Popliteus Tendon Irritation in Runners

If you've got a pain at the back or outer-back of your knee that doesn't quite match anything you've read about online, you're not going crazy. This is one of the most commonly overlooked knee injuries in runners, and its unusual location is exactly why it gets missed so often. The good news is that once you know what it is, the path forward is straightforward.

What's Going On

The popliteus is a small but important muscle tucked behind your knee. Its main job is to "unlock" your knee from the fully straight position — it initiates the bending motion at the start of each stride's swing phase. It also acts as a rotational stabilizer, preventing your shin bone from twisting outward when you're standing on one leg, which is essentially what you're doing with every stride.

Despite its small size, the popliteus works hard during running. It's particularly stressed during downhill running, where it has to control both knee extension and rotation simultaneously, and on cambered (sloped) road surfaces, where asymmetric forces put extra rotational demand on the knee. Because this muscle's pain pattern — back or outer-back of the knee — doesn't fit the well-known profiles of runner's knee or IT band syndrome, many runners (and even some clinicians) struggle to identify it. It often gets labeled as "mystery knee pain," which is frustrating when you're trying to figure out what's wrong.

The good news is that popliteus irritation is primarily a loading issue. Reduce the specific demands that are overloading it (hills and uneven terrain), and it settles down. It doesn't tend to be a career-threatening injury — it's more of an annoying one that needs the right diagnosis to resolve.

Why This Happens

• Downhill running — this is the biggest culprit. The popliteus works overtime to control knee extension and rotation on descents, and steep or prolonged downhills can overwhelm it.
• Cambered roads — running on a sloped surface puts uneven rotational forces on the knee. If you always run the same route in the same direction, one leg's popliteus is consistently working harder than the other.
• Hill running — both uphills and downhills stress the popliteus, just in different ways. Uphills demand more flexion work; downhills demand more extension control.
• Trail running — uneven terrain means the knee is constantly adjusting to unexpected angles and surfaces, increasing rotational stress on the popliteus.
• Sudden increase in hilly training — like all muscles, the popliteus needs time to adapt to new demands. Jumping from flat runs to a hilly block too quickly doesn't give it time to build tolerance.
• Worn shoes — when shoes lose their support, other structures have to compensate. The popliteus is one of the first muscles to pick up the slack for inadequate footwear.

How to Recognize It

• Pain at the back or outer-back of the knee — a location that doesn't match the more common "front of knee" or "outside of knee" patterns most runners are familiar with.
• It's noticeably worse going downhill or running on uneven ground.
• You might describe it as a deep ache behind the knee rather than a sharp surface pain.
• The pain typically appears during or after running, particularly runs with significant elevation change.
• Pressing deep into the back of the knee will usually find a tender spot.
• Many runners describe this as "mystery knee pain" because the location just doesn't seem to fit anything they've heard of.

When to Get Help

If you've eliminated hills and switched to flat-terrain running for 2-3 weeks and the pain isn't improving, it's time to see a professional. A sports-focused clinician is more likely to recognize this diagnosis than a general practitioner — the popliteus isn't on every clinician's radar.

• Pain that persists beyond 2-3 weeks of flat-terrain running
• Symptoms that are getting worse rather than staying stable
• If previous generic "knee pain" treatments haven't worked — it's worth specifically asking about the popliteus tendon

How to Adjust Your Training

Mild

You can still run, but switch entirely to flat, predictable surfaces. No hills — not up, not down — and avoid cambered roads. This might mean using a treadmill for a couple of weeks, and that's fine. The popliteus recovers relatively quickly once you remove the specific stresses that are aggravating it. Gentle calf and hamstring mobility work can help reduce compensatory tension around the knee.

Moderate

Flat, even surfaces only — this means no trails, no gravel paths, nothing that puts unpredictable rotational demand on the knee. Add gentle knee flexion mobility work and some glute and hip stability exercises (single-leg bridges, clamshells) to reduce the compensatory demand on the popliteus. You might feel limited by the terrain restriction, but think of it as a short-term investment — a few weeks of flat running now prevents a much longer disruption later.

Severe

When popliteus irritation has been going on for a while without proper management, it's time to get a professional evaluation. Be prepared to advocate for yourself — this injury is often misdiagnosed as generic posterior knee pain or lumped in with hamstring issues. A sports medicine specialist can differentiate popliteus problems from other conditions and set you up with targeted treatment. Getting the right diagnosis now means a faster, more direct recovery.

Staying Ahead of It

• Gradual hill introduction — when adding hill sessions to your training, build volume and intensity over weeks, not days. Your popliteus needs progressive exposure to adapt safely.
• Flat surface maintenance running — keep the majority of your base running on flat, predictable terrain. Hills should be a targeted training stimulus, not the default.
• Alternate cambered road sides — if you run on roads with a camber, switch direction regularly so both legs share the rotational load evenly. Running the same direction every day is an easy habit that creates an unnecessary imbalance.
• Knee stability work — exercises that challenge rotational control — like single-leg stands with gentle perturbations or step-downs with controlled knee alignment — build the popliteus's capacity alongside the larger stabilizers.
• Proper footwear — adequate shoe support reduces the compensatory demand on small stabilizing muscles like the popliteus. When your shoes start breaking down, your popliteus starts working overtime.

The Bottom Line

Popliteus tendon irritation is a common injury that's uncommonly diagnosed. If you've been dealing with "mystery pain" at the back of your knee that gets worse on hills, there's a good chance this is your answer. The fix is straightforward: take the hills out of the equation for a few weeks, do some targeted stability work, and the popliteus typically settles down. You'll be back on your favorite hilly routes before long.