Pain behind the knee usually comes from soft tissue problems: a fluid-filled cyst, a strained muscle, or irritation from a meniscus tear. Less commonly, it signals a blood vessel or circulation issue that needs prompt attention. The location narrows things down because only a few structures sit in that space, a small diamond-shaped area doctors call the popliteal fossa.
Baker’s Cyst: The Most Common Cause
A Baker’s cyst is a pocket of joint fluid that bulges out the back of your knee. It forms because the knee joint can only drain excess fluid in one direction, toward the back. When something damages the joint or surrounding tissue, the body produces extra fluid, and that buildup collects into a sac behind the knee. The two most common triggers are arthritis and knee injuries.
With arthritis, the gradual breakdown of cartilage causes chronic low-grade inflammation that keeps fluid production elevated. With an acute injury, like a ligament sprain or meniscus tear, the sudden swelling pushes fluid into the back of the joint. Either way, you’ll typically notice a soft, egg-shaped lump behind your knee that feels tight when you fully bend or straighten your leg. Small cysts sometimes cause no symptoms at all and are found incidentally on imaging. Larger ones create a constant aching pressure, especially after prolonged standing or walking.
Occasionally a Baker’s cyst ruptures, releasing fluid down into the calf. This causes sudden sharp pain and swelling in the calf that can look and feel alarmingly similar to a blood clot, which is why ruptured cysts often prompt an urgent ultrasound to rule out something more serious.
Meniscus Tears
Each knee has two C-shaped pads of cartilage (menisci) that cushion the joint. The back portion of the inner meniscus sits close to the rear of the knee, so tears in that area often produce pain that localizes behind or deep inside the joint. These tears are common in athletes who twist or pivot, but they also happen gradually in older adults as the cartilage weakens with age.
The hallmark signs of a meniscus tear include a popping sensation at the time of injury, swelling that develops over several hours, pain when twisting or rotating the knee, and a feeling that the knee catches or locks when you try to straighten it. Some people describe the knee “giving way” during walking or going downstairs. If the torn fragment flips into the back of the joint, the pain concentrates behind the knee and worsens with deep bending, like squatting or kneeling.
Hamstring and Popliteus Muscle Strain
Three hamstring muscles run down the back of your thigh and attach by tendons just below and behind the knee. A strain at or near those attachment points causes pain that centers on the back of the knee rather than higher up in the thigh. This is especially common in runners, sprinters, and anyone who suddenly increases their training volume. The pain typically worsens when you bend your knee against resistance or stretch the hamstring by straightening your leg with your foot flexed.
A smaller, deeper muscle called the popliteus also sits directly behind the knee joint. Its job is to “unlock” the knee from a fully straight position so you can begin bending it. When this muscle or its tendon is strained, the pain sits right in the crease behind the knee and flares during downhill running or walking down stairs, both of which load the popliteus heavily. Because this muscle is deep and small, popliteus injuries are frequently overlooked or mistaken for other problems. On MRI, a complete tear can look like a small mass surrounded by fluid, which sometimes gets misidentified as a cyst.
Popliteal Artery Entrapment
This is a rare but important cause of behind-the-knee pain, affecting less than 1% of people. It occurs most often in young athletes between 15 and 25 years old, particularly runners. In this condition, the main artery behind the knee gets compressed by surrounding muscle during exercise. The repeated compression causes the artery to spasm, reducing blood flow to the calf and foot.
The reduced blood flow creates a buildup of metabolic waste in the muscles, producing heaviness, achiness, tiredness, and sometimes numbness in the calf and foot during activity. Symptoms typically improve within three to five minutes of stopping exercise. That pattern of pain with exertion and relief with rest is the key feature. Without treatment, the problem tends to worsen over time: pain starts sooner into a workout, and recovery takes longer afterward.
Diagnosis involves checking the pulse in your foot and behind your knee at rest versus during exercise. At rest, pulses are usually normal. During activity, or when pushing your foot up and down against resistance, the pulse diminishes or disappears.
Blood Clots in the Leg
Deep vein thrombosis, a blood clot in one of the leg’s deep veins, can cause pain behind or below the knee along with swelling, warmth, and redness. This is the cause you don’t want to miss, because a clot can break loose and travel to the lungs.
Certain factors raise the risk significantly: recent surgery (especially in the prior 12 weeks), being bedridden for more than three days, active cancer treatment, a history of previous clots, or recent immobilization of the leg in a cast or brace. Clinicians look for specific physical signs to gauge probability. Swelling in one calf that measures more than 3 centimeters larger than the other is a red flag, as is pitting edema (when pressing the skin leaves a dent) confined to the symptomatic leg. Visible surface veins that weren’t there before also raise suspicion.
If you have behind-the-knee pain with new calf swelling on one side, especially after a period of immobility or surgery, that combination warrants same-day evaluation. An ultrasound can confirm or rule out a clot quickly.
How to Tell These Apart
The pattern of your pain gives the best initial clue. A Baker’s cyst produces a tight, pressure-like ache that worsens at the extremes of bending or straightening, and you can often feel a lump. A meniscus tear causes catching, locking, or a sensation that something is mechanically wrong inside the joint. Hamstring or popliteus strains hurt with specific movements, like bending against resistance or walking downhill, and often trace back to a clear moment of overexertion.
Popliteal artery entrapment has the most distinctive pattern: symptoms appear only during exercise and vanish a few minutes after stopping. Blood clots tend to cause constant pain with visible swelling that doesn’t go away with position changes or rest, and they often develop after a clear risk event like surgery, travel, or prolonged bed rest.
Pain that’s been present for weeks, worsens gradually, and responds to rest and gentle stretching is less likely to be dangerous. Pain that appears suddenly with significant swelling, skin color changes, or warmth in one leg needs urgent attention regardless of what you think is causing it.