Pain behind the knee is common and usually comes from one of a handful of causes: a fluid-filled cyst, a muscle or tendon strain, a meniscus tear, arthritis, or occasionally a ligament injury. The location, timing, and quality of your pain can help narrow down what’s going on. Most causes are manageable without surgery, but a few deserve prompt attention.
Baker’s Cyst: The Most Common Culprit
A Baker’s cyst is a pocket of fluid that forms behind the knee. Your knee joint naturally produces a lubricating fluid to keep things moving smoothly, but when something irritates the joint (arthritis, a cartilage tear, or general overuse), the knee can produce too much of it. That excess fluid migrates into a small sac called a bursa at the back of the knee, creating a visible or palpable bulge.
The hallmark sensation is tightness behind the knee, sometimes with a noticeable lump you can feel when your leg is straight. Stiffness makes it hard to fully bend the knee, and pain tends to worsen after you’ve been on your feet for a while or after physical activity. Fully straightening or bending the knee can intensify the discomfort. Baker’s cysts themselves aren’t dangerous, but they signal that something else in the knee is producing extra fluid, so the underlying cause often needs attention too.
Hamstring and Calf Muscle Strains
Two muscle groups converge right behind the knee: the hamstrings coming down from the thigh and the calf muscles (specifically the gastrocnemius) rising up from below. A strain in either one can send pain directly into the back of the knee, making them easy to confuse with a joint problem.
A calf strain typically strikes at the junction where the muscle meets the tendon, roughly halfway between the knee and the heel. You’ll feel a sudden sharp pain in the back of the lower leg, and standing on your toes becomes difficult or painful. Swelling or bruising in the calf muscle usually follows. A hamstring strain, by contrast, tends to produce pain higher up, closer to the back of the thigh, though it can radiate downward toward the knee. Both injuries commonly happen during sudden movements: sprinting, jumping, or even an awkward step off a curb.
The simplest way to tell the difference at home is to test what movement reproduces the pain. If pushing up onto your toes hurts, the calf is more likely involved. If bending your knee against resistance or stretching with a straight leg hurts in the back of the thigh, it’s probably a hamstring issue.
Popliteus Tendon Pain
There’s a small but important muscle behind the knee called the popliteus. It acts as a kind of “key” that unlocks the knee from a fully straight position to allow bending. Some researchers consider its tendon important enough to call it the fifth major ligament of the knee.
When this tendon becomes inflamed, the pain shows up at the outer-back part of the knee and can be quite sharp. The area from the crease behind the knee to the outside of the joint becomes tender to touch, and you may hear a crackling sound when moving the knee. Bending the knee against resistance, especially in the first 15 to 30 degrees, hurts. Walking downhill is a reliable trigger because the popliteus has to work harder to control your body weight on a decline. Stairs, running, and even regular walking can be painful in the early stages.
Meniscus Tears
Each knee has two C-shaped pieces of cartilage (menisci) that act as shock absorbers. The rear portion of these cartilage pads sits near the back of the knee, and tears in this area often produce posterior knee pain. A meniscus tear can happen during a sport, but it just as easily occurs from an awkward twist while getting out of a car or slipping on a wet floor.
The distinguishing feature of a meniscus tear is a mechanical sensation: catching, locking, or clicking in the knee. If your knee occasionally feels like it gets stuck mid-motion or you hear a popping or snapping sound when rotating the leg, a tear in the back portion of the meniscus is a strong possibility. Swelling typically develops over several hours rather than immediately, and you may notice the knee feels unstable or gives way at unpredictable moments.
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 the more well-known ACL tears, and they usually happen with significant force: a hard blow to the front of a bent knee (the classic “dashboard injury” in a car accident), a fall directly onto a bent knee, or a bad landing after a jump.
If your pain behind the knee started after a specific high-impact event, a PCL injury is worth considering. The knee may feel unstable, especially when going down stairs or decelerating while running. Swelling tends to develop quickly. Mild PCL sprains can sometimes be managed without surgery, but imaging is usually needed to determine the severity.
Arthritis
Both osteoarthritis (wear-and-tear) and rheumatoid arthritis can produce pain at the back of the knee. Osteoarthritis tends to develop gradually in people over 50 or in younger people who’ve had prior knee injuries. The pain is typically worse in the morning or after sitting for a long time, then eases up with gentle movement before worsening again with prolonged activity. Stiffness and a grinding sensation are common companions.
Arthritis also frequently leads to Baker’s cysts, so if you have both a fluid-filled bulge and chronic aching behind the knee, the two problems may be connected.
When to Take It Seriously
Most posterior knee pain is musculoskeletal and improves with rest, ice, and gradual rehabilitation. But one cause demands urgency: a deep vein thrombosis (DVT), or blood clot, in the vein behind the knee. A DVT can feel like a muscle cramp, which is why people sometimes dismiss it.
The warning signs that point toward a clot rather than a strain include swelling that affects the entire lower leg (not just the knee), skin that looks reddish or feels noticeably warm to the touch, and tenderness that doesn’t correspond to a specific injury or movement. These symptoms often appear without any preceding physical trauma. A DVT is a medical emergency because a clot can break free and travel to the lungs. If your symptoms match this pattern, especially if you’ve recently been immobile for long periods (a long flight, bed rest after surgery), get evaluated the same day.
Nerve Compression Behind the Knee
Less commonly, pain behind the knee involves nerve irritation. The tibial nerve runs through the popliteal fossa (the soft diamond-shaped area behind the knee), and swelling, cysts, or muscle injuries in that space can compress it. The telltale signs are neurological: numbness or altered sensation on the sole of your foot, difficulty flexing your toes or ankle, or a tingling quality to the pain that feels different from a typical ache or strain. If your behind-the-knee pain comes with foot numbness, that’s a signal that something is pressing on a nerve and warrants investigation.
Managing Posterior Knee Pain at Home
For most non-traumatic causes, a consistent rehabilitation approach makes a significant difference. The American Academy of Orthopaedic Surgeons recommends a conditioning program lasting 4 to 6 weeks, performed two to three days per week. Each session starts with 5 to 10 minutes of gentle warmup (walking or a stationary bike works well), followed by stretching, then strengthening exercises targeting the quadriceps, hamstrings, inner and outer thigh muscles, and glutes. You finish with the same stretches you started with.
The key muscles to strengthen aren’t just the ones that hurt. Weakness in the quadriceps or glutes often forces the hamstrings and calf muscles to compensate, overloading the structures behind the knee. Building balanced strength around the entire joint takes pressure off the posterior knee. Pain during any exercise is a signal to stop or modify, not to push through. After the initial recovery period, continuing these exercises two to three times a week helps protect the knee long-term.
For acute pain in the first 48 to 72 hours, rest, ice (15 to 20 minutes at a time), gentle compression, and keeping the leg elevated all help reduce swelling and discomfort. Avoid activities that reproduce the pain, but don’t immobilize the knee completely. Gentle, pain-free movement promotes healing better than total rest.