drives excess fluid production. If you’re over 50 and the pain has been building gradually over months or years without a clear injury, arthritis is high on the list.
Nerve Pain From the Lower Back
Sometimes the source of the pain isn’t the knee at all. A pinched nerve in the lower spine can send pain, numbness, or tingling down the back of the leg, and the back of the knee is a common spot where people feel it. The sciatic nerve, the longest nerve in the body, starts in the lower back and runs down the back of each leg. When a herniated disc or bone spur compresses one of the nerve roots where the sciatic nerve originates, the result is sciatica: pain that radiates from the low back through the buttock and down the leg.
The giveaway is that this pain tends to travel. It rarely stays isolated to the back of the knee. You might feel it in the buttock, back of the thigh, or calf as well. Coughing, sneezing, or sitting for long periods can make it worse. If your posterior knee pain came on without any knee injury and you also have low back symptoms, nerve compression is worth investigating.
Popliteus Muscle Strain
The popliteus is a small, often-overlooked muscle that sits right behind the knee joint and helps “unlock” the knee from a fully straight position. When this muscle or its tendon becomes strained or inflamed, it causes pain at the outer-back part of the knee. It’s more common in runners and hikers, particularly on downhill terrain.
Popliteus tendinopathy tends to cause localized tenderness and sometimes a crunchy or grinding feeling with movement. Pain is typically worst with resisted knee bending or rotation. It can feel similar to other causes of posterior knee pain, so it’s often diagnosed only after meniscus tears, Baker’s cysts, and other common culprits have been ruled out.
Managing the Pain at Home
Regardless of the specific cause, the initial approach to posterior knee pain is similar. Activity modification is the starting point: avoid movements that aggravate the pain while keeping the leg gently moving to maintain circulation and prevent stiffness. Light muscle activation, like slow knee bending and straightening within a comfortable range, helps more than complete immobilization. Ice applied for 15 to 20 minutes at a time can reduce swelling and provide short-term relief. Over-the-counter anti-inflammatory medications like ibuprofen or naproxen are effective first-line options for pain and inflammation.
If the pain doesn’t improve within a week or two, physical therapy becomes the next step. Active rehabilitation, meaning exercises you perform rather than passive treatments done to you, is consistently more effective. The focus is on restoring strength, flexibility, and normal movement patterns. For posterior knee pain specifically, this often means hamstring and calf stretching, quadriceps strengthening to better support the joint, and balance training if the knee feels unstable. Two to three sessions per week is a typical starting frequency, with many people noticing improvement within the first few weeks.
Corticosteroid injections into the knee joint are an option when oral medications and therapy haven’t provided enough relief. They work by reducing inflammation directly at the source. Current guidelines limit these to three injections per year to the same site, and they tend to provide temporary rather than permanent relief.
Signs That Need Prompt Attention
Most posterior knee pain resolves with conservative treatment. A few patterns, though, warrant faster evaluation: sudden calf swelling and redness (which could indicate a ruptured Baker’s cyst or a blood clot), inability to bear weight on the leg, a locked knee that won’t fully straighten, or significant instability where the knee buckles or gives way. Numbness, tingling, or weakness spreading down the leg also warrants a visit, since these suggest nerve involvement that benefits from earlier intervention.