Pain behind the knee during running usually comes from overworked tendons, strained muscles, or irritated soft tissue in the back of the joint. The most common culprits are hamstring tendon problems, calf muscle strain at the knee, popliteus tendon irritation, Baker’s cysts, and posterior meniscus tears. Each feels slightly different, responds to different triggers, and calls for a different approach to recovery.
Hamstring Tendon Pain at the Knee
Your hamstrings don’t just run down the back of your thigh. They attach below and behind the knee, and the tendons at those attachment points take a beating during running. The inner hamstring tendon (the semimembranosus) is the one most often involved in posterior knee pain. It typically shows up as an aching, localized pain on the inner-back side of the knee, and the area feels tender when you press on it just below the joint line.
This is a repetitive stress injury. It develops gradually over weeks of training, especially if you’ve recently increased your mileage or intensity. Sprinting places particular demand on the hamstrings, so speed work or interval sessions can push these tendons past their tolerance. The pain often starts mild, worsens during a run, and eventually lingers afterward.
Popliteus Tendon and Downhill Running
The popliteus is a small muscle deep behind the knee that helps control rotation and deceleration. If your posterior knee pain flares specifically on downhill stretches, this muscle is a prime suspect. Downhill running forces the popliteus to work hard as a brake, placing repeated stress on its tendon where it attaches near the outer-back portion of the knee.
Popliteus tendinopathy is an overuse injury at the muscle-tendon junction. The pain tends to be deep and hard to pinpoint, sitting behind or slightly to the outside of the knee. Runners who train on hilly terrain, trail runners descending steep grades, and those who suddenly add downhill routes to their training are most vulnerable. Reducing the volume and grade of downhill running is the most direct way to let this tendon recover.
Calf Strain at Its Upper Attachment
Your main calf muscle, the gastrocnemius, actually originates at the back of the femur, right above the knee joint. A strain or irritation at this upper attachment point produces pain that feels like it’s in the knee itself rather than the calf. Symptoms develop gradually: localized tenderness, some swelling at the back of the knee or upper calf, weakness during push-off, and limited range of motion when you try to fully straighten the knee or flex your foot upward.
This injury is more common in runners who do a lot of speed work or hill repeats, since both place heavy loads on the calf at full stretch. It can be confused with hamstring tendon pain because the location overlaps, but calf-origin strains tend to hurt more with activities that load the ankle (like calf raises or pushing off the ground) rather than with knee bending.
Baker’s Cyst
A Baker’s cyst is a fluid-filled sac that forms in the hollow behind the knee, called the popliteal fossa. It happens when excess joint fluid escapes into a bursa at the back of the knee, causing it to swell. The lump typically feels soft and squashy, roughly the size of a golf ball, and the pain is most noticeable when you bend the knee deeply because the swollen bursa gets compressed.
Here’s the key detail many runners miss: a Baker’s cyst is almost always a symptom of something else going on inside the knee. Cartilage tears, early arthritis, or chronic joint inflammation cause the knee to produce extra fluid, and that fluid migrates to the path of least resistance at the back of the joint. Treating the cyst alone without addressing the underlying problem means it will likely return. If you notice a visible or palpable lump behind your knee along with the pain, that distinction matters for getting the right diagnosis.
Posterior Meniscus Tears
The meniscus is a crescent-shaped piece of cartilage that cushions the knee joint. Tears in the back portion of the meniscus can produce pain behind the knee, though this is less common than tears that cause pain along the sides of the joint. The hallmark symptom is a catching or locking sensation, where the torn flap of cartilage gets pinched between the bones during certain movements.
Pain from a posterior meniscus tear tends to worsen with twisting, squatting, or straightening the knee after sitting for a while. Stairs and getting in and out of cars can also be painful. During running, the repetitive impact and slight rotational forces at the knee can aggravate the tear. If your posterior knee pain comes with mechanical symptoms like catching, clicking, or the knee briefly “giving way,” a meniscus tear is worth investigating with imaging.
When the Problem Is Vascular
Rarely, pain behind the knee during running signals a blood flow problem rather than a musculoskeletal one. Popliteal artery entrapment syndrome (PAES) occurs when the calf muscle compresses the main artery running behind the knee, restricting blood flow during exercise. It’s most common in young male athletes, particularly those in their late teens and 20s, and is rarely diagnosed in people over 40.
The distinguishing features of PAES are important to recognize. The pain or cramping occurs specifically during exercise and goes away completely with rest. Your feet may feel cold after running. You might notice tingling, burning, or numbness in the calf. If the nearby vein is also compressed, you could develop nighttime calf cramping, swelling, or skin color changes. Runners, cyclists, and people doing high-intensity training are at highest risk. PAES doesn’t resolve with the usual rest-and-rehab approach for muscle injuries, so if your symptoms match this pattern and aren’t improving, it needs specific evaluation.
Sorting Out Your Specific Cause
Location and timing are your best clues. Pain on the inner-back side of the knee that builds over a run points toward hamstring tendon issues. Pain concentrated on the outer-back side that flares on downhills suggests the popliteus. Tenderness right where the knee meets the upper calf, especially with push-off, favors a calf-origin strain. A visible lump that hurts with deep bending is likely a Baker’s cyst. Catching or locking sensations raise suspicion for a meniscus tear.
Also consider what changed in your training. Most posterior knee pain in runners is overuse, meaning it followed an increase in mileage, intensity, hill work, or speed sessions. The pain typically develops over days to weeks rather than appearing suddenly in one stride. A sudden, sharp onset during a sprint or awkward landing is more consistent with an acute muscle strain or ligament injury.
Recovery and Getting Back to Running
The initial approach is the same regardless of the specific structure involved: reduce the training load that provoked the pain. That doesn’t necessarily mean stopping entirely. Cutting mileage, avoiding hills, slowing your pace, or temporarily switching to flat terrain can be enough to let the irritated tissue calm down while you maintain fitness.
Targeted exercises help restore strength and flexibility to the structures behind the knee. Calf stretches are a good starting point: stand facing a wall with the affected leg straight behind you, heel flat on the floor, and lean your hips forward to feel the stretch through your calf and into your heel. Hold for 30 seconds, repeat a few times, and do this daily. Standing quadriceps stretches (pulling your heel toward your buttock) help maintain balance across the joint. Warm up with 5 to 10 minutes of walking or stationary cycling before any stretching or strengthening work.
As pain improves, progressive strengthening of the hamstrings, calves, and glutes helps build the tissue tolerance needed to handle running loads again. Eccentric exercises, where you slowly lower a weight rather than lift it, are particularly effective for tendon injuries. Single-leg calf raises (lowering slowly), Nordic hamstring curls, and single-leg bridges target the structures most commonly involved in posterior knee pain.
Return to running gradually. A common mistake is resuming your previous training volume the moment the pain subsides. Tendons and muscles need progressive loading to rebuild, and jumping back too quickly restarts the cycle. Increase your weekly mileage by no more than 10 percent per week, reintroduce hills and speed work only after easy running is pain-free, and pay attention to any return of symptoms as an early warning signal.
Red Flags That Need Prompt Attention
Most posterior knee pain in runners resolves with load management and targeted rehab, but certain symptoms warrant prompt evaluation. Significant swelling that appears without a clear injury, pain that wakes you at night, knee locking where you physically cannot straighten or bend it, or symptoms accompanied by fever or feeling generally unwell all fall outside the range of typical overuse injuries. Unexplained weight loss, fatigue, or pain that affects multiple joints simultaneously could point toward systemic conditions that require different workup entirely.