Your knees squash a little after a run, then bounce back

Running is one of the most accessible forms of exercise, and its general health benefits are well established. Yet the fear that pounding pavement wears out knees and drives osteoarthritis persists, and it stops plenty of people from running at all. Cartilage is the tissue at the centre of that fear, and modern MRI can now measure its thickness, volume and composition with enough precision to detect changes from a single bout of exercise. This review set out to establish what actually happens to hip and knee cartilage in the minutes and hours after a run.

The authors systematically reviewed studies that used MRI to measure change in hip or knee cartilage within 48 hours before and after running, separating immediate effects (within 20 minutes) from delayed effects (20 minutes to 48 hours). Twenty-four studies were included, evaluating 446 knees; no hip data were available. Risk of bias was assessed with a modified rating scale and only one third of studies scored as low risk. Percentage changes in cartilage outcomes were pooled using random-effects meta-analysis, and the certainty of the evidence was formally graded.

Knee cartilage thickness and volume decreased immediately after running, with declines ranging from 3.3% for weight-bearing femoral cartilage volume to 4.9% for patellar cartilage volume. MRI relaxation times, which reflect cartilage composition, also fell immediately after running, with the largest decline of 13.1% in femoral trochlear cartilage. Crucially, the changes did not last: tibiofemoral cartilage relaxation times recovered to baseline within 91 minutes, and existing cartilage defects were unchanged within 48 hours of a run. The authors graded the overall evidence as very low certainty, and hip cartilage effects remain unknown.

This directly addresses the popular belief that running damages knees, and the answer for a single run is reassuring: cartilage compresses measurably but recovers quickly, behaving more like a tissue responding to load than one being worn away. The very low certainty rating matters, though, and this review cannot speak to the bigger question of whether years of running raise or lower osteoarthritis risk, since it only measured responses within 48 hours. Hip cartilage was not studied at all, and people with existing knee disease were not the focus. Longer-term imaging studies are the obvious next step.

If worry about wearing out your knees has kept you from running, this is a reason to be less afraid of any single run: the cartilage changes measured were small, appeared transient, and existing defects did not worsen within 48 hours. It is not proof that a lifetime of running is protective or harmful, because this evidence only covers the hours around one bout, and the authors themselves rate its certainty as very low. Anyone with an existing knee condition sits outside what these studies measured.