Pain behind the knee usually comes from one of a handful of common causes: a fluid-filled cyst, a cartilage tear, a ligament injury, or arthritis irritating the joint. The area behind your knee, sometimes called the popliteal fossa, is where your hamstring and calf muscles meet, and it creases every time you bend your leg. That makes it vulnerable to both overuse problems and acute injuries. The specific pattern of your pain, when it started, and what makes it worse can help narrow down what’s going on.
Baker’s Cyst
A Baker’s cyst is one of the most common reasons for a noticeable ache or tightness behind the knee. It’s a pocket of fluid that forms at the back of the joint, creating a visible bulge you can sometimes feel with your fingers. The cyst itself isn’t the root problem. It develops because something else inside the knee, usually arthritis or a cartilage tear, causes the joint to produce excess fluid. That fluid migrates to the back of the knee and collects there.
Symptoms include swelling behind the knee (and sometimes down into the calf), stiffness that makes it hard to fully bend the leg, and a general feeling of pressure or tightness. Small cysts may cause no symptoms at all and are found incidentally on imaging. Larger ones can make squatting or kneeling uncomfortable. If a Baker’s cyst ruptures, fluid leaks into the calf and can cause sudden sharp pain and swelling that mimics a blood clot, which is why a ruptured cyst sometimes triggers an urgent medical visit. Ultrasound is just as accurate as MRI for diagnosing a Baker’s cyst and can also detect whether one has ruptured.
Treatment focuses on the underlying knee problem rather than the cyst itself. When the source of excess fluid (the arthritis or torn cartilage) is managed, the cyst often shrinks on its own.
Meniscus Tears
Each knee has two C-shaped pads of cartilage called menisci that cushion the joint. Tears in the back portion of these pads are a frequent source of posterior knee pain, especially pain that worsens with twisting, deep squatting, or kneeling. You don’t need a dramatic sports injury to tear a meniscus. Even heavy lifting or a deep squat can do it, particularly as the cartilage weakens with age.
Common signs of a meniscus tear include a popping sensation at the time of injury, swelling or stiffness that develops over a day or two, pain when twisting or rotating the knee, difficulty fully straightening the leg, and a feeling that the knee is locking in place or giving way. Some people describe it as a “catching” sensation when they walk. If the torn fragment flips into the joint space, the knee can lock suddenly in a bent position.
Minor tears often improve with rest, icing, and gradual strengthening exercises over a few weeks. Larger tears, or those causing mechanical locking, may need surgical repair. An MRI is the standard imaging tool for confirming the tear’s location and size.
PCL Injuries
The posterior cruciate ligament (PCL) sits at the back of the knee and keeps the shinbone from sliding too far backward. It’s most commonly injured when the front of a bent knee takes a direct hit. The classic scenario is a car accident where the knee strikes the dashboard, but athletes in football, soccer, basketball, and rugby tear it by falling forward onto a bent knee.
What makes PCL injuries tricky is that they’re often subtle. Symptoms tend to be milder than other ligament tears. You might notice mild swelling, a vague sense that the knee doesn’t feel right, and pain behind the knee that gets worse when you kneel. Some people develop pain at the front of the knee when running or decelerating, but this can take a week or two to appear. Many athletes finish a game on a partially torn PCL because the initial symptoms are so nonspecific. Some never seek treatment at all until the injury shows up on imaging done for a separate knee problem.
Mild PCL sprains generally heal with rest and rehabilitation over several weeks. More severe tears, especially those combined with damage to other ligaments, may require surgical reconstruction.
Arthritis
Both osteoarthritis and rheumatoid arthritis can produce pain behind the knee. Osteoarthritis is the wear-and-tear type that develops gradually, usually after age 50 or following years of heavy physical activity. Rheumatoid arthritis is an autoimmune condition that can affect the knee at any age. Either type can cause the joint to produce excess fluid, which often pools at the back of the knee and may form a Baker’s cyst.
Arthritis pain tends to build slowly over months or years. It’s typically worse after periods of inactivity (first thing in the morning, or after sitting for a long time) and improves somewhat with gentle movement. Stiffness, swelling, and a gradual loss of range of motion are hallmarks. If you’re over 50 and the pain came on without a specific injury, arthritis is one of the most likely explanations.
Muscle and Tendon Strains
The hamstring tendons attach just behind the knee, and the calf muscles (specifically the gastrocnemius) originate there. Strains in either group are a common cause of posterior knee pain, especially in runners, cyclists, and people who’ve recently increased their activity level. The pain is usually sharp during the activity that caused it and sore or achy afterward. You might feel it most when bending the knee against resistance or pushing off while walking uphill.
Minor strains and sprains typically recover within a few days to a couple of weeks with rest, gentle stretching, and gradual return to activity. More severe strains with significant bruising or weakness can take longer and may benefit from guided physical therapy.
Popliteal Artery Entrapment
This is a less common cause, but worth knowing about if you’re a younger athlete with cramping or aching behind the knee that only happens during exercise. Popliteal artery entrapment syndrome (PAES) occurs when the main artery behind the knee gets compressed by surrounding muscle. It primarily affects people under 40, is much more common in young men, and tends to show up in runners, cyclists, and athletes doing high-intensity weight training or circuit work.
The key distinguishing feature is that the pain or cramping comes on during exercise and goes away completely with rest. It can mimic shin splints or compartment syndrome, so it’s often misdiagnosed initially. If you’re young, active, and experiencing exercise-induced calf or posterior knee pain that doesn’t improve with typical treatment, this is worth bringing up with your doctor.
When the Pain Could Be a Blood Clot
Most pain behind the knee is musculoskeletal and not dangerous. But deep vein thrombosis (DVT), a blood clot in one of the deep veins of the leg, can cause similar symptoms and requires urgent treatment. DVT pain typically starts in the calf and presents as cramping or soreness. Other signs include leg swelling (often in just one leg), skin that looks red or purple, and a feeling of warmth in the affected area.
DVT is more likely if you’ve been immobile for a long period (a long flight, bed rest after surgery), are on hormonal birth control, have a clotting disorder, or have recently had surgery. If your posterior knee or calf pain is accompanied by significant one-sided leg swelling, skin color changes, or warmth, get it evaluated promptly. A ruptured Baker’s cyst can look identical to a DVT from the outside, so imaging is often needed to tell the two apart.