Pain behind the knee most often comes from irritation or strain in one of several tendons that cross the back of the joint. The hamstring tendons, the calf muscle tendons, and a smaller stabilizing tendon called the popliteus all attach in this area, and any of them can become inflamed through overuse, sudden force, or gradual wear. Less commonly, the pain isn’t from a tendon at all but from a fluid-filled cyst or a cartilage tear mimicking tendon trouble.
Figuring out which structure is causing your pain depends on exactly where it hurts, what movements make it worse, and whether you notice swelling or mechanical symptoms like catching or locking.
Hamstring Tendon Pain at the Inner Back of the Knee
The hamstrings run down the back of your thigh and attach just below the knee joint. When the tendon on the inner (medial) side becomes irritated, it produces an aching pain localized to the inner back of the knee, with tenderness when you press just below the joint line. This is the most common tendon-related cause of posterior knee pain, especially in active people.
The pain tends to flare during activities that demand heavy hamstring work: running, cycling, walking downstairs, climbing, or suddenly dropping into a deep squat. In its acute form the pain can be severe enough to limit everyday movement. Risk factors include tight or weak hamstrings, poor warm-up habits, fatigue during exercise, imbalances in flexibility or strength between legs, and sports that involve sprinting or sudden direction changes. Runners who ramp up mileage too quickly and cyclists who push big gears are particularly susceptible.
Popliteus Tendon Pain on the Outer Back of the Knee
The popliteus is a small muscle behind the knee that helps control rotation and deceleration. Its tendon attaches on the outer (lateral) side, and when it’s overworked it produces sharp pain at the outer back of the knee. You may also notice localized swelling, redness, and a crackling sound when you bend or straighten the leg.
Downhill running and walking are the classic triggers. Going downhill forces the popliteus to work overtime to brake your body weight against gravity, and repeated stress leads to inflammation of the tendon sheath. A direct blow that pushes the knee inward, a sudden hyperextension, or even overpronation of the foot (which rotates the shin bone outward) can also set it off. People with this problem often find they can’t fully straighten the knee and feel pain when resisting rotation or bending the knee between about 15 and 30 degrees.
Calf Tendon Pain at the Upper Attachment
Your calf muscle (the gastrocnemius) actually originates above the knee, with tendons attaching to the lower end of the thighbone on both the inner and outer sides. When these upper attachments degenerate or tear, you feel pain, tenderness, and sometimes swelling at the back of the knee that develops gradually over weeks.
This type of tendon trouble typically results from chronic overuse, a prior calf injury that never fully healed, or underlying instability in the knee joint. Weakness and limited range of motion often accompany the pain. If the condition progresses without treatment, the tendon can develop partial tears or, rarely, a complete rupture. The gradual onset distinguishes it from acute hamstring or popliteus injuries, which tend to appear more suddenly.
Baker’s Cyst: Not a Tendon, but Easily Confused
A Baker’s cyst is a pocket of fluid that forms in the hollow behind the knee. It creates a visible or palpable bulge along with a feeling of tightness, and the pain gets worse with activity or when you try to fully bend or straighten the leg. Standing for long periods also aggravates it. Unlike tendon pain, which usually hurts in a specific spot when you press on it, a Baker’s cyst produces more diffuse pressure and stiffness.
Most Baker’s cysts develop because of an underlying knee problem, such as arthritis or a cartilage injury, that causes the joint to produce excess fluid. In rare cases, the cyst can rupture, sending fluid into the calf. This causes sudden sharp pain in the knee, calf swelling, and sometimes a sensation of water running down the back of the lower leg. A ruptured cyst can look and feel similar to a blood clot in the leg, so sudden calf swelling after knee pain warrants prompt evaluation.
Meniscus Tears Can Mimic Tendon Pain
A tear in the cartilage pad inside the knee, particularly the posterior horn of the meniscus, can send pain to the back of the knee and fool you into thinking it’s a tendon issue. The key difference is mechanical symptoms. Meniscus tears often cause catching, locking, or a sensation of the knee buckling or giving way. The pain is usually worst when bearing weight and when twisting or pivoting, and walking up or down stairs tends to increase both pain and swelling.
Tendon problems, by contrast, don’t produce locking or catching. If your knee feels like it’s getting stuck or you sense something shifting inside the joint during movement, that points more toward cartilage than tendon damage.
How to Narrow Down the Cause
Location is your best initial clue. Pain on the inner back of the knee suggests the hamstring tendons. Pain on the outer back of the knee, especially if it worsens going downhill, points to the popliteus. A gradual, diffuse ache with weakness may indicate the calf tendon attachments. A bulge with tightness suggests a Baker’s cyst, and any catching or locking raises the possibility of a meniscus tear.
Activity patterns matter too. Pain that started after increasing running volume, switching to hillier terrain, or returning to sport after time off is more likely tendon-related. Pain that appeared after a specific twisting injury, especially with immediate swelling, leans toward a meniscus or ligament issue.
Recovery and Rehabilitation
Tendon pain behind the knee generally responds well to a structured rehab approach, but the timeline is longer than most people expect. The initial phase focuses on reducing load: pulling back from the activities that triggered the pain and letting acute inflammation settle. Ice, gentle range-of-motion work, and avoiding deep knee bending or heavy hamstring loading in the first week or two help calm things down.
Once acute pain subsides, progressive strengthening becomes the priority. Eccentric exercises, where you slowly lower a weight rather than lift it, are the cornerstone of tendon rehab because they stimulate the tendon to remodel and tolerate load again. Calf stretches held for 30 seconds per side, step-back lunges done in sets of 15 with three sets total, and gradual increases in resistance all play a role. The goal is to rebuild the tendon’s capacity to handle the forces you need it to handle.
Returning to full activity should be gradual. A reasonable guideline is to start at about 50% of your previous training volume and increase by roughly 10% each week, as long as symptoms stay manageable. If you’re having trouble going up or down stairs, your training load still needs significant modification. Rushing the process is the most common reason people end up with recurring posterior knee pain. Some tendon injuries resolve in a few weeks with early intervention, while others take several months of consistent rehab to fully settle.
For Baker’s cysts and meniscus tears, treatment depends on the underlying cause. Cysts often shrink once the joint condition driving excess fluid production is addressed. Small meniscus tears may heal with rest and physical therapy, while larger tears that cause persistent locking or catching sometimes require surgical repair.