Pain behind the knee usually signals one of a handful of common issues: a fluid-filled cyst, a muscle or tendon strain, or irritation from overuse. Less often, it points to a ligament injury or, rarely, a blood clot that needs immediate attention. The location and type of pain, whether it’s sharp, dull, achy, or accompanied by swelling, narrows down 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 the knee. It creates a noticeable bulge and a feeling of tightness, almost like a water balloon sitting behind the joint. The pain typically worsens with activity or when you fully straighten or deeply bend the knee. You might also notice stiffness that limits your range of motion.
These cysts don’t appear on their own. They’re almost always caused by an underlying knee problem, most commonly osteoarthritis or a cartilage tear. When the knee joint is irritated, it produces excess lubricating fluid, and that fluid can pool into a sac at the back of the knee. Treating the cyst itself often means treating whatever is irritating the joint in the first place. Small cysts sometimes resolve on their own once the underlying inflammation settles. Larger or persistent ones may need to be drained.
In some cases, a Baker’s cyst can press on nearby nerves. When this happens, you might feel tingling or numbness that radiates down toward the sole of the foot or the outer ankle, which is caused by compression of the nerves that run through the back of the knee.
Hamstring Strains
Your hamstring muscles run along the back of your thigh and attach just below the knee. A strain at the lower end of these muscles produces pain right at the back of the knee, especially when you try to bend or straighten the leg against resistance. Hamstring injuries are extremely common in sports that involve sprinting, jumping, or sudden direction changes, including soccer, basketball, and track.
The classic scenario is a sudden, sharp pain during a full sprint. The hamstring contracts while it’s being stretched, and that combination of lengthening and loading is what tears the muscle fibers. But you don’t have to be an athlete. Simply overstretching during a lunge, a sudden slip, or even a forceful kick can strain the lower hamstring enough to cause pain behind the knee. Mild strains typically improve within a few weeks with rest and gradual stretching. More severe tears, where the muscle partially or fully separates from the bone, take significantly longer.
Tendon Problems Behind the Knee
Several tendons cross the back of the knee, and any of them can become inflamed from repetitive stress. One worth knowing about is the popliteus tendon, a small but important stabilizer that wraps from the back of the knee to the outer side of the joint. When this tendon gets irritated, the pain tends to sit at the outer-back corner of the knee and flares up specifically when going downhill, whether running or walking.
Downhill movement forces the popliteus to work harder to control the knee’s position, which is why this injury shows up most often in runners who train on hills or banked surfaces. Flat-footed runners are also more prone to it because overpronation of the foot creates extra rotational stress on the tendon. The area behind and to the outside of the knee will feel tender to the touch, and pain typically increases when you try to rotate your foot outward against resistance.
General tendinitis behind the knee from other tendons follows a similar pattern: gradual onset, pain that worsens with activity and improves with rest, and localized tenderness. It usually responds to reducing the activity that triggered it, icing the area, and slowly building back up.
Ligament Injuries
The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shinbone from sliding backward. Injuring it causes posterior knee pain and a feeling of instability, though isolated PCL injuries can be surprisingly subtle. Many people with a mild PCL tear don’t realize the ligament is damaged because the knee doesn’t always swell dramatically the way it does with a torn ACL.
PCL injuries typically happen from a direct blow to the front of the shin while the knee is bent, like hitting the dashboard in a car accident, or from a fall onto a bent knee during sports. If the knee feels like it’s “giving way” when you walk or change direction, especially on stairs or uneven ground, a ligament problem is worth investigating. A doctor can test for a PCL injury by checking whether the shinbone shifts backward more than it should when pressure is applied.
When Back-of-Knee Pain Is a Red Flag
Most causes of posterior knee pain are musculoskeletal and, while uncomfortable, aren’t dangerous. A deep vein thrombosis (DVT), or blood clot in a leg vein, is the exception. DVT can produce pain, cramping, or soreness that often starts in the calf and may extend to the back of the knee. The key differences from a muscle strain are the accompanying signs: noticeable swelling in the leg (not just the knee), skin that feels warm to the touch, and a change in skin color to red or purple.
DVT becomes life-threatening if the clot breaks free and travels to the lungs, causing a pulmonary embolism. Warning signs of that include sudden shortness of breath, chest pain that worsens when you breathe deeply or cough, a rapid pulse, dizziness, or coughing up blood. If you have leg swelling with warmth and discoloration, especially after a period of immobility like a long flight, surgery, or bed rest, seek emergency care.
What the Type of Pain Tells You
The character of your pain offers real clues. A dull ache with visible swelling and stiffness points toward a Baker’s cyst, particularly if you already have arthritis or a known cartilage problem. Sharp pain that came on suddenly during physical activity suggests a muscle or tendon strain. Pain that creeps in gradually and worsens with specific activities like downhill walking is more consistent with tendon overuse. Instability or a feeling that the knee “shifts” when you move hints at a ligament issue.
Pain accompanied by tingling, numbness, or weakness in the foot or ankle suggests nerve involvement. The nerves that supply the lower leg and foot pass directly through the back of the knee, and anything that compresses them, whether a cyst, swelling from an injury, or scar tissue, can produce sensory symptoms well below the knee itself. If you’re experiencing numbness on the sole of your foot alongside posterior knee pain, that combination is worth mentioning to your doctor because it changes the diagnostic picture.
Managing Pain at Home
For most non-emergency causes, the initial approach is the same: reduce the load on the knee, apply ice for 15 to 20 minutes several times a day, and avoid the specific movement that triggers the pain. Compression with a wrap or sleeve can help control swelling, and keeping the leg elevated when resting encourages fluid to drain away from the joint.
Gentle range-of-motion exercises help prevent stiffness from setting in, especially with Baker’s cysts and mild strains. Straightening and bending the knee through a comfortable range, without forcing it, keeps the joint mobile while it heals. For tendon issues, stretching the calf and hamstring muscles reduces the pulling force on the tendons behind the knee.
Most mild posterior knee pain improves within two to four weeks with these measures. If the pain hasn’t improved after that window, if it’s getting worse instead of better, or if swelling persists, imaging like an ultrasound or MRI can identify what’s going on inside the joint. Baker’s cysts, meniscus tears, and ligament injuries all show up clearly on these scans, which makes the next steps much more targeted.