Pain behind the knee usually points to a soft tissue problem: a fluid-filled cyst, a strained muscle, or irritation in a tendon or ligament. Less commonly, it signals something more serious like a blood clot or a vascular issue. The location, quality of pain, and how it started are the biggest clues to what’s going on.
Baker’s Cyst: The Most Common Culprit
A Baker’s cyst is a pocket of fluid that forms in the hollow behind your knee. Your knee joint naturally produces a lubricating fluid that reduces friction when you bend and straighten your leg. When something irritates the joint, like arthritis or a cartilage tear, the knee overproduces that fluid. The excess drains backward into a small sac behind the knee, causing it to swell into a visible, sometimes golf-ball-sized bump.
The telltale sign is a feeling of pressure or fullness behind the knee that gets worse when you fully bend or straighten the leg. Some people describe it as a bubble that won’t go away. The cyst itself isn’t dangerous, but it can rupture. A ruptured Baker’s cyst sends fluid down into the calf, causing sudden swelling, warmth, and bruising that looks and feels a lot like a blood clot. That overlap matters, because a blood clot needs urgent treatment and a ruptured cyst doesn’t. Ultrasound can reliably distinguish the two, and studies show it’s just as accurate as MRI for detecting Baker’s cysts while being faster, cheaper, and more widely available.
PCL Injuries
The posterior cruciate ligament (PCL) sits at the back of the knee, connecting your thighbone to your shinbone. It’s most often injured by a direct blow to the front of a bent knee. Think: your knee hitting the dashboard in a car accident, or landing hard on a bent knee during sports. It can also tear from a twisting or hyperextension injury.
PCL tears tend to cause deep, aching pain behind the knee along with a feeling of instability. Your knee may feel like it could give out, especially going downstairs or decelerating from a run. On examination, the injured knee often appears to sag backward compared to the uninjured side, and the shinbone slides too far back when the knee is bent past 90 degrees. Mild PCL sprains often heal with physical therapy, but complete tears sometimes require surgical reconstruction.
Popliteus Muscle Strain
The popliteus is a small but important muscle at the back of the knee. Its job is to “unlock” the knee from a fully straightened position by rotating the shinbone slightly inward as you start to bend. Without it, the first few degrees of bending would feel stuck.
When this muscle is strained, you’ll notice pain at the back of the knee and along the outer joint line. Weight-bearing hurts, and stairs are particularly uncomfortable. You may also notice that your lower leg rotates outward in an unnatural way when you bend the knee. Swelling and limited range of motion are common. This injury is easy to overlook because the muscle is small and deep, but it responds well to rest, ice, and targeted strengthening exercises.
Meniscus Tears and Tendinitis
A torn meniscus, the C-shaped cartilage cushion inside the knee, can refer pain to the back of the joint depending on which part of the cartilage is damaged. These tears often happen during a sudden twist, whether from a sports collision or something as ordinary as catching your foot on a rug. You’ll typically notice clicking or popping along with the pain, and the knee may catch or lock during movement.
Tendinitis behind the knee involves inflammation in one of the hamstring tendons where they attach near the back of the joint. It develops gradually from overuse, running, or cycling with poor bike fit. The pain is usually sore and aching, worse with activity and better with rest, and you may feel warmth over the irritated tendon.
Arthritis
Both osteoarthritis and rheumatoid arthritis can produce pain behind the knee. Osteoarthritis wears down the cartilage inside the joint over years, and the resulting inflammation often triggers excess fluid production, which can cause or worsen a Baker’s cyst. Rheumatoid arthritis attacks the joint lining itself, producing swelling and stiffness that radiates throughout the knee, including the back.
Arthritis-related pain tends to be worse in the morning or after sitting for a long time, then loosens up with gentle movement. Over time, you may notice the knee becoming stiffer, more swollen, or harder to fully straighten.
Nerve Compression
The tibial nerve runs through the space behind the knee, and when it’s compressed or irritated, it can cause pain in that area along with some distinctive nerve symptoms. The hallmark is weakness in the muscles that point your foot downward and curl your toes, combined with numbness or tingling on the sole of your foot. If your behind-the-knee pain comes with these sensory changes in the foot, nerve involvement is likely part of the picture.
Vascular Problems
Rarely, pain behind the knee comes from a blood vessel issue rather than a muscle or joint problem. A popliteal artery aneurysm, a bulge in the artery running behind the knee, can cause knee pain, swelling, and a pulsing feeling you can detect by pressing into the back of the joint. The bigger risk is that these aneurysms can produce blood clots that cut off circulation to the lower leg. Warning signs of reduced blood flow include skin that feels cold or changes color, numbness, loss of the pulse behind the knee, and inability to move the foot.
Deep vein thrombosis (DVT) is another vascular concern. A blood clot forming in the veins behind the knee causes pain, swelling, and warmth in the calf and lower leg. The skin may appear red or darkened. On its own, a DVT is treatable, but if a piece of the clot breaks off and travels to the lungs, it becomes a pulmonary embolism, which causes chest pain and breathlessness and requires emergency care.
When the Pain Needs Urgent Attention
Most behind-the-knee pain improves with rest, ice, and time. But certain combinations of symptoms point to something that can’t wait. Sudden, severe pain with immediate swelling and an inability to bear weight after an injury suggests a fracture, dislocation, or major ligament rupture. A hot, swollen, red knee with fever raises concern for a joint infection. And leg swelling paired with warmth, skin color changes, chest pain, or shortness of breath warrants emergency evaluation for a blood clot.
For pain that’s milder but lingers beyond a couple of weeks, or that comes with clicking, locking, or a visible bump, imaging can help sort out the cause. Ultrasound is a practical first step for suspected cysts, fluid collections, and soft tissue problems. MRI provides a more detailed look when a ligament tear, meniscus injury, or cartilage damage is suspected.