Pain in the back of your knee most commonly comes from a Baker’s cyst, a meniscus tear, hamstring tendon irritation, or osteoarthritis. Less often, it signals a blood clot or a vascular problem that needs prompt attention. The back of the knee (called the popliteal fossa) is a compact space where muscles, tendons, blood vessels, and nerves all converge, so several different structures can be the source.
Baker’s Cyst
A Baker’s cyst is the single most common cause of posterior knee pain. It forms when excess joint fluid gets pushed out of the knee and pools in a pocket between two muscles behind the joint. The fluid becomes trapped there because the surrounding muscles act like a one-way valve: movement pushes fluid into the pocket, but it can’t easily flow back.
In adults, Baker’s cysts almost always develop alongside another knee problem. Degenerative meniscus tears are one of the most frequent triggers. Osteoarthritis, rheumatoid arthritis, cartilage injuries, and ligament tears can all produce the excess fluid that feeds the cyst. In children, these cysts tend to form on their own without an underlying injury.
A Baker’s cyst typically feels like tightness or fullness behind the knee, and it’s often more noticeable when you’re standing with your leg fully straight. Bending the knee to about 45 degrees usually relieves the tension. Pain tends to increase with activity and can limit how far you bend or straighten the joint. If the cyst gets large enough, it can press on nearby veins and cause swelling in your lower leg.
In some cases, a Baker’s cyst ruptures. When that happens, you’ll feel a sharp pain in the knee and calf, followed by swelling, redness, and a sensation like warm water running down your calf. A ruptured cyst can look and feel very similar to a blood clot, so it needs medical evaluation to tell the two apart.
Meniscus Tears
Each knee has two crescent-shaped pads of cartilage called menisci that cushion the joint. The back portion of the inner (medial) meniscus is relatively immobile compared to the rest, which makes it especially vulnerable to tearing. Posterior horn tears of the medial meniscus are strongly associated with older age, female sex, and higher body weight. They also tend to come with more cartilage wear than other types of meniscus tears.
A posterior meniscus tear typically causes pain at the back or inner-back part of the knee that worsens with twisting, squatting, or deep bending. You may feel a catching or clicking sensation, and the knee can feel like it momentarily locks. Tenderness along the joint line is a hallmark finding. A torn meniscus can also generate the excess fluid that leads to a Baker’s cyst, meaning you can end up with both problems at once.
Physical therapy is the first-line approach for degenerative meniscus tears, and research shows that both exercise therapy and surgery produce comparable self-reported outcomes for many patients. Larger, more complex tears, particularly in younger patients, may benefit from earlier surgical repair.
Hamstring and Calf Muscle Problems
Your hamstring muscles run down the back of your thigh and attach via tendons just behind and below the knee. The calf muscle (gastrocnemius) originates from the bottom of the thighbone, right at the back of the knee. Irritation, strain, or tendon inflammation in either group can produce pain that centers on the back of the knee.
Hamstring tendonitis tends to develop gradually in runners, cyclists, or anyone who repeatedly loads the muscles during bending and straightening motions. It causes tenderness when you press on the tendons behind the knee, and you’ll often feel it when bending your leg against resistance or rising up on your toes. A sudden hamstring strain, by contrast, comes on sharply during a sprint, jump, or sudden stretch.
Gastrocnemius strain or irritation feels similar but is centered slightly lower, right at the crease behind the knee or at the top of the calf. Pain typically flares with pushing off during walking, running, or going up stairs.
Osteoarthritis
Knee osteoarthritis is often thought of as front-of-knee pain, but it frequently causes posterior pain too. As the cartilage in the joint wears down, the body produces extra joint fluid, which increases pressure inside the knee. That pressure can stretch and irritate the posterior capsule (the thick tissue lining the back of the joint) and, as described above, push fluid into the popliteal space to form a Baker’s cyst.
If you have osteoarthritis contributing to posterior knee pain, treating the underlying inflammation can help. Corticosteroid injections into the joint have been shown to reduce both the size and wall thickness of associated Baker’s cysts, likely because the medication migrates from the joint space into the cyst itself.
Popliteal Artery Entrapment
This is a less common but important cause, particularly in young, athletic people. The popliteal artery runs through the back of the knee, and in some individuals, surrounding muscle or connective tissue compresses it during activity. The condition can be present from birth due to an anatomical variation, or it can develop over time as the calf muscle grows from activities like running or marching.
The hallmark symptom is cramping or aching in the calf and foot during exercise that goes away with rest, similar to what happens when blood flow is restricted. You might also notice numbness, tingling, skin color changes, or a cool feeling in the lower leg. On examination, the calf muscles are often visibly developed, and foot pulses may disappear when you flex or extend the foot. If untreated, the artery can become permanently narrowed or develop an aneurysm.
Blood Clots
A deep vein thrombosis (DVT) in the popliteal vein can cause pain, swelling, and warmth behind the knee and in the calf. This is the most serious cause on this list because a clot can break loose and travel to the lungs.
Certain factors raise your risk significantly: recent surgery requiring anesthesia, being bedridden for three or more days, active cancer treatment, a history of previous blood clots, or recent leg immobilization in a cast. Physical signs that point toward DVT include swelling of the entire leg, calf swelling that measures at least 3 centimeters more than the other side, pitting edema in just the affected leg, and tenderness along the deep vein pathway. If you have pain behind your knee along with any of these features, especially after surgery or a period of immobility, seek medical evaluation promptly.
How Posterior Knee Pain Is Evaluated
A physical exam is usually the starting point. Your provider will watch you stand and walk, press on specific structures behind the knee, and move the joint through its range of motion. Several hands-on tests help narrow the diagnosis:
- McMurray test: The examiner bends your knee fully, then rotates and straightens it while pressing along the joint line. A pop or click with pain suggests a meniscus tear.
- Fluid tests: Pressing on one side of the kneecap and watching the other side bulge, or tapping the kneecap down to feel it bounce off fluid, confirms excess joint fluid.
- Popliteal palpation: Feeling for a soft, fluid-filled mass behind the knee that shrinks when you bend to 45 degrees is consistent with a Baker’s cyst.
Imaging depends on what the exam suggests. An ultrasound can confirm a Baker’s cyst, check for a blood clot, and assess blood flow in the popliteal artery. An MRI provides detailed views of the menisci, cartilage, and surrounding soft tissues. For suspected artery entrapment, a specialized ultrasound with foot-flexion maneuvers is the typical first screening test.
Exercises That Help
For most muscular and degenerative causes of posterior knee pain, a structured conditioning program over four to six weeks can significantly reduce symptoms. The American Academy of Orthopaedic Surgeons recommends exercising two to three days per week at minimum to maintain strength and range of motion. Here’s what a typical program looks like:
Stretching
Hamstring stretches, done lying on your back with the leg raised, should be held for 30 to 60 seconds per repetition, two to three reps, four to five days a week. Calf stretches (heel cord stretches against a wall) are held for 30 seconds, done in sets of two with four repetitions, six to seven days a week. Quadriceps stretches, 30 to 60 seconds each, round out the flexibility work.
Strengthening
Hamstring curls and half squats form the core of posterior knee strengthening: three sets of 10, four to five days per week, starting with 5-pound weights and working up to 10 pounds. Calf raises (two sets of 10, nearly daily) target the gastrocnemius. Straight-leg raises in both face-up and face-down positions build the quadriceps and hip extensors that support the knee from the front and back. Hip abduction and adduction exercises, three sets of 20, address the lateral stabilizers that influence how force travels through the knee.
Progress gradually. If a specific exercise reproduces sharp pain behind the knee, back off and try a lighter resistance or smaller range of motion before building back up.