Why the front of your knee hurts

Patellofemoral pain — the clinical name for what most runners simply call runner's knee — is the single most common knee complaint in running. The patella sits in a shallow groove at the front of the femur and slides up and down that groove every time you bend and straighten your leg. Running loads that joint repeatedly, and when the load arrives faster than your tissues can adapt to it, you get pain.

Typically it feels like a dull ache around, behind or under the kneecap. It's often worst going downhill, on stairs, after a long car journey, or in the first few steps after sitting with a bent knee. Some runners notice a grinding or crackling sensation, which is usually harmless in itself but worth mentioning to a physiotherapist if it comes with pain or swelling.

Here's the reassuring part: in most cases there is no structural damage. It's a load-tolerance problem, which means it responds well to the right exercise and sensible pacing.

What usually triggers it

Runner's knee rarely appears out of nowhere. Look back over the four to six weeks before the pain started and you'll often find one of these:

• A jump in weekly distance or frequency — the classic "10% rule" exists for a reason, and even that is too much for some runners.
• A sudden increase in hills, particularly descents, or a switch to faster intervals.
• New shoes, a change of surface, or a return to running after a break.
• Higher training loads combined with poor sleep, a stressful period at work, or a recent illness.
• Hip and thigh muscles that fatigue early, letting the kneecap drift slightly out of its ideal path.

None of these is a moral failing. They're simply clues about where the load crept up.

Strengthening that actually helps

Rehabilitation for runner's knee isn't about resting until it feels better. It's about building the capacity of the leg to tolerate running. Two muscle groups matter most: the quadriceps, which controls the kneecap, and the glutes, which control the whole leg.

• Heavy, slow quads: seated knee extensions, leg press or split squats, using a load you could manage for about 8–12 repetitions. Slow the lowering phase to three seconds. Two to three sets, two to three times a week.
• Glute strength: hip thrusts, step-ups and single-leg bridges. Aim for control rather than speed, and add load as it becomes easy.
• Calf and ankle work: heel raises and single-leg balance help the whole chain absorb force, which takes pressure off the knee.

Some discomfort during exercise is acceptable — a 3 or 4 out of 10 that settles within 24 hours. Sharp pain, or pain that lingers into the next day, means the dose was too high.

Pacing and the 24-hour rule

You don't usually need to stop running altogether. A better approach is to trim the load while you build strength.

• Cut weekly volume by roughly 30–50% to start, keeping pain at or below 3/10 during the run.
• Use the 24-hour rule: if the knee is no worse the morning after a run, the load was fine. If it's stiffer or more painful, reduce something next time — distance, pace or hills.
• Walk the descents rather than run them, and swap one long run for two shorter ones.
• Keep easy runs genuinely easy. Most runners' "easy" pace is closer to their moderate pace, which adds load for very little benefit.

Getting back to full training

Progress one variable at a time. Increase distance for a couple of weeks, then pace, then hills. A reasonable pattern looks like this:

• Weeks 1–2: reduce volume, run on flat ground, and strengthen twice weekly.
• Weeks 3–6: rebuild distance by around 10% a week, staying off steep descents.
• Week 7 onwards: reintroduce faster sessions and hills if the knee has stayed quiet for at least two weeks.

Expect the process to take six to twelve weeks. Patience here usually costs far less time than repeatedly restarting and flaring up again.

When to see a physiotherapist

Book an assessment if the pain has lasted more than three weeks despite reduced training, or if the knee swells, locks, gives way, or you can't walk comfortably. A good physiotherapist will look at your hip, knee, ankle and running technique, then give you a plan tailored to your training rather than a generic sheet of exercises.

Runner's knee is stubborn but rarely serious. Build the leg, respect the 24-hour rule, and progress one thing at a time — most runners are back to normal training within a couple of months.