Pain behind the knee has several possible causes, ranging from a fluid-filled cyst to muscle strain to, less commonly, a blood clot. The space behind your knee, called the popliteal fossa, is packed with muscles, tendons, nerves, and blood vessels, and irritation to any of these structures can produce that deep, nagging ache. The cause usually depends on when the pain started, what makes it worse, and whether you notice swelling or stiffness alongside it.
What’s Behind Your Knee
The back of the knee is a diamond-shaped hollow formed by the muscles of your thigh above and your calf below. The hamstring muscles frame the top, while the two heads of the calf muscle form the bottom edges. Running through this space are the popliteal artery and vein (the main blood supply to your lower leg), the tibial nerve, and the common peroneal nerve, both of which branch off from the sciatic nerve just above the knee. A small, deep muscle called the popliteus also sits here, helping rotate and stabilize the joint.
Because so many structures share this tight corridor, pain behind the knee can feel vague and hard to pin down. The source might be a joint problem, a muscle or tendon issue, a nerve problem, or a vascular condition. Here are the most likely explanations, starting with the most common.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most frequent causes of a noticeable lump or fullness behind the knee. It’s a pocket of joint fluid that collects between the calf muscle and one of the hamstring tendons, forming a soft, grape-to-golf-ball-sized bulge in the back of the knee. It typically feels tight when you straighten your leg and may ache after prolonged standing or walking.
In adults, Baker’s cysts rarely appear on their own. They almost always develop alongside another knee problem that causes the joint to produce excess fluid. The most common trigger is a degenerative meniscus tear, where the damaged cartilage acts like a one-way valve, pushing fluid out of the joint and into the space behind the knee. Osteoarthritis and rheumatoid arthritis are other frequent causes. Baker’s cysts occur most often in adults between 35 and 70 years old.
A small cyst may not need treatment beyond addressing the underlying knee condition. Occasionally, a Baker’s cyst can rupture, sending fluid down into the calf. This causes sudden pain and swelling in the calf that can mimic a blood clot, so it’s worth getting checked if that happens.
Posterior Meniscus Tear
Each knee has two C-shaped pads of cartilage (menisci) that cushion the joint. Tears at the back end of the meniscus, called posterior horn tears, are the most common type and often produce pain specifically behind or along the sides of the knee. You might also notice catching, clicking, or a locking sensation where the knee briefly refuses to straighten. Swelling, stiffness, and a feeling of weakness or instability are other hallmarks.
In younger people, posterior horn tears usually result from a twisting injury during sports. In people over 40, the cartilage can fray and tear gradually with normal activity. If a torn meniscus fragment shifts out of position, it can block the knee from fully extending, which is the “locking” sensation people describe. Because posterior horn tears also drive fluid production, they’re a leading cause of the Baker’s cysts described above.
Hamstring or Calf Muscle Strain
The hamstring tendons attach just above the back of the knee, and the calf muscle originates just below it. A strain to either group can produce pain that feels like it’s coming from directly behind the knee joint. Hamstring strains typically hurt more with bending the knee against resistance or stretching the leg straight, while calf strains tend to flare with pushing off during walking or running.
These injuries often follow a sudden increase in activity, insufficient warm-up, or overtraining. The pain is usually worst in the first few days and improves steadily with rest, gentle stretching, and gradual return to movement over two to six weeks depending on severity.
Popliteus Tendinopathy
The popliteus is a small muscle deep behind the knee that helps rotate the lower leg inward and stabilize the joint, particularly when your foot hits the ground during walking. When its tendon becomes irritated, the pain tends to localize to the outer-back part of the knee. The telltale clue is that downhill walking or running makes it significantly worse, because the popliteus works harder to decelerate your body weight on a slope.
People with popliteus tendinopathy often feel pain when the knee is slightly bent (around 15 to 30 degrees of flexion) and may have difficulty fully straightening the leg due to muscle spasm. It’s less well-known than conditions like runner’s knee, but it’s worth considering if your pain clearly worsens on descents or when twisting on a planted foot.
Deep Vein Thrombosis
A blood clot in the deep veins of the leg, known as DVT, is a less common but more serious cause of pain behind the knee and in the calf. The pain typically feels like a deep cramp or soreness that doesn’t improve with rest the way a muscle strain would. Other signs include swelling in one leg (not both), skin that looks red or purple, and warmth over the affected area. Some DVTs produce no obvious symptoms at all.
Risk factors include recent surgery or hospitalization, prolonged immobility (long flights, bed rest), use of hormonal birth control, pregnancy, cancer, and a personal or family history of clots. DVT is a medical emergency because the clot can break loose and travel to the lungs. If you have new, unexplained calf swelling with pain and warmth in one leg, seek medical evaluation promptly.
Nerve-Related Causes
The tibial nerve passes through the space behind the knee before continuing down into the calf and foot. Compression or irritation of this nerve at or near the knee can produce pain behind the knee along with tingling, numbness, or burning on the sole of the foot and toes. The pain may worsen with prolonged sitting or activities that press on the back of the knee.
Nerve pain tends to feel different from joint or muscle pain. It’s often described as sharp, electric, or burning rather than dull or achy, and it frequently travels below the knee into the lower leg or foot. A Baker’s cyst, swelling from another knee injury, or even tight calf muscles can all press on the tibial nerve and trigger these symptoms.
Popliteal Artery Entrapment
This is a rare condition, but it’s worth knowing about if you’re a young, active person with calf pain during exercise that goes away with rest. Popliteal artery entrapment syndrome (PAES) happens when the popliteal artery gets compressed by surrounding muscle, reducing blood flow to the lower leg during activity. About 85% of cases occur in males, and nearly 60% are diagnosed in athletes in their 20s. Symptoms develop gradually and include cramping calf pain with exertion, foot numbness, and sometimes coolness or color changes in the lower leg. It can be mistaken for shin splints or compartment syndrome.
How to Narrow Down the Cause
A few patterns can help you and your provider figure out what’s going on:
- Pain with a visible bulge that worsens when the knee is straightened suggests a Baker’s cyst.
- Catching, locking, or clicking with pain along the back or sides of the knee points toward a meniscus tear.
- Pain that worsens going downhill but not on flat ground is characteristic of popliteus tendinopathy.
- One-sided swelling with warmth and skin color changes raises concern for DVT.
- Tingling or numbness in the sole of the foot alongside knee pain suggests nerve involvement.
- Calf cramping during exercise that resolves with rest in a young athlete may indicate arterial entrapment.
Physical examination typically includes checking for swelling, testing range of motion, and specific maneuvers to stress the meniscus, ligaments, and tendons. Imaging with ultrasound can identify a Baker’s cyst or blood clot quickly. An MRI provides a detailed look at the meniscus, ligaments, and surrounding soft tissues when the diagnosis isn’t clear from the exam alone.