Pain behind the knee usually comes from soft tissue problems: a fluid-filled cyst, a strained muscle or tendon, or irritation from overuse. Less commonly, it signals a ligament injury, nerve compression, or a blood clot. The location, timing, and type of pain narrow down the cause quickly, and most cases improve without surgery.
Baker’s Cyst
A Baker’s cyst is the single most common reason for a noticeable lump or aching fullness behind the knee. It forms when the knee produces too much synovial fluid, the natural lubricant inside the joint. That excess fluid can only drain in one direction, out the back of the joint, where it collects in a small sac called a bursa. The result is a soft, fluid-filled bulge in the crease behind your knee.
You may feel stiffness, mild pain, and difficulty bending the knee all the way. These symptoms typically get worse after standing for a long time or after physical activity. Some people also notice swelling that extends down into the calf or up into the thigh.
A Baker’s cyst is almost always a secondary problem. It develops because something else is irritating the knee, most often arthritis or a meniscus tear. Treating the underlying issue usually resolves the cyst on its own. In some cases, a doctor drains the fluid with a needle or injects a corticosteroid to reduce inflammation, but the cyst often returns if the root cause isn’t addressed.
Hamstring Strain or Tendon Irritation
Three hamstring muscles run down the back of your thigh and attach near the back of the knee. When one of these tendons becomes irritated or partially torn, you’ll feel an aching pain on the inner back side of the knee, often with tenderness when you press just below the joint line. A sudden strain typically causes a sharp, tearing sensation during activity, followed by tightness, weakness, and reduced range of motion.
Hamstring tendon problems are common in runners, sprinters, and anyone who suddenly ramps up their activity level. Mild strains heal within a few weeks with rest, ice, and gradual strengthening. More severe tears can take several months.
Calf Muscle Injury
The large calf muscle (gastrocnemius) actually crosses the back of the knee joint before running down to the heel. Because of this, a strain at the top of the calf can feel like it’s coming from behind the knee itself. The key difference from a hamstring problem: calf pain is worse when you rise onto your toes or resist bending the knee against pressure, while hamstring pain is worse when you try to straighten the leg against resistance.
This type of injury is especially common in sports that involve sudden pushing off, like tennis, basketball, or sprinting. Many people describe feeling a “pop” at the moment of injury.
Popliteus Tendon Problems
The popliteus is a small muscle deep behind the knee that helps unlock the joint when you start to bend it from a straight position. Overuse of this muscle, particularly from running or training on hills or banked surfaces, can inflame the tendon and cause pain at the outer back corner of the knee. People with flat feet or excessive inward rolling of the foot are more prone to this because the foot position changes how forces travel through the knee.
Popliteus tendinopathy causes tenderness along the outer back of the knee, sometimes with mild swelling and redness. You may notice a crackling sound when moving the joint. Pain typically worsens when walking downhill or decelerating, since that’s when the popliteus works hardest to control knee rotation.
Posterior Cruciate Ligament Injury
The posterior cruciate ligament (PCL) sits deep inside the knee and prevents the shinbone from sliding too far backward. Injuring it requires significant force: a direct blow to the front of a bent knee (the classic “dashboard injury” in a car crash), a hard fall onto a bent knee during sports, or a severe twisting or hyperextension event.
Unlike an ACL tear, which usually announces itself with a dramatic pop and immediate swelling, a PCL injury can be more subtle. You may notice deep pain behind the knee, instability when going downstairs, and a sense that the knee “sags” backward when bent. The injured knee may slide too far backward, especially when bent beyond 90 degrees. Partial PCL tears often heal with bracing and rehabilitation, but complete tears sometimes require surgical reconstruction.
Meniscus Tear
The meniscus is a C-shaped piece of cartilage that acts as a shock absorber between the thighbone and shinbone. A tear in the back portion of the meniscus can produce pain specifically behind the knee, along with catching, locking, or a clicking sensation during movement. Meniscus tears happen from twisting the knee under load (common in sports) or simply from wear and tear as the cartilage weakens with age.
Small tears often improve with rest and physical therapy over four to six weeks. Larger or mechanically unstable tears, especially those that cause the knee to lock, may need arthroscopic surgery to trim or repair the damaged cartilage.
Nerve Compression
The space behind the knee, called the popliteal fossa, is a narrow corridor packed with nerves, arteries, and veins. Swelling, a cyst, or even tight surrounding muscles can compress the tibial nerve as it passes through this area. Nerve compression causes tingling, burning, or numbness that radiates into the calf. In severe or long-standing cases, the muscles and nerves in the lower leg can sustain lasting damage.
Popliteal Artery Entrapment
This is a rare but important cause of posterior knee pain, mostly affecting young, otherwise healthy and physically active people. In popliteal artery entrapment syndrome, the calf muscle is positioned in a way that pinches the main artery behind the knee during exercise. The result is cramping, aching, or numbness in the calf that comes on with activity and fades with rest, mimicking other exercise-related conditions.
Because these patients look healthy on a standard exam, diagnosis can be tricky. Testing typically involves measuring blood pressure in the ankle before and after exercise on a treadmill. In someone with this condition, ankle pressure drops during exertion because the artery is being compressed. Ultrasound or specialized imaging done while flexing the foot can confirm whether the muscle is trapping the artery.
Deep Vein Thrombosis
A blood clot in a deep leg vein is the one cause of posterior knee pain that can become a medical emergency. Deep vein thrombosis (DVT) causes leg swelling, pain or cramping (often starting in the calf), skin color changes (reddish or purplish), and a feeling of warmth in the affected leg. Some clots produce no noticeable symptoms at all.
The danger is that a clot can break free and travel to the lungs, causing a pulmonary embolism. Warning signs of that complication include sudden shortness of breath, chest pain that worsens with deep breathing or coughing, dizziness, fainting, a rapid pulse, or coughing up blood. Risk factors for DVT include prolonged immobility (long flights, bed rest after surgery), recent surgery, obesity, pregnancy, and use of hormonal birth control.
How to Narrow Down Your Cause
A few patterns help distinguish these conditions from each other:
- Gradual onset with a visible lump: most likely a Baker’s cyst, especially if you have a history of arthritis or knee problems.
- Sharp pain during a specific movement or sport: points toward a hamstring strain, calf injury, or meniscus tear.
- Pain that worsens going downhill or on banked surfaces: suggests popliteus tendon irritation.
- Deep instability after a high-force impact: raises concern for a PCL injury.
- Tingling, burning, or numbness into the calf: suggests nerve compression or artery entrapment.
- Swelling, warmth, and skin color change without an injury: warrants urgent evaluation for a blood clot.
Most posterior knee pain improves with rest, ice, gentle stretching, and gradual return to activity over a few weeks. Persistent pain, significant swelling, locking, instability, or any signs of a blood clot all warrant imaging and professional evaluation to rule out something that needs more targeted treatment.