Pain behind the knee when you bend it usually comes from a handful of common conditions: a Baker’s cyst, a meniscus tear, hamstring tendonitis, a ligament injury, or arthritis. The specific cause depends on how the pain started, exactly where you feel it, and whether you notice swelling, clicking, or instability along with it.
The back of your knee is a compact space where tendons, ligaments, a small muscle, and a fluid-filled joint capsule all converge. Bending your knee compresses that space, which is why so many different problems announce themselves during flexion. Here’s how to start narrowing down what’s going on.
Baker’s Cyst
A Baker’s cyst is a fluid-filled pouch that forms in the hollow behind your knee. It develops when something damages the joint (often arthritis or a cartilage tear) and extra fluid drains backward into a small pocket. The cyst typically feels firm when your leg is straight and softens when you bend it, but the swelling itself creates a sense of fullness or tightness that gets worse the deeper you flex. Some people describe it as a water balloon pressing against the back of the knee.
Small cysts can be painless and go unnoticed. Larger ones make squatting, kneeling, or even sitting with your knee bent for a long time uncomfortable. If a Baker’s cyst ruptures, fluid leaks into the calf, causing sudden sharp pain and swelling that can mimic a blood clot. Treatment focuses on the underlying problem that created the extra fluid in the first place. Draining the cyst alone usually leads to it refilling.
Meniscus Tears
Each knee has two C-shaped pieces of cartilage (menisci) that cushion the joint. Tears in the back portion of either one are a frequent source of pain during bending, because flexion pinches the torn edge between the bones. Swelling often makes it physically hard to bend the knee past a certain point.
A tear in the back of the outer meniscus commonly produces a snapping, clicking, or clunking sound when the knee moves through its range. A “ramp lesion,” which is a tear at the very back of the inner meniscus, tends to cause deeper, less obvious pain and is easier to miss on imaging. Meniscus tears can happen during a sudden twist or pivot, but they also occur from everyday slips, trips, or simply age-related wear. If your knee catches, locks, or gives way along with the pain behind it, a meniscus tear is high on the list.
Hamstring Tendonitis
Your hamstring muscles run down the back of your thigh and attach to bones around the knee and lower leg via tendons. When those tendons become inflamed near their lower attachment points, the result is a dull ache around the back of the knee that sharpens when you bend or straighten your leg against resistance. Stiffness during walking is common, and the pain often builds gradually rather than appearing after a single event.
Hamstring tendonitis is especially common in runners, cyclists, and anyone who rapidly increases their activity level. You may also notice leg stiffness first thing in the morning that loosens up after a few minutes of movement, then returns after prolonged sitting. Sharp pain with a sudden stretch of the hamstring suggests a more acute strain rather than chronic tendon irritation.
PCL Injury
The posterior cruciate ligament (PCL) sits at the back of the knee and prevents the shinbone from sliding too far backward. The classic injury mechanism is a direct blow to the front of a bent knee: a knee hitting the dashboard in a car accident, or a football player falling forward onto a flexed knee. Severe twisting or hyperextension injuries can also damage it.
A PCL tear often causes a vague, deep ache behind the knee rather than the dramatic swelling and instability people associate with ligament injuries. One hallmark sign is that the injured knee appears to sag backward when bent, and it may slide too far back when flexed beyond 90 degrees. Mild PCL sprains can heal with rehabilitation, but complete tears sometimes require surgical reconstruction, especially if the knee feels unstable going downstairs or decelerating while running.
Popliteus Tendinopathy
There’s a small, often-overlooked muscle behind the knee called the popliteus. Its job is to “unlock” the knee from a fully straight position so you can start bending it, and it stabilizes the back of the joint during flexion. When its tendon becomes irritated, you feel a sharp pain on the outer-back part of the knee.
Popliteus problems are particularly aggravated by downhill walking or running, because the muscle works overtime to decelerate your body weight on slopes. You may hear a crackling sound when the tendon moves, and bending the knee even 15 to 30 degrees against resistance can reproduce the pain. Continued loading without rest leads to scar tissue formation, which makes every subsequent activity more painful. This is one of the less commonly diagnosed causes, but it’s worth considering if your pain is specifically on the outer-back corner of the knee and worsens on declines.
Arthritis
Both osteoarthritis (wear-and-tear breakdown of cartilage) and rheumatoid arthritis (an autoimmune condition) can cause posterior knee pain. With arthritis, bending hurts because the protective cartilage that normally cushions bone surfaces has thinned or worn away, leaving bone-on-bone contact or inflamed joint lining. The pain tends to be worse after periods of inactivity and improves slightly with gentle movement, then flares again with prolonged use. Morning stiffness lasting more than a few minutes is typical. Arthritis also frequently causes the excess joint fluid that leads to Baker’s cysts, so the two conditions often show up together.
How to Tell These Apart
Location and onset are your best clues. A Baker’s cyst creates visible or palpable swelling in the center of the hollow behind the knee. Meniscus tears tend to produce clicking or locking along with pain that started during a twist or awkward step. Hamstring tendonitis aches broadly across the back of the thigh and knee and builds over days or weeks. PCL injuries follow a clear impact event and may make the knee feel loose. Popliteus tendinopathy targets the outer-back corner and flares specifically on downhill terrain.
During a physical exam, clinicians use specific maneuvers to isolate the problem. The McMurray test, for instance, involves bending and rotating the knee while pressing along the joint line to detect meniscus tears. A pop, click, or pain during this movement points toward torn cartilage. For ligament injuries, the knee is bent to about 20 to 30 degrees and the shinbone is pulled forward or pushed backward to check for abnormal looseness. Imaging, usually an MRI, confirms what the physical exam suggests.
Stretches and Early Management
For muscular and tendon-related causes, a few targeted stretches can reduce tension in the structures behind the knee. None of these should increase your pain. If they do, stop and get an evaluation before continuing.
Supine hamstring stretch: Lie on your back with both knees bent. Lift the affected leg, clasp your hands behind your thigh just below the knee, then slowly straighten the leg and gently pull it toward your head until you feel a stretch at the back of the thigh and behind the knee. Hold for 30 seconds. Do 2 to 3 repetitions, 4 to 5 days per week.
Heel cord stretch: Stand facing a wall with your unaffected leg forward and slightly bent. Place the affected leg straight behind you with the heel flat on the floor and toes angled slightly inward. Press your hips forward toward the wall until you feel a stretch through the calf and into the heel. Hold 30 seconds, relax 30 seconds, and repeat for 2 sets of 4. This helps because tight calf muscles pull on structures that cross the back of the knee.
Standing quad stretch: Hold a wall or chair for balance, bend your affected knee, and grasp your ankle to pull your heel toward your buttock. Hold 30 to 60 seconds. Tight quads alter how the kneecap tracks and can shift stress to the back of the joint. Do 2 to 3 repetitions, 4 to 5 days per week.
Beyond stretching, reducing the aggravating activity (not eliminating all movement) is the most effective early step. Ice for 15 to 20 minutes after activity helps with acute inflammation. Compression wraps or sleeves can limit swelling. If the pain hasn’t improved after two to three weeks of consistent self-care, or if you have significant swelling, locking, instability, or pain that wakes you at night, that’s a signal to get imaging and a hands-on evaluation.