Pain behind the knee after running usually comes from overworked tendons, strained muscles, or fluid buildup in the joint. The back of the knee, called the popliteal area, is a crowded intersection of tendons, muscles, and small fluid-filled sacs, and repetitive impact from running can irritate any of them. Most causes are manageable and respond well to rest and targeted strengthening, but a few warrant prompt medical attention.
Hamstring Tendon Irritation
The hamstrings don’t just attach at the back of your thigh. They run all the way down and anchor to the bones just below the knee joint. When one of those lower attachment points gets inflamed, it produces an aching pain on the inner back side of the knee, right below the joint line. This is called semimembranosus tendinopathy, and it’s one of the most common causes of posterior knee pain in runners.
The pain tends to increase with any activity that demands heavy hamstring work: running (especially uphill), walking down stairs, cycling, and deep knee bends. You’ll typically notice tenderness when you press the area just below and behind the inner knee. It often starts as a mild ache during or after runs and gradually worsens if you keep training through it. Overstriding, where your foot lands well ahead of your body, places extra load on the hamstrings at this attachment point because they have to work harder to decelerate each stride.
Popliteus Tendon Stress
A small, triangular muscle called the popliteus sits deep behind the knee and helps control rotation as you bend and straighten it. Its tendon runs from the back of the shinbone up to the outer side of the thighbone, and it takes on extra work during downhill running because it helps prevent the knee from over-rotating under braking forces. Chronic repetitive stress causes the tendon to thicken and develop internal damage, a condition called popliteus tendinosis.
If your posterior knee pain started after adding downhill routes, trail running, or a sudden increase in mileage, this tendon is a likely culprit. The pain is usually felt deeper than hamstring issues and sits more toward the outer back corner of the knee.
Baker’s Cyst
A Baker’s cyst is a pocket of fluid that forms behind the knee when the joint produces too much lubricating fluid. This overproduction is usually triggered by an underlying problem like cartilage wear or a small meniscus tear. The excess fluid migrates to the back of the knee and pools in a small sac, creating a visible or palpable swelling.
The hallmark of a Baker’s cyst is a feeling of tightness or fullness behind the knee that worsens with activity and prolonged standing. Fully bending or straightening the knee also increases pain. Running doesn’t directly cause Baker’s cysts, but it can aggravate the underlying joint issue that produces the extra fluid. If you notice a soft, egg-shaped lump behind your knee along with the pain, that’s a strong clue. In some cases the cyst can rupture, sending fluid down into the calf and causing sudden sharp pain and swelling that mimics a blood clot.
Calf Muscle Strain at the Knee
Your calf muscles originate at the base of your thighbone, right above the back of the knee, before running down to your heel. A strain in the upper portion of the calf can feel like knee pain rather than calf pain, which makes it easy to misidentify. This type of strain often comes on suddenly during a sprint or hill repeat, with a sharp, tearing sensation. You may also have difficulty bending the knee fully or pushing off with that foot. Pressing on the upper calf just below the crease of the knee will reproduce the pain.
Meniscus Tears
The meniscus is a C-shaped piece of cartilage that cushions the space between your thighbone and shinbone. Tears in the rear portion of the meniscus can produce pain that localizes to the back of the knee. The distinguishing feature of a meniscus tear is mechanical symptoms: catching, clicking, popping, or a sensation that the knee momentarily locks up. Pain accompanied by a snapping or clunking sound during bending strongly suggests a tear in the rear section of the meniscus.
Small meniscus tears can develop gradually from repetitive loading over months or years of running, not just from a single traumatic event. If your posterior knee pain comes with any locking or catching sensation, imaging is usually needed to confirm the diagnosis and determine whether the tear requires surgical repair.
A Serious Cause Worth Ruling Out
Deep vein thrombosis, a blood clot in the leg, can produce swelling, warmth, and pain behind the knee that looks remarkably similar to a Baker’s cyst or muscle strain. Clinical examination alone is often not enough to tell them apart. In a study of patients evaluated for Baker’s cysts, about 7% also had a coexistent blood clot, and some cysts were actually compressing the vein behind the knee, which can itself trigger clot formation.
If you have sudden swelling in the calf along with warmth, redness, or pain that doesn’t match the pattern of a typical running injury (especially if you’ve recently traveled, been immobile, or are on hormonal birth control), seek medical evaluation promptly. An ultrasound can quickly distinguish between a cyst, a strain, and a clot.
Running Form and Common Triggers
Several training habits put extra stress on the structures behind the knee. Overstriding forces the hamstrings to absorb braking forces at their most stretched position, loading the tendons near the knee with every footstrike. Downhill running increases demand on the popliteus tendon and the hamstring attachment points. Sudden jumps in weekly mileage or intensity don’t give tendons time to adapt, since tendons remodel more slowly than muscles.
Shortening your stride so your foot lands closer to your center of gravity reduces the load on the posterior knee. If downhill routes are a regular part of your training, building into them gradually rather than adding long descents all at once gives the popliteus and hamstring tendons time to strengthen.
Strengthening for Prevention and Recovery
The posterior chain, your glutes, hamstrings, and calves, works as a unit to stabilize the knee during running. Weakness in any link shifts extra load to the tendons and joint structures behind the knee. A few simple exercises, done consistently, can make a meaningful difference.
- Wall squats: Stand about a foot from a wall with feet hip-width apart and toes slightly turned out. Slide your back down the wall by bending your knees, keeping them behind your toes. Focus on engaging the muscles above your kneecap and your glutes as you push back up.
- Bodyweight squats: Stand with feet shoulder-width apart, lower yourself as if sitting in a chair, bending knees to no more than 90 degrees. Keep your back straight and knees tracking over your toes.
- Lunges: Start in a split stance, lower until your front knee reaches roughly 90 degrees, then push back up through your heels. Three sets of five repetitions per leg is a good starting point.
These exercises target the glutes and quadriceps, which take pressure off the hamstrings and popliteus by sharing the work of knee stabilization. Adding eccentric hamstring work (like Nordic hamstring curls or slow single-leg deadlifts) specifically strengthens the hamstring tendons at their vulnerable lower attachment points. Start with low volume and increase gradually, since tendon adaptation takes weeks, not days.
For most posterior knee pain in runners, reducing mileage (not necessarily stopping entirely), correcting overstriding, and building posterior chain strength resolves the problem within a few weeks. Pain that persists beyond two to three weeks, comes with swelling or locking, or wakes you at night is worth having evaluated with imaging to rule out structural damage or fluid-related causes.