Pain behind the knee usually comes from one of a small number of structures. The most common causes are a Baker’s cyst, a hamstring strain, patellofemoral pain, or irritation of the popliteus muscle or tendon. Less often, the source is a blood clot, artery problem, or nerve issue — and those need urgent medical care.
That short list matters because the back of the knee is crowded. Tendons, nerves, blood vessels, and fluid-filled sacs all sit close together. Where the pain lands, and what makes it better or worse, often points to which structure is involved.
What Causes Pain Behind The Knee Common Conditions?
Four conditions account for most cases of pain in the back of the knee. Each has a distinct pattern of symptoms and a clear reason for hurting where it does.
Baker’s cyst (popliteal cyst)
A Baker’s cyst is a fluid-filled sac that bulges out of the back of the knee joint. It forms when joint fluid escapes through a small one-way valve between the joint and a bursa — a fluid-filled sac — behind the knee.
The cyst itself is usually not the real problem. It is a sign that the knee is producing extra fluid, often from arthritis, a meniscus tear, or another problem inside the joint. Treating the cyst without addressing the underlying cause rarely helps.
Typical signs include a tight or full feeling behind the knee, swelling that gets worse with activity, and stiffness when bending or straightening the leg. Some people notice it most when standing up from a chair. A large cyst can press on nearby structures and cause calf discomfort.
Hamstring strain
The hamstrings run down the back of the thigh and attach around the knee. A strain — a stretch or partial tear of muscle fibers — can cause pain at the back of the knee, especially near the inner side where the tendons attach.
This injury usually follows a specific event: a sudden sprint, a lunge, or a quick change of direction. Pain with walking, tightness, and tenderness to touch are common. Mild strains often settle with rest and gradual return to activity. Severe strains, where walking is difficult or bruising appears, should be assessed by a clinician.
Patellofemoral pain
Patellofemoral pain means pain around the kneecap. It sounds like a front-of-knee problem, and it usually is. But many people feel it behind the knee too, especially when squatting, climbing stairs, or sitting for long periods.
The cause is not fully settled. Research points to a mix of factors — muscle strength, movement patterns, and how load is spread across the joint. It is common in runners and in people who spend a lot of time on their feet, but it can affect anyone.
Popliteus muscle or tendon irritation
The popliteus is a small muscle at the back of the knee. It helps unlock the knee when you start to bend it and stabilizes the joint during rotation.
Overuse — from running downhill, repeated twisting, or sudden increases in training — can irritate the muscle or its tendon. Pain tends to sit in the center of the back of the knee and gets worse with downhill walking or descending stairs. It is less common than the other three conditions but often missed.
How Do You Tell These Conditions Apart?
The pattern of your symptoms is often the best clue. Location, timing, and what makes the pain worse all point toward different structures.
| Condition | Where it hurts | What makes it worse | Other clues |
|---|---|---|---|
| Baker’s cyst | Center-back of knee, may spread to calf | Activity, standing, bending | Visible swelling, tightness, underlying knee problem |
| Hamstring strain | Back of knee near inner side | Sprinting, lunging, stretching | Sudden onset, tenderness, tightness |
| Patellofemoral pain | Around and behind kneecap | Squatting, stairs, sitting long | Grinding or clicking, diffuse ache |
| Popliteus irritation | Center-back of knee | Downhill walking, twisting | Gradual onset, no swelling |
This table is a guide, not a diagnosis. The same symptom can come from more than one structure, and some people have more than one problem at once. A physical exam is usually what settles it.
When Is Pain Behind The Knee a Medical Emergency?
Some causes of back-of-knee pain are dangerous and need urgent care. A blood clot in a deep leg vein — deep vein thrombosis, or DVT — can cause pain, swelling, warmth, and redness in the calf or behind the knee. A clot can travel to the lungs, which is life-threatening.
Seek emergency care for these signs:
- Sudden swelling in one leg, especially with warmth or redness
- Calf pain that feels like a cramp or charley horse and does not ease
- Shortness of breath, chest pain, or coughing up blood
- Pain that is severe, sudden, and keeps you from bearing weight
- Cold, pale, or numb lower leg or foot
A popliteal artery aneurysm — a bulging artery behind the knee — is rare but serious. So is nerve compression in the popliteal fossa, the hollow at the back of the knee. Neither is common, but both can cause pain that does not fit the patterns above.
If the pain is new, severe, or comes with any of those warning signs, do not wait to see whether it settles on its own.
What Does a Baker’s Cyst Feel Like?
A Baker’s cyst usually feels like a firm, tight fullness behind the knee. It may look like a visible bulge. Bending the knee can make it feel more prominent, and straightening it can feel stiff.
Many people describe it as a pressure rather than a sharp pain. It tends to be worse after activity and better with rest and elevation. Some cysts cause no symptoms at all and are found by chance on an imaging scan.
Here is a detail that surprises many people: a Baker’s cyst is not a tumor, and it is not cancer. The name sounds alarming, but it simply describes a fluid collection. That said, because the cyst signals extra fluid in the joint, it is worth finding out why the knee is producing it.
How Is Pain Behind The Knee Evaluated?
Evaluation starts with a conversation and a physical exam. A clinician will ask when the pain started, what makes it better or worse, and whether there was an injury. They will feel the back of the knee, check range of motion, and test strength.
Imaging is used when the exam points to a specific cause. Ultrasound is often the first choice for a suspected Baker’s cyst or blood clot. An MRI can show soft tissue — tendons, ligaments, and cartilage — in detail. X-rays show bone but not soft tissue, so they are more useful for ruling out arthritis or fractures.
Blood tests may be ordered if a clot or infection is suspected. Most cases do not need every test. The goal is to confirm the likely cause and rule out the dangerous ones.
Can Pain Behind The Knee Be Prevented?
Some causes can be reduced, and some cannot. Injury-related pain — hamstring strains and popliteus irritation — is more likely when training load jumps too fast or when muscles are weak or tight.
General measures that make sense for most people:
- Increase activity gradually rather than in sudden jumps
- Warm up before exercise and cool down after
- Keep the muscles around the knee — quadriceps, hamstrings, and calves — strong
- Wear supportive footwear and avoid training on very hard or uneven surfaces
These steps lower risk. They do not guarantee prevention. Baker’s cysts and patellofemoral pain often relate to underlying joint or movement factors that are not fully controllable.
No supplement or device has been shown in clinical trials to prevent pain behind the knee. Claims otherwise are marketing, not evidence.
What Should You Do About Pain Behind The Knee?
Rest, ice, and avoiding activities that make it worse are reasonable first steps for mild, recent pain. Over-the-counter pain relievers may help with discomfort, but they do not treat the underlying cause. Anyone with stomach, kidney, or heart conditions should check with a clinician or pharmacist before using them.
See a clinician if the pain lasts more than a few days, keeps getting worse, or limits walking. See one sooner if there is swelling, warmth, or any of the emergency signs listed above.
Treatment depends on the cause. A Baker’s cyst often improves when the underlying knee problem is treated. Hamstring strains usually heal with rest and a graded return to activity. Patellofemoral pain often responds to strengthening and movement retraining. Popliteus irritation typically settles with load management.
Some clinicians drain a large, painful Baker’s cyst, but this is not a cure and the cyst can return if the joint problem remains. The evidence for draining versus leaving it alone is limited, and practice varies.
Frequently Asked Questions
What is the most common cause of pain behind the knee?
A Baker’s cyst is the most common cause of a bulge or fullness behind the knee. Hamstring strains and patellofemoral pain are also frequent causes of pain in that area.
Is pain behind the knee serious?
Most cases are not serious and come from a cyst, strain, or joint irritation. Sudden swelling, warmth, calf pain, or shortness of breath can signal a blood clot and need emergency care.
Can a Baker’s cyst go away on its own?
Some do shrink or disappear, especially if the underlying knee problem settles. Others persist or return if the joint keeps producing extra fluid.
When should I see a doctor for pain behind the knee?
See a clinician if the pain lasts more than a few days, worsens, or limits walking. Seek emergency care for sudden swelling, warmth, or breathing problems.
