Why Is the Back of My Knee Hurting? 5 Causes & Fixes

Pain behind your knee usually comes from a Baker's cyst, a hamstring or calf strain, or a posterior meniscus tear — but sudden swelling, warmth, or redness back there can signal a blood clot and needs same-day medical attention.

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The back of the knee (the popliteal region) is a tight space packed with tendons, a major artery and vein, and a thin pocket of joint fluid — so pain there tends to come from a short, specific list of causes rather than the dozens of possibilities you'd see with general knee pain. Most cases are mechanical and improve with rest, but a handful of red-flag symptoms mean you should not wait it out. This guide walks through what's actually going on back there, how to tell the causes apart, and exactly when posterior knee pain stops being a "watch and wait" problem.

What Causes Pain Behind the Knee?

The most common causes of pain behind the knee are a Baker's cyst (fluid buildup), a hamstring or popliteus strain, and a tear in the back of the meniscus — each produces a slightly different pattern of pain, swelling, and stiffness.

According to the Cleveland Clinic, several distinct problems converge in this small area. A Baker's cyst forms when "something damages your knee and extra fluid drains into a small bump behind your knee," producing a soft, fluid-filled bulge that's most noticeable when the knee is fully straight. Hamstring strains are another frequent culprit — the Cleveland Clinic notes that "hamstring strains or calf muscle injuries can send pain into other areas of the back of your leg," since the hamstring tendons attach right at the back of the knee joint. A torn meniscus, the cartilage cushion between the thigh bone and shin bone, can also radiate pain here — the Cleveland Clinic explains that "a torn meniscus can happen when you twist your leg suddenly."

"Baker's cysts develop when something damages your knee and extra fluid drains into a small bump behind your knee." — Cleveland Clinic staff, Back of Knee Pain

A fourth, less commonly discussed cause is a popliteus strain. The popliteus is a small, deep muscle that sits directly behind the knee joint and helps "unlock" the knee when you start to bend it from a fully straight position. Sports medicine literature describes it as part of the posterolateral corner of the knee, and injuries to it are graded as first-, second-, or third-degree strains depending on severity — pain typically sits at the outer-back portion of the knee and worsens when walking downhill or pivoting.

Is It a Baker's Cyst, a Hamstring Strain, or Something Else?

A Baker's cyst usually feels like a soft, fluid-filled bulge with tightness when the knee is fully extended; a hamstring or popliteus strain feels more like a pulling, aching pain that's worse with specific movements rather than a visible lump.

The location and texture of the discomfort are the biggest clues. A Baker's cyst tends to sit centrally behind the knee and may shrink slightly when you bend the knee partway, because that's when the joint capsule has the most room. The Cleveland Clinic lists the core symptoms as "knee pain," "stiffness," "trouble bending your knee as far as you usually can," and "swelling in your knee, thigh or calf" — note that the swelling can travel down into the calf, which is one reason Baker's cysts sometimes get confused with a blood clot (more on that below).

"The bump that forms behind your knee is the most obvious symptom of a Baker cyst." — Cleveland Clinic staff, Baker's Cyst (Popliteal Cyst)

A hamstring or calf (gastrocnemius) strain, by contrast, usually has a clear trigger — a sprint, a lunge, an awkward stair step — and the pain is sharper at the moment of injury, then settles into a dull ache and tightness that worsens when you stretch or contract the muscle. A popliteus strain often shows up after downhill running, deceleration sports, or a twisting injury, and tends to hurt specifically when pivoting on a slightly bent knee rather than when the leg is straight.

When Back-of-Knee Pain Is a Medical Emergency (DVT)

Sudden swelling, warmth, redness, or tenderness behind the knee or in the calf — especially after a long flight, surgery, or prolonged bed rest — can mean a deep vein thrombosis (DVT), a blood clot that requires emergency care, not home treatment.

This is the one cause of posterior knee pain you should never wait out. The popliteal vein runs directly behind the knee, and clots can form there or travel up from the calf. According to the Cleveland Clinic, the warning signs of DVT include "swelling of your leg or arm on one side (sometimes, this happens suddenly)," "warmth in the area of your leg or arm that's swollen or painful," "discolored or red skin," and "pain or tenderness in your leg or arm" that may only show up when standing or walking. A DVT can feel like a deep, cramping ache — easy to mistake for a pulled muscle — but the combination of swelling, warmth, and redness on just one leg is the pattern that sets it apart.

"It's important to call your provider right away or go to the emergency room if you have deep vein thrombosis or PE symptoms. Don't wait to see if your symptoms go away. Get treatment right away to prevent serious complications." — Cleveland Clinic staff, Deep Vein Thrombosis (DVT)

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Risk factors that raise your odds of a DVT include recent surgery, a long flight or car ride without moving, pregnancy, certain birth control or hormone medications, smoking, obesity, and a personal or family history of clotting disorders. If a piece of the clot breaks loose, it can travel to the lungs and cause a pulmonary embolism — a life-threatening emergency marked by sudden shortness of breath, chest pain, or coughing up blood. If you notice one-sided leg swelling with warmth or redness, treat it as urgent: go to an emergency room or urgent care the same day rather than scheduling a routine appointment.

Could It Be a Meniscus Tear or a Calf Strain?

A posterior meniscus tear usually causes catching, locking, or a popping sensation at the moment of injury, followed by swelling that builds over two to three days — a pattern that's different from the gradual ache of a cyst or tendon strain.

The meniscus is the C-shaped cartilage that cushions the knee joint, and its back portion (the posterior horn) is one of the most commonly torn sections, especially with a sudden twisting motion while the foot is planted. According to OrthoInfo, the patient information resource published by the American Academy of Orthopaedic Surgeons (AAOS), common symptoms include "pain, stiffness and swelling, catching or locking of your knee, the sensation of your knee giving way," and many people report that "you might feel a pop when you tear the meniscus." Importantly, the swelling is often delayed — the same source notes that "over 2 to 3 days, however, the knee will gradually become stiffer and more swollen," which helps distinguish it from a cyst that's been building gradually for weeks.

"You might feel a pop when you tear the meniscus." — OrthoInfo, American Academy of Orthopaedic Surgeons, Meniscus Tears

A gastrocnemius (calf) strain — sometimes called "tennis leg" — produces a sudden, sharp pain at the top of the calf, right where the muscle meets the back of the knee, often during a push-off or jumping movement. It can be confused with a Baker's cyst because both cause swelling that pools in roughly the same area, but a calf strain comes with a clear injury moment and bruising, while a cyst builds slowly and isn't usually associated with a specific traumatic event.

What to Do About Pain Behind Your Knee in 2026

Most mechanical causes of posterior knee pain respond to rest, ice, compression, and gentle range-of-motion work over one to two weeks — but persistent swelling, a visible lump, locking, or any DVT warning sign should prompt a same-day or next-day medical evaluation.

For straightforward strains, start with the standard R.I.C.E. approach: rest from the aggravating activity, ice for 15-20 minutes several times a day for the first 48 hours, a light compression wrap if swelling is present, and elevation when sitting. Gentle stretching of the hamstring and calf — never pushing into sharp pain — usually resumes within a few days once the acute soreness fades.

Visit a healthcare provider if any of the following apply, as the Cleveland Clinic recommends checking in if you have "pain behind your knee for more than a few days in a row," if "you notice swelling, a growth or a bump you can see and feel behind your knee," or if "you have pain along with a fever, or your knee feels warm to the touch." Locking, catching, or a knee that gives way under you are also signs that warrant imaging rather than continued home treatment, since they suggest a meniscus or ligament issue that may not resolve on its own.

In Short

Pain behind the knee most often comes from a Baker's cyst, a hamstring or popliteus strain, or a posterior meniscus tear, and each has a distinct pattern: a soft bulge for a cyst, a pulling ache for a muscle strain, and catching or delayed swelling for a meniscus tear. The one cause you should never ignore is a deep vein thrombosis — sudden swelling, warmth, or redness behind the knee or in the calf, especially after travel, surgery, or prolonged sitting, needs same-day emergency evaluation. For everything else, a short course of rest, ice, and gentle movement resolves most cases within one to two weeks, and ongoing swelling, locking, or a visible lump are your cue to see a healthcare provider for an exam or imaging.

What You Also May Want To Know

Why does the back of my knee hurt when I straighten my leg?

This is a classic sign of a Baker's cyst, since the fluid-filled sac is most noticeable and most tense when the knee is fully extended. It can also happen with a hamstring strain, since straightening the leg stretches the injured tendon. If straightening also causes catching or locking, a meniscus tear is worth ruling out.

Can a Baker's cyst go away on its own?

Yes, a Baker's cyst can shrink or disappear on its own, particularly if the underlying cause — such as mild arthritis or a minor cartilage irritation — resolves. Avoiding activities that trigger swelling, such as deep squatting, often helps mild cases. Persistent, painful, or growing cysts should be evaluated, since they can occasionally rupture and mimic a blood clot.

How do I know if my knee pain is a blood clot and not a muscle strain?

A blood clot (DVT) typically causes one-sided swelling, warmth, and redness that builds over hours, often without a specific injury moment. A muscle strain usually has a clear trigger — a sprint, a lunge, a sudden stretch — and the pain is more localized to the muscle rather than diffuse swelling of the whole calf. When in doubt, especially after a long flight, surgery, or period of inactivity, treat it as a possible clot and seek same-day medical care.

What does a popliteus strain feel like?

A popliteus strain causes an ache on the outer-back portion of the knee that worsens with pivoting, downhill running, or twisting while the knee is slightly bent. It's a less common diagnosis than a hamstring strain or Baker's cyst and is often missed on a first exam, since the muscle sits deep behind the joint. Pain that doesn't improve with typical hamstring-strain treatment is a reason to ask specifically about the popliteus.

When should I see a doctor for pain behind my knee?

See a provider if the pain lasts more than a few days, if you notice a visible bump or swelling, or if the knee feels warm or you have a fever alongside the pain. Locking, catching, or a knee that gives way also warrant an evaluation. Sudden one-sided swelling with warmth or redness is an emergency and should not wait for a scheduled appointment.

Reviewed and Updated on June 23, 2026 by George Wright