Pain behind the knee usually comes from one of a handful of common causes: a fluid-filled cyst, a meniscus tear, a hamstring or calf strain, or a ligament injury. Less often, it signals something vascular like a blood clot. The back of the knee, called the popliteal fossa, is a compact space packed with nerves, blood vessels, tendons, and small fluid sacs, so several different problems can produce pain in the same spot.
Figuring out which structure is involved comes down to how the pain started, what makes it worse, and whether you notice swelling, stiffness, or changes in sensation. Here’s a breakdown of the most likely causes and what each one feels like.
Baker’s Cyst
A Baker’s cyst is the single most common reason people notice a distinct lump or fullness behind the knee. It forms when the knee joint produces too much synovial fluid, the lubricant that normally reduces friction inside the joint. That excess fluid migrates into a small sac (called a bursa) at the back of the knee, causing it to swell into a visible, sometimes golf-ball-sized bulge.
The cyst itself isn’t the root problem. It’s almost always driven by something else going on inside the knee: osteoarthritis, rheumatoid arthritis, or a cartilage tear. The underlying condition triggers overproduction of fluid, and the cyst is where that fluid ends up. You’ll typically feel tightness or aching behind the knee that worsens when you fully straighten or bend the leg. Some cysts cause no pain at all and are discovered by accident. Others rupture, sending fluid down into the calf and causing sudden sharp pain and swelling that can look alarming.
Small, painless Baker’s cysts often resolve on their own once the underlying knee issue is managed. Larger or painful ones may be drained with a needle, but they tend to refill unless the root cause, like arthritis or a tear, is addressed.
Meniscus Tears
Each knee has two C-shaped pads of cartilage (menisci) that sit between the thighbone and shinbone. When the back portion of one of these pads tears, pain concentrates behind or along the side of the knee. Posterior horn tears of the medial meniscus are especially common and are a frequent source of nagging pain in the back of the knee.
Classic signs of a meniscus tear include a popping sensation at the time of injury, swelling or stiffness that develops over the next day or two, pain that flares when you twist or rotate the knee, and a feeling of the knee locking or catching when you try to straighten it. Some people describe the knee “giving way” during walking or turning. In younger people, meniscus tears usually result from a sudden twist during sports. In people over 40, the cartilage has weakened with age and can tear during ordinary movements like squatting or stepping off a curb.
Minor tears sometimes heal with rest, ice, and physical therapy over four to six weeks. Tears that cause locking, persistent swelling, or instability often need surgical repair, typically done through small incisions with a camera (arthroscopy). Recovery after surgery ranges from a few weeks for a simple trim to three or four months when the torn cartilage is stitched back together.
Hamstring and Calf Strains
The hamstring muscles run down the back of the thigh and attach via tendons just behind and below the knee. The two heads of the calf muscle (gastrocnemius) originate just above the back of the knee. A strain in either group can produce pain right in the popliteal area, and the two can be tricky to tell apart.
A distal hamstring strain tends to hurt higher, closer to the crease behind the knee, and flares when you bend the knee against resistance or stretch the leg straight. A proximal calf strain sits slightly lower, behind the top of the shinbone, and worsens when you push off your toes or go up stairs. Both can come on suddenly during a sprint or jump, or build gradually from overuse in runners and cyclists.
Mild strains (a pulled-muscle feeling without significant swelling) typically improve in two to three weeks with rest, gentle stretching, and gradual return to activity. Moderate strains with bruising or noticeable weakness may take six to eight weeks. A complete tear of either muscle or tendon is rare but requires longer rehabilitation and occasionally surgery.
PCL Injuries
The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shinbone from sliding backward. It’s injured far less often than the ACL, but when it does tear, pain and swelling concentrate in the back of the knee. The most common mechanism is a direct blow to the front of the shin while the knee is bent, like hitting the dashboard in a car accident or falling onto a bent knee during sports.
PCL injuries are graded on a scale of one to four. A grade I tear is a partial tear with mild looseness. Grade II involves a partial tear with more noticeable instability. Grade III is a complete rupture, and grade IV means the PCL plus at least one other ligament is damaged. Lower-grade injuries often heal with bracing and physical therapy over two to four months. Higher-grade tears, especially when other ligaments are involved, may require surgical reconstruction.
One distinctive feature of PCL injuries is that the knee may not feel dramatically unstable the way it does with an ACL tear. Instead, you might notice a vague sense of insecurity when walking downhill or on uneven ground, along with persistent aching behind the knee.
Popliteus Tendinopathy
The popliteus is a small, deep muscle behind the knee that helps unlock the joint from a fully straight position and stabilizes it during rotation. When its tendon becomes irritated, you feel pain at the outer-back corner of the knee, right near the bony bump on the outside of the thighbone. It’s most common in runners, hikers who descend steep terrain, and anyone whose activity involves repeated knee bending under load.
Pain from the popliteus tendon is typically pinpointed to the posterolateral (outer-back) area and worsens with downhill walking or running. It may also flare when you sit cross-legged or rotate your lower leg inward. This condition is often mistaken for a lateral meniscus tear or an iliotibial band problem because of the overlapping pain location. Physical therapy focused on strengthening the muscles that control knee rotation, combined with temporary reduction in downhill activity, usually resolves it within four to eight weeks.
Nerve Compression
The tibial nerve runs through the back of the knee just behind the popliteus muscle, making it vulnerable to compression from swelling, cysts, or muscle injuries in the area. When this nerve is pinched, you may feel pain in the calf along with numbness or tingling on the sole of the foot. In more severe cases, you might notice weakness when trying to flex your toes or point your foot downward, and the Achilles reflex can diminish.
The common peroneal nerve also passes through the outer edge of this space before wrapping around the top of the fibula bone. Compression here causes different symptoms: numbness on the top of the foot and difficulty lifting the foot upward (foot drop). Nerve involvement behind the knee is much less common than muscular or joint causes, but the combination of behind-the-knee pain plus changes in foot sensation is a clear signal that a nerve is being affected and warrants prompt evaluation.
Blood Clots: The Red Flag
A deep vein thrombosis (DVT) in the popliteal vein can produce pain, swelling, and warmth behind the knee that closely mimics a Baker’s cyst or a muscle strain. The overlap is significant enough that clinical examination alone often can’t reliably distinguish between the two. A ruptured Baker’s cyst, in particular, can look almost identical to a DVT, with sudden calf swelling and tenderness.
Certain features raise the suspicion for a clot: swelling that affects the entire calf rather than just the knee area, skin that feels warm to the touch or appears reddish, pain that worsens when you flex your foot upward, and risk factors like recent surgery, prolonged immobility, use of hormonal birth control, or a personal or family history of clotting. A DVT is a medical emergency because a clot can break loose and travel to the lungs. If your behind-the-knee pain is accompanied by unexplained calf swelling and warmth, get evaluated the same day. An ultrasound can confirm or rule out a clot quickly.
What the Pain Pattern Tells You
The timing and character of your pain narrows the possibilities considerably:
- Gradual onset with visible swelling behind the knee: most likely a Baker’s cyst, especially if you have known arthritis or a previous knee injury.
- Sharp onset during a twist or pivot: suggests a meniscus tear or, if there was a direct blow, a PCL injury.
- Pain during or after running that fades with rest: points toward a hamstring or calf strain, or popliteus tendinopathy if the pain is on the outer side.
- Pain plus numbness or tingling in the foot: raises concern for nerve compression behind the knee.
- Swelling, warmth, and redness spreading down the calf: needs urgent evaluation for a possible blood clot.
An MRI is the most useful imaging tool for most of these conditions, as it shows soft tissues like cartilage, ligaments, cysts, and muscles in detail. X-rays are helpful mainly to check for arthritis or fractures. For suspected blood clots, an ultrasound is the first-line test and gives results within minutes.