Pain behind the knee has several possible causes, ranging from fluid-filled cysts and muscle strains to blood clots that need urgent attention. The space behind your knee, called the popliteal fossa, is a tightly packed area containing major blood vessels, nerves, tendons, and muscles. Problems with any of these structures can produce pain in this specific location.
Why the Back of the Knee Is Vulnerable
The hollow behind your knee is a crossroads for some of your leg’s most important structures. The main artery supplying your lower leg passes through here, along with the large vein that returns blood back up toward your heart. The sciatic nerve splits into two branches just above this space, and tendons from both your thigh and calf muscles converge at its borders. The hamstring tendons form the upper boundary, while the two heads of the calf muscle create the lower edges.
Because so many structures overlap in a small area, pain behind the knee can originate from muscles, tendons, nerves, blood vessels, or the knee joint itself. Pinpointing the cause often depends on how the pain started, what makes it worse, and whether you have swelling or other symptoms alongside it.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most common reasons for a noticeable lump or fullness behind the knee. It forms when the knee joint produces too much synovial fluid, the lubricant that reduces friction inside the joint. That excess fluid migrates backward into a small sac called a bursa, causing it to swell into a visible, fluid-filled bulge.
The cyst itself is usually a secondary problem. Arthritis, cartilage tears, and other joint conditions trigger the overproduction of fluid in the first place. Symptoms include swelling behind the knee, stiffness that makes it hard to fully bend the joint, and an aching pain that worsens with activity. Some Baker’s cysts cause no symptoms at all and are discovered incidentally on imaging.
If a Baker’s cyst is drained but the underlying knee problem isn’t addressed, it will usually refill. About one-third of cysts don’t return after drainage alone, but the majority come back unless the root cause, such as a cartilage tear or arthritis, is treated. A ruptured Baker’s cyst can mimic a blood clot, causing sudden calf pain and swelling. Ultrasound is typically used to tell the two apart.
Meniscus Tears
Each knee has two C-shaped cartilage pads called menisci that sit between your thighbone and shinbone, acting as shock absorbers. Tears in the back portion of the meniscus are a frequent source of deep posterior knee pain, especially during squatting or full bending of the knee.
When the back (posterior horn) of the meniscus tears, particularly at its root where it anchors to bone, the cartilage loses its ability to distribute weight evenly. Normally, the meniscus spreads load across the joint like a washer under a bolt. A root tear severs the fibers responsible for this, allowing the torn section to push outward from the joint. This increases pressure directly on the cartilage surfaces of the bones and accelerates wear.
Some people with posterior meniscus tears feel pain along the joint line, while others only notice deep knee pain when bending past 90 degrees. The pain tends to worsen with deeper flexion because the damaged meniscus shifts further backward as the knee bends, irritating surrounding tissues.
Hamstring Strains
The three hamstring muscles run along the back of your thigh from the hip to just below the knee, where their tendons attach to the shinbone. A strain in the lower portion of any of these muscles can produce sharp pain that feels like it’s behind the knee rather than in the thigh.
Hamstring injuries typically cause sudden, sharp pain, sometimes with a popping or tearing sensation. Swelling and tenderness develop within a few hours. Bruising may appear along the back of the leg, and putting weight on the injured leg can be difficult. These injuries are common in sports that involve sprinting, sudden stops, or rapid changes of direction. Unlike joint-related causes, the pain is usually linked to a specific moment of injury rather than a gradual onset.
Popliteus Tendon Problems
The popliteus is a small but important muscle deep behind the knee that helps unlock the joint from a fully straight position and stabilizes it during rotation. When its tendon becomes irritated or inflamed, it produces a persistent ache on the outer-back side of the knee.
Popliteus tendinopathy is particularly common in runners and hikers. The hallmark pattern is pain that worsens with downhill running, descending stairs, and other deceleration movements where the quadriceps fatigue and the popliteus takes on extra stabilizing work. On examination, the area is tender to touch, and rotating the lower leg inward or outward reproduces the pain. Repetitive twisting movements and sudden hyperextension of the knee increase the risk of this injury.
Deep Vein Thrombosis
A deep vein thrombosis (DVT) is a blood clot that forms in one of the deep veins of the leg. The popliteal vein runs directly through the back of the knee, so a clot here can cause pain, swelling, and warmth that feels localized behind the knee or in the calf. This is the most serious cause of posterior knee pain because a clot can break loose and travel to the lungs.
DVT symptoms include leg swelling, cramping or soreness (often starting in the calf), skin color changes such as redness or purple discoloration, and a feeling of warmth in the affected area. In some cases, DVT produces no noticeable symptoms at all. Risk factors include being over 60, obesity, smoking, recent surgery, prolonged immobility (such as a long flight or bed rest), cancer, heart failure, and genetic clotting disorders like factor V Leiden.
Because a ruptured Baker’s cyst and a DVT can look nearly identical, with sudden calf swelling and pain, imaging is essential to distinguish between them. If you develop new leg swelling with warmth, skin color changes, or pain that came on after a period of immobility, seek medical evaluation promptly.
Nerve Compression
The sciatic nerve divides into two major branches just before the popliteal fossa, and compression of either branch can produce symptoms behind the knee that feel different from typical musculoskeletal pain. Instead of a sharp or aching pain, nerve compression tends to cause tingling, burning, or numbness that may radiate into the calf or foot.
In popliteal artery entrapment syndrome, an abnormally positioned muscle compresses not only the artery but sometimes the nearby nerves as well. Symptoms include cold feet after exercise, tingling or burning in the calf, and numbness. This condition is most common in young, athletic individuals and can cause permanent nerve and muscle damage if left undiagnosed.
Posterior Cruciate Ligament Injury
The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shinbone from sliding too far backward relative to the thighbone. Injuries to this ligament, often from a direct blow to the front of the bent knee (like hitting a dashboard in a car accident), produce pain and a sense of instability behind the knee. PCL injuries are less common than ACL tears but can cause chronic posterior knee pain if untreated. Doctors test for PCL damage by pushing the lower leg backward while the knee is bent and checking whether it sags compared to the other side.
Sorting Out the Cause
The pattern of your pain offers useful clues. Pain that worsens with deep squatting or full knee bending points toward a meniscus tear. Pain specifically during downhill walking or stair descent suggests popliteus tendinopathy. A visible lump that increases with activity likely indicates a Baker’s cyst. Sudden onset with a popping sensation during athletics is characteristic of a hamstring strain. And leg swelling with warmth or skin color changes raises concern for a blood clot.
Seek evaluation if your pain makes it difficult to bend or move the knee, persists for more than a few days, involves visible swelling or a palpable lump, or is accompanied by fever or warmth to the touch. Sudden calf swelling after a period of immobility warrants same-day assessment to rule out DVT.