Pain in the back of the knee most often comes from a Baker’s cyst, a meniscus tear, or a muscle strain, though less common causes like blood clots and ligament injuries can also be responsible. The location narrows the list considerably compared to general knee pain, and each cause has a distinct pattern that can help you figure out what’s going on.
Baker’s Cyst
A Baker’s cyst is one of the most common reasons for pain or tightness behind the knee. It forms when excess joint fluid collects in a small sac called a bursa at the back of the knee. You might feel a fluid-filled lump behind the knee that becomes more noticeable when you straighten your leg, along with stiffness that makes it hard to fully bend or extend the joint.
The cyst itself isn’t the root problem. It’s almost always triggered by something else going on inside the knee, most often osteoarthritis, rheumatoid arthritis, or a cartilage tear. These conditions cause the knee to produce too much synovial fluid, the lubricant that normally helps the joint move smoothly. That excess fluid migrates to the back of the knee and pools there. Treating the underlying cause, whether that’s managing arthritis or repairing torn cartilage, typically resolves the cyst over time. Some cysts rupture on their own, which can cause sudden sharp pain and swelling in the calf that mimics a blood clot.
Meniscus Tears
Each knee has two C-shaped pads of cartilage called menisci that cushion the joint. When a tear occurs in the back portion of the inner meniscus (called a posterior horn tear or ramp lesion), pain tends to concentrate behind or along the back of the knee rather than at the front or sides.
The hallmark symptoms are pain that worsens with twisting or squatting, a catching or locking sensation when you try to straighten the leg, and a feeling that the knee might buckle or give way. A fresh tear from a sports injury or awkward pivot usually causes noticeable swelling within hours, and the knee will feel warm to the touch. Older, degenerative tears that develop gradually from wear and tear tend to be less dramatically swollen but can still cause persistent pain behind the knee.
Smaller tears in the back of the meniscus sometimes heal without surgery because that area has better blood flow than other parts of the cartilage. Conservative treatment with rest, ice, and physical therapy typically takes about 6 to 8 weeks for recovery, though this varies depending on the size and location of the tear.
Posterior Cruciate Ligament (PCL) Injury
The posterior cruciate ligament sits deep inside the knee and prevents the shinbone from sliding too far backward. Injuring it causes pain that radiates through the back of the knee, along with swelling that builds steadily over hours. The knee may feel unstable, as though it could give out, and walking becomes difficult.
PCL injuries are rarely caused by a simple misstep. They require significant force: a direct blow to the front of a bent knee (the classic “dashboard injury” in a car accident), a hard fall onto a bent knee during sports, or a severe twisting or hyperextension. If you felt a pop at the time of injury followed by rapid swelling and you can’t bear weight, that combination points toward a ligament tear and warrants prompt evaluation.
Popliteus Muscle Strain
The popliteus is a small muscle tucked behind the knee that most people have never heard of. Its job is to “unlock” the knee joint when you start to bend it from a fully straight position, and it helps stabilize the outer side of the knee during movement. Straining it produces pain at the back and outer edge of the knee, especially when climbing stairs, bearing weight, or bending the knee.
A telltale sign of a popliteus strain is an unnatural outward rotation of the lower leg when you try to bend the knee. You may also notice swelling behind the knee and reduced range of motion. This injury is more common in runners and hikers, particularly on downhill terrain where the muscle works hardest to control knee flexion.
Hamstring Tendon Issues
Your hamstring muscles run down the back of the thigh and attach via tendons just behind and below the knee. When those tendons become irritated or inflamed from overuse, the pain localizes right at the back of the knee. This is especially common in runners, cyclists, and anyone who suddenly increases their activity level. The pain typically worsens during exercise and eases with rest, and you may feel tightness or tenderness when pressing into the tendons behind the knee. Unlike a meniscus tear, there’s usually no locking, catching, or feeling of instability.
Blood Clots in the Leg
A deep vein thrombosis, or blood clot, in the popliteal vein (the major vein running behind the knee) can cause pain, swelling, and warmth in the back of the knee and calf. The skin may change color, turning red or purple. Some blood clots produce no noticeable symptoms at all, which is part of what makes them dangerous.
This is the cause on this list that demands the most urgency. A clot that breaks free can travel to the lungs and become life-threatening. Risk factors include prolonged sitting or bed rest, recent surgery, pregnancy, and use of certain hormonal medications. If you have back-of-knee pain with calf swelling and warmth, particularly if it came on without any injury, get it evaluated quickly.
Popliteal Artery Entrapment
This uncommon condition occurs when a calf muscle compresses the main artery behind the knee during exercise. The signature symptom is cramping or pain in the back of the lower leg that appears during activity and disappears with rest. You might also notice cold feet after exercise, or tingling and numbness in the calf. If the nearby vein also gets compressed, you can develop nighttime calf cramps, swelling, and skin color changes.
Popliteal artery entrapment is most often diagnosed in young male athletes, typically in their late teens or twenties, and is rarely seen in people over 40. It’s worth considering if you’re in that demographic and experiencing exercise-related calf and knee pain that doesn’t match a typical muscle strain.
How to Tell These Apart
The circumstances around your pain offer the strongest clues. Pain that appeared after a specific injury, especially with a popping sound, swelling, and inability to bear weight, points toward a meniscus or ligament tear. A soft, fluid-filled lump you can feel behind the knee suggests a Baker’s cyst. Pain that only shows up during exercise and fades with rest fits a muscle strain, tendon issue, or (in younger athletes) artery entrapment. Swelling with warmth and no clear injury history raises concern for a blood clot.
A doctor evaluating back-of-knee pain will typically test how the joint moves under stress, checking for excess looseness in the ligaments and for the catching sensation associated with meniscus tears. Imaging with an MRI can confirm soft tissue injuries, while ultrasound is often used to identify Baker’s cysts and blood clots. Most causes of posterior knee pain respond well to conservative treatment: rest, physical therapy, and gradual return to activity. Surgery is reserved for large meniscus tears that lock the joint, significant ligament damage, and vascular conditions that don’t resolve on their own.