Pain behind the knee usually comes from one of a handful of common causes: a fluid-filled cyst, a muscle or tendon strain, a torn piece of cartilage, or arthritis. The back of the knee is a busy intersection where muscles, tendons, ligaments, and nerves all cross paths, so pinpointing the source depends on how the pain started and what makes it worse.
Baker’s Cyst: The Most Common Culprit
A Baker’s cyst is a pocket of fluid that forms in the hollow space behind your knee. It happens when something inside the joint, like arthritis or a cartilage tear, triggers excess production of the lubricating fluid that normally keeps your knee moving smoothly. That extra fluid travels through a one-way valve into the back of the knee and pools there, creating a soft, sometimes visible bulge.
The cyst itself can feel like tightness or fullness behind your knee, especially when you bend it all the way or straighten it completely. Small cysts sometimes go unnoticed until they grow large enough to press on surrounding tissue. Most Baker’s cysts resolve within a few weeks once the underlying swelling in the joint settles down. The cyst is really a symptom, not the root problem, so treatment focuses on whatever is irritating the knee in the first place.
Hamstring and Calf Muscle Strains
Three hamstring muscles run down the back of your thigh and attach near the back of the knee. When one of them is strained, the pain often concentrates right at or just above the crease behind the knee. This tends to happen during activities that involve sudden acceleration, like sprinting, or overstretching, like reaching for something on the ground.
The upper calf muscle also originates behind the knee. Strains in this muscle typically occur during the push-off phase of running or jumping, when the knee is straight and the foot is flexed upward. That combination puts the muscle at maximum stretch while it’s contracting, which is exactly when tears happen. If your pain started suddenly during exercise and gets worse when you rise onto your toes or push off while walking, a calf strain is a likely explanation.
Meniscus Tears
Each knee has two C-shaped pads of cartilage called menisci that sit between the thighbone and shinbone. A tear in the back portion of either pad, called the posterior horn, sends pain directly to the back of the knee. You might also notice a clicking or catching sensation when you bend your knee past about 90 degrees.
Meniscus tears don’t always require a dramatic injury. They can happen from an awkward twist during everyday movement, stepping off a curb, or a simple slip. In people over 40, the cartilage gradually weakens and can tear with relatively little force. The telltale signs are pain with deep bending, occasional locking where the knee gets stuck mid-motion, and swelling that builds over several hours rather than instantly.
PCL Injuries
The posterior cruciate ligament (PCL) sits at the back of the knee and keeps the shinbone from sliding too far backward. It’s tougher than its more famous counterpart, the ACL, so PCL tears are less common. They typically happen from a direct blow to the front of the shin, like hitting the dashboard in a car accident, or from landing badly after a jump.
PCL injuries often feel different from other knee problems. The pain tends to worsen over time rather than hitting all at once, and the knee can feel unstable or unreliable, especially when going downstairs or decelerating. Providers grade PCL injuries from Grade I (a partial tear with the ligament still intact) through Grade IV (a complete tear involving other structures). Partial tears frequently heal with physical therapy alone, while complete tears sometimes need surgical repair.
Tendon Inflammation
Several tendons cross the back of the knee, and any of them can become inflamed from overuse. Hamstring tendinitis is common in runners and cyclists who ramp up their training too quickly. The pain is usually a dull ache that worsens with activity and improves with rest, and you can often pinpoint it by pressing on the spot where the tendon meets bone just behind the knee.
Unlike a sudden muscle tear, tendinitis builds gradually. You might notice it only after long runs or bike rides at first, then during shorter sessions, and eventually during everyday walking if it goes untreated. Reducing activity volume, icing after exercise, and targeted strengthening exercises resolve most cases over several weeks.
Arthritis
Both osteoarthritis (from wear and tear over time) and rheumatoid arthritis (from the immune system attacking the joint) can produce pain behind the knee. Osteoarthritis is by far the more common of the two. The pain tends to be worst after periods of inactivity, like first thing in the morning, and may ease once the joint warms up with movement.
Arthritis-related posterior knee pain often comes with general stiffness, a grating sensation during movement, and gradual loss of range of motion. It also frequently triggers Baker’s cysts, so you may be dealing with both at once. The key clue is that the pain developed slowly over months or years rather than after a single event.
When the Pain Could Be Something Serious
Rarely, pain behind the knee signals a vascular problem. A blood clot in the deep veins of the leg can cause swelling, warmth, and aching in the calf and behind the knee, usually in one leg only. A popliteal artery aneurysm, which is a bulging of the artery behind the knee, can cause pain along with numbness, coldness, skin paleness, and muscle weakness in the lower leg if the blood supply becomes compromised.
Nerve compression is another less common possibility. The main nerve running through the back of the knee supplies sensation to the sole of your foot. If something presses on it, like a cyst or swelling, you may notice numbness or tingling on the bottom of your foot along with pain behind the knee.
Sudden onset of leg coldness, paleness, numbness, or an inability to move your foot warrants emergency care, as these suggest disrupted blood flow or severe nerve damage that can worsen quickly.
Narrowing Down Your Cause
Paying attention to a few details can help you and your provider figure out what’s going on:
- Pain after a sudden twist or pop: points toward a meniscus tear or muscle strain
- A visible or palpable lump: suggests a Baker’s cyst
- Pain that builds gradually with activity: fits tendinitis or early arthritis
- Instability or the knee giving way: raises concern for a PCL injury
- Swelling, warmth, and redness in one leg: warrants evaluation for a blood clot
- Numbness on the sole of your foot: suggests nerve involvement
Physical examination tests for meniscus tears are reasonably good at detecting the injury. A study of 255 patients found that the most commonly used clinical tests caught meniscus tears 80 to 98 percent of the time, though they also produced a fair number of false positives. MRI remains the gold standard when the diagnosis is uncertain, but many posterior knee problems can be identified through a careful physical exam and a clear description of how the pain behaves.