Runner's Knee (PFPS): Causes, Treatment, and How to Run Through It Safely

Runner's knee — patellofemoral pain syndrome (PFPS) — produces pain at the front of the knee, around or behind the kneecap, that typically worsens during downhill running, prolonged sitting with knees bent, or stairs. It is the most common running injury in recreational runners and one of the most consistently mismanaged: the instinct to rest completely, wait for it to resolve, then return to full training is precisely the cycle that keeps it recurring.

The kneecap (patella) tracks in a groove on the femur. When the surrounding musculature is balanced, it tracks centrally. When hip abductors and external rotators are weak — which is the default state for most runners who don't do specific hip strength work — the hip internally rotates under load, the kneecap is pulled laterally, and every landing creates friction at the lateral edge of the patellofemoral joint. The pain is in the knee. The problem is in the hip.

The diagnostic test: single-leg squat on the affected side while watching your knee in a mirror. If the knee drifts medially — collapsing inward over the foot — hip abductor weakness is confirmed. This test is also the treatment: strengthening the hip abductors until the single-leg squat produces a stacked, stable position.

Treatment protocol: clam shells (3 sets of 20, daily), side-lying hip abductions (3 sets of 15), and single-leg squats (3 sets of 8-10 per side, progressing to body weight then weighted). These are not exciting exercises. They are the exercises that resolve PFPS without requiring a physiotherapist, a cortisone injection, or a 6-week running hiatus.

Running modification during recovery: reduce mileage by 30-50%, eliminate downhill running entirely (the highest patellofemoral loading pattern), and run on softer surfaces. Increase cadence by 5-10% — the shorter stride that results meaningfully lowers peak patellofemoral joint force: increasing step rate to 110% of preferred reduced peak patellofemoral force by roughly 14% in one biomechanics study (Heiderscheit et al., Medicine & Science in Sports & Exercise, 2011), and modelling work has estimated up to a 20% reduction at a 10% higher cadence (Lenhart et al., 2014). Ice post-run for 15-20 minutes can ease acute pain, though it treats the symptom rather than the underlying hip weakness.