Pain behind the knees is common and usually comes from one of a handful of causes: a fluid-filled cyst, inflamed tendons, a muscle strain, or arthritis. Less often, it signals a ligament tear or, rarely, a blood clot. The space behind your knee is packed with muscles, tendons, nerves, and blood vessels, so pinpointing the source depends on how the pain started, what makes it worse, and whether you notice any swelling.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most recognizable causes of pain behind the knee. It forms when excess fluid from inside the knee joint drains backward into a small pouch, creating a squishy bump you can sometimes see or feel. The cyst itself is benign and is never cancerous.
Baker’s cysts usually develop because something else is irritating the joint, like arthritis or a cartilage tear that triggers extra fluid production. You might notice stiffness, mild swelling that extends into your calf, and difficulty bending the knee all the way. The bump tends to feel more noticeable when you fully straighten your leg. In many cases a Baker’s cyst shrinks on its own once the underlying knee problem is managed, though larger cysts sometimes need to be drained.
Hamstring and Calf Muscle Strains
The hamstring muscles run down the back of your thigh and attach just below the knee, while the two heads of the calf muscle anchor just above it. A strain in either group can send pain straight into the crease behind the knee, even if the actual tissue damage is a few inches away.
Hamstring tendinitis develops gradually from overuse and typically shows up as a dull ache around the back of the thigh or knee that worsens with stairs, lunges, or sprinting. Athletes who do a lot of start-stop running are at highest risk, particularly sprinters, basketball and soccer players, and dancers. Studies suggest athletes between 16 and 25 have the highest rates of hamstring injuries, though people over 40 and anyone with tight quads or a previous hamstring injury are also vulnerable.
Calf strains tend to come on more suddenly, often with a sharp twinge during a push-off movement. The pain radiates upward into the back of the knee and worsens when you point your toes or rise onto the balls of your feet. Mild strains of either muscle group typically improve within a few weeks with rest, icing, and gentle stretching. More severe tears can take several months.
Meniscus Tears
Each knee has two C-shaped pads of cartilage (menisci) that cushion the joint. A tear in the back portion of either one produces pain that localizes behind the knee, often accompanied by a catching or locking sensation when you try to bend or straighten the leg. Meniscus tears can happen during a sudden twist, an awkward step off a curb, or a sports collision. They also develop gradually in older adults as the cartilage wears thin.
Small tears sometimes heal with rest and physical therapy. Larger tears, especially those that cause the knee to lock, often require a procedure to trim or repair the damaged cartilage.
Posterior Cruciate Ligament (PCL) Injuries
The posterior cruciate ligament sits deep inside the knee and prevents the shinbone from sliding too far backward. It takes a lot of force to injure it. The two classic scenarios are falling hard onto a bent knee and hitting the front of the knee against a dashboard in a car accident.
A PCL tear causes pain and swelling behind the knee, along with a vague feeling of instability, as if the knee might give out during walking or going downstairs. Partial tears are often managed with bracing and months of focused strengthening. Complete tears in active people sometimes require surgical reconstruction, with a full return to sports taking six months to a year.
Arthritis
Osteoarthritis and rheumatoid arthritis can both produce pain that wraps around to the back of the knee. The joint lining becomes inflamed, excess fluid builds up, and the swelling presses into the popliteal space. Morning stiffness that eases after 15 to 30 minutes of movement is a hallmark of osteoarthritis, while rheumatoid arthritis tends to cause longer-lasting stiffness and affects both knees at once. Arthritis-related posterior knee pain generally worsens over weeks or months rather than appearing overnight, and it often coexists with a Baker’s cyst.
Popliteus Tendinopathy
The popliteus is a small muscle buried in the back of the knee that helps “unlock” the joint when you start to bend it from a fully straight position. Its tendon can become inflamed from overuse, particularly in runners who train on hills or banked surfaces. Downhill running is a consistent trigger because the popliteus has to work overtime to decelerate your body weight on each step.
The pain is sharp, located on the outer-back corner of the knee, and often accompanied by tenderness and sometimes a crackling sound when the tendon moves. Flat feet (overpronation) can contribute because the altered foot position rotates the shinbone in a way that stresses the tendon further. Rest, temporary avoidance of hills, and correction of foot mechanics with supportive shoes or insoles usually resolve it.
Nerve-Related Pain
The large sciatic nerve splits into two branches right behind the knee. If either branch is compressed or irritated in that space, you may feel burning, tingling, or shooting pain that radiates up into the thigh or down into the calf. This type of pain can be triggered by prolonged standing, full extension of the knee, or direct pressure on the back of the knee. It is far less common than muscular or joint causes, but the tingling and radiating quality of the pain distinguishes it from a simple strain or cyst.
When It Could Be a Blood Clot
A deep vein thrombosis (DVT) in the popliteal vein behind the knee can mimic a Baker’s cyst or muscle strain, which is why it’s worth knowing the warning signs. DVT typically causes persistent leg swelling, cramping or soreness that starts in the calf, skin that turns red or purple, and a noticeable warmth in the affected leg. It can also occur without obvious symptoms. Risk is higher after surgery, long flights, bed rest, or in people who take hormonal medications. DVT is a medical emergency because the clot can travel to the lungs. If you have unexplained calf swelling along with warmth and skin color changes, seek immediate evaluation.
Sorting Out the Cause
A few details help narrow down what’s going on. Pain that came on suddenly during activity points toward a muscle strain, meniscus tear, or ligament injury. Pain that built up gradually over days or weeks suggests tendinitis, arthritis, or a growing cyst. A visible bump behind the knee is almost always a Baker’s cyst. Tingling or shooting pain suggests nerve involvement. And pain combined with diffuse calf swelling and warmth raises the possibility of a clot.
Most causes of posterior knee pain respond well to initial rest, ice, and gentle range-of-motion exercises. Avoiding deep squats, heavy stair climbing, and high-impact activity for a week or two gives inflamed tissues a chance to settle. If the pain persists beyond two to three weeks, worsens, or is accompanied by significant swelling, locking, or instability, imaging (usually an ultrasound or MRI) can identify the specific structure involved and guide the next steps.