Pain behind the knee usually comes from one of a handful of common structures: a fluid-filled cyst, a torn meniscus, a strained muscle or tendon, or a ligament injury. Less often, it signals a vascular problem that needs urgent attention. The back of the knee, called the popliteal fossa, is a compact space packed with muscles, tendons, nerves, and blood vessels, so pinpointing the source matters.
Baker’s Cyst
A Baker’s cyst (also called a popliteal cyst) is one of the most recognized causes of posterior knee pain. It’s a small, fluid-filled bump that forms on the back of the knee when something damages the joint and triggers excess fluid production. The two most common triggers are knee arthritis and acute injuries. Arthritis gradually breaks down joint tissue, prompting the body to produce extra fluid that pools behind the knee. An injury can do the same thing more abruptly, with swelling driving fluid into the back of the joint where it balloons into a cyst.
Many Baker’s cysts cause only a feeling of tightness or fullness behind the knee, especially when you straighten your leg fully or bend it deeply. Larger cysts can feel like a grape-sized or golf ball-sized lump you can press on. If a cyst ruptures, it can send fluid down into the calf, mimicking the symptoms of a blood clot: sudden pain, swelling, and redness. That overlap makes imaging important. Ultrasound is remarkably accurate here. A study in the American Journal of Roentgenology found that ultrasound detected Baker’s cysts with 100% sensitivity and 94% overall accuracy compared to MRI, making it a fast, reliable first step.
Meniscus Tears
The meniscus is a C-shaped piece of cartilage that cushions and stabilizes the knee. Each knee has two, and the inner (medial) meniscus is the one most often torn. Its back portion, the posterior horn, is the thickest part and absorbs the most weight, which is why it’s also the most frequently injured section. A tear here sends pain directly to the back of the knee, particularly during deep squatting.
Unlike a Baker’s cyst, a meniscus tear often comes with mechanical symptoms: catching, locking, or a feeling that the knee might give out. Swelling and stiffness typically build gradually over hours to days after the injury rather than appearing immediately. You may also find it difficult to fully straighten the knee. These tears can happen during a sudden twist while playing sports, but they also develop slowly in people with age-related cartilage wear, sometimes without a single obvious injury.
Hamstring and Calf Strains
The hamstring muscles run down the back of the thigh and attach near the back of the knee, while the calf muscles (the gastrocnemius in particular) originate just above the back of the knee. A strain in either group can radiate pain into the posterior knee area, even if the actual muscle damage is higher or lower on the leg. You’ll typically notice this pain most with activities like walking uphill, running, or bending the knee against resistance.
Most mild to moderate strains improve with rest, ice, and gradual return to activity over a few weeks. More severe strains, where a significant number of muscle fibers tear, can take several months and may need guided rehabilitation to recover fully.
Popliteus Muscle Injury
The popliteus is a small but important muscle buried deep behind the knee. Its main job is to “unlock” the knee from a fully straight position by initiating a slight inward rotation of the shinbone. It also helps prevent the thighbone from sliding forward on the shin and acts as a stabilizer for the outer-back corner of the knee.
When this muscle is strained, pain concentrates at the back of the knee and along the outer joint line. Weight-bearing and stair climbing typically make it worse. One distinctive clue is pain with outward rotation of the lower leg, or discomfort when you internally rotate the affected leg (for instance, the motion you’d use to reach across and take off the opposite shoe). Swelling behind the knee is common, and the lower leg may rotate outward more than normal when bending the knee.
PCL Injuries
The posterior cruciate ligament (PCL) is the ligament that runs along the back of the knee, preventing the shinbone from sliding too far backward relative to the thighbone. PCL injuries are less common than ACL tears, but they do happen, most often from a direct blow to the front of the upper shin while the knee is bent. The classic scenario is a car accident where the knee strikes the dashboard, or a fall onto the ground during sports where the knee hits the turf in a bent position. Hyperextending the knee forcefully can also damage the PCL.
A PCL injury typically causes deep pain behind the knee, swelling, and a sense of instability, especially when walking downhill or decelerating. Mild PCL sprains often respond well to bracing and physical therapy. Complete tears, particularly when combined with damage to other structures, may need surgical reconstruction.
Tendinitis
Several tendons cross the back of the knee, and any of them can become inflamed from overuse. Hamstring tendinitis at the lower attachment point and popliteus tendinopathy are among the most common culprits. The pain tends to come on gradually, worsens with repetitive activities like running or cycling, and eases with rest. You may notice stiffness first thing in the morning that loosens up as you move. Tendinitis generally responds to activity modification, targeted stretching, and strengthening exercises over a period of weeks.
Nerve Compression
The tibial nerve runs through the popliteal fossa on its way to the foot, where it supplies sensation to the sole and controls the muscles that flex the toes and ankle. When something compresses this nerve behind the knee (swelling, a cyst, or muscle injury in the area), the symptoms can extend well beyond the knee itself. You may feel numbness or tingling on the bottom of the foot, weakness when pointing the toes downward, or pain in the calf that seems out of proportion to any visible injury.
In some cases, the pattern follows a two-phase course: an initial burst of intense calf pain that fades, followed within 24 hours by a return of even stronger pain along with progressive numbness in the sole of the foot and difficulty flexing the toes. This pattern suggests pressure is building on the nerve and warrants prompt evaluation.
Vascular Causes Worth Knowing
Two vascular conditions can produce pain behind the knee, and both deserve quick medical attention. A deep vein thrombosis (DVT) is a blood clot that forms in the veins of the leg. It causes swelling, warmth, redness, and aching that often concentrates behind the knee or in the calf. The pain usually doesn’t change with knee movement the way a muscle injury would.
A popliteal artery aneurysm is a bulge in the artery behind the knee. Most produce no symptoms at all until they cause trouble. If the aneurysm ruptures, you’ll notice pain, swelling, and bruising behind the knee. If it sends a clot downstream or blocks off entirely, the leg beyond the knee may become painful, pale, cold, numb, or weak. These aneurysms can be difficult to distinguish from a Baker’s cyst or DVT on physical exam alone, which is one reason imaging is so valuable. A provider may be able to feel the enlarged artery pulsing behind the knee, which is a helpful distinguishing sign.
The red flags to watch for are: sudden severe swelling with skin color changes, coldness or numbness in the lower leg or foot, or pain that comes with a palpable throbbing mass behind the knee. Any of these combinations warrants emergency evaluation.
How the Cause Is Identified
A physical exam narrows the possibilities quickly. Testing flexibility by measuring the angle behind the knee while lying on your back can reveal hamstring tightness. Specific rotation maneuvers of the lower leg help isolate the popliteus muscle or meniscus. Checking whether the shinbone slides backward when pushed helps assess the PCL.
For imaging, ultrasound is often the first step because it’s fast, inexpensive, and highly accurate for detecting cysts and distinguishing them from solid masses or blood clots. MRI is typically reserved for cases where a meniscus tear, ligament injury, or other internal structural damage is suspected, since it provides a more detailed view of cartilage, ligaments, and bone.
Because the back of the knee houses so many structures in a small space, the same symptom (a dull ache that worsens with bending) can point to very different problems. Paying attention to how the pain started, what makes it worse, and whether you have any symptoms below the knee gives your provider the clearest picture of what’s going on.