Runner’s knee — known medically as patellofemoral pain syndrome — is pain around the front of the knee, usually around or just behind the kneecap. It’s one of the most common knee complaints in runners, but you don’t have to be a runner to get it.

You’ll typically feel it going downstairs, squatting, sitting for long periods with the knee bent, or during and after a run. That last one is the classic pattern — you can start a run feeling completely fine, then have the pain creep in after a mile or two and linger afterwards.

So what’s actually happening underneath? Your kneecap sits in a small groove at the bottom of the thigh bone, and it’s designed to glide smoothly up and down that groove every time you bend and straighten your knee. When it’s tracking properly, you don’t feel a thing. But when it’s being pulled slightly off to one side, it no longer glides cleanly — it creates friction and irritation on the underside of the kneecap. Over time, that friction builds into the nagging, sometimes sharp pain that just won’t settle.

That’s the heart of it. The fundamental issue is a mal-tracking kneecap — a kneecap that isn’t sitting and moving where it should. Almost everything else we’re about to cover comes back to that one thing.

So if a mal-tracking kneecap is the root problem, the real question is: why does the kneecap start tracking badly in the first place? There are four main causes of runner’s knee, and in a lot of people it’s more than one happening at the same time.

1. Weak quads — particularly the VMO. The VMO is the teardrop-shaped muscle on the inner side of your quad, just above the kneecap. Its job is to pull the kneecap inward and keep it centred in the groove. When it’s weak or not firing properly, the kneecap drifts outward and starts to track badly. This is one of the most common reasons people develop runner’s knee, and it’s why targeted quad strengthening — done correctly — is so often part of the solution.

2. Foot pronation. When your foot collapses inward as you run, which is what pronation is, it sets off a chain reaction up the leg. The shin bone rotates inward, which in turn rotates the thigh bone inward. Meanwhile, the soft tissue on the outside of the knee is pulling the kneecap outward. You’ve now got rotation dragging the leg one way and soft-tissue tension pulling the other — and the kneecap is caught in the middle, being tugged off track with every single stride.

3. Weak glutes. Your glutes control how the thigh bone moves. When they’re weak, the femur drops and rotates inward as you run — the tell-tale sign is the knee caving inward mid-stride. The result is the same as the others: the kneecap is pulled away from where it should be tracking. Weak glutes and foot pronation often work against the knee at the same time, which is a big reason runner’s knee can feel so persistent.

4. Structural factors. This one is different from the other three. Sometimes the reason the kneecap doesn’t track properly has nothing to do with muscle strength or foot mechanics — it comes down to how the knee is actually built. The position of the kneecap, or the angle at which the patellar tendon attaches relative to where the kneecap sits, can mean that no amount of exercise will fully correct the tracking. It’s the reason some runners do everything right and still can’t fully shake it — and it’s something genuinely worth understanding before you spend months wondering why nothing’s working.

Here’s the part most people miss. When you look at the causes of runner’s knee individually, each one is fairly simple. But in real life they rarely show up alone.

A runner with a weak VMO often has weak glutes too. Throw in a bit of foot pronation and you’ve got three separate forces all dragging the kneecap off track at once. They stack on top of each other, and the combined effect is far greater than any one of them on its own. That’s why runner’s knee can grumble on for months and feel so resistant to the odd stretch or generic exercise — you might be addressing one piece of the puzzle while two others carry on unchecked.

It also explains why two people with the same diagnosis can need quite different approaches. If your main driver is weak glutes, the fix looks different than it would for someone whose main issue is pronation. This is exactly why a proper assessment matters — working out which of these factors is driving your knee, rather than guessing, is what turns a frustrating, on-and-off problem into one you can actually resolve. If you’ve been struggling for a while, it’s well worth getting a professional opinion to pinpoint your specific causes.

If all of that sounds a little disheartening, here’s the encouraging part. For the large majority of runners, runner’s knee is very treatable. It isn’t usually a sign of serious structural damage like a tear or a fracture — it’s a tracking and loading problem, and tracking problems respond well to the right work.

Once you understand what’s driving your knee and address the right things — strengthening the VMO, switching on the glutes, managing pronation where it’s a factor — the pain often settles surprisingly quickly. The key word there is right. Generic knee exercises pulled off the internet may help a little, but the runners who get the best results are the ones targeting their specific causes rather than throwing everything at the wall.

So if you take one thing from this article, let it be this: runner’s knee comes down to a kneecap that isn’t tracking properly, and the most common reasons are a weak VMO, foot pronation, and weak glutes — often all three at once, with structural factors playing a part for some. Understand which of those apply to you, address them properly, and you give yourself the best possible chance of finally getting on top of it.

Joe Sharp
BSc (Hons) Physiotherapy