The Berlin Marathon is less than two weeks away, and after the finish many runners will be asking the same old question: my knees ache — is that normal, or have I broken something? Japanese orthopaedic surgeons have, for the first time, looked at what happens to the menisci after 42 km not in one-off scans but day by day. The result is reassuring and also hints at when to return to running.

What meniscus extrusion is

The menisci are two C-shaped cartilage pads between the femur and the tibia. They distribute load and work like a hoop: under pressure the meniscus tends to “spread apart”, while strong circumferential fibres hold it together.

When a loaded meniscus shifts outwards beyond the edge of the tibia, this is called extrusion. It is measured with standing ultrasound as the distance from the edge of the tibial plateau to the outer edge of the meniscus. Persistent extrusion is a sign that the “hoop” is holding less well; it is studied as a marker in meniscal injuries and osteoarthritis.

It was already known that even jogging causes temporary extrusion. In a study by Ishii and colleagues (Journal of Medical Ultrasonics, 2023), the meniscus in 13 young volunteers shifted after uphill and downhill running and was back in place the next day. The shift was greater the higher the cumulative load on the inner part of the knee (r = 0.68), rather than the peak load.

A marathon is a load an order of magnitude larger. The question was how much stronger the response would be and how long recovery would take.

What the researchers did

Sohei Dobashi and colleagues from Hokkaido University (Scandinavian Journal of Medicine & Science in Sports, September 2026) examined 100 amateur runners — 200 knees, mean age 47.2 years, 77 men and 23 women — at five marathons.

Standing ultrasound with the leg fully extended: before the start and immediately after the finish.
90 people (180 knees) were scanned again on days 1–5 after the race.
Before the start, each runner's load on the inner part of the knee during walking — the knee adduction moment impulse (KAM) — was measured with a sensor on the shin, adjusted for body mass and height.

What they found

Immediately after the finish, medial meniscus extrusion rose from 1.14 mm (95% CI 1.06–1.22) to 1.57 mm (1.49–1.66) — by 38%, or an average of 0.44 mm (p < 0.001).

Recovery time course:

days 1–3 — extrusion still significantly above baseline (p < 0.01);
from day 4 — no different from the pre-race level.

Who shifted more (independent factors in a multivariable model):

higher load on the inner part of the knee during walking — β = 0.24;
higher body mass index — β = 0.22;
greater monthly running volume — β = 0.18;
age — borderline significant (β = 0.15).

The link with volume looks unexpected: you would think trained knees should react less. The study does not explain why. It is a correlation, and there could be several explanations — from faster paces among high-volume runners to accumulated load.

How to read this

This is a normal, reversible response to a big load. The meniscus behaves roughly like muscles after a long race: it temporarily “gives” and then recovers. It is the same picture as in Ishii's jogging study, only stronger and longer-lasting — three days instead of one.

The clinical significance is unknown — the authors say so plainly. Whether this temporary shift has consequences, and whether a specific recovery interval is needed, remains to be established. There was no long-term follow-up.

Running by itself does not destroy knee cartilage — another 2026 study backs this up. Lukáš Slovák and colleagues from the University of Ostrava (Medicine & Science in Sports & Exercise) compared knee MRIs of 183 adults without symptoms: 61 triads matched by sex and age — recreational runners, non-runners and people with obesity (BMI ≥ 30). Runners had higher T2 mapping values in some outer regions — this reflects cartilage composition, not damage. But cartilage abnormalities and poorer knee function were associated with obesity, not with recreational running. The study is cross-sectional, so it does not prove causation.

Limitations of the marathon study: ultrasound only, no MRI; a mean age of 47 and three-quarters of participants men; the published data do not link extrusion to pain; conditions differed across the five marathons.

How to apply this

No running for the first three days after a marathon. The authors do not prescribe a ready-made interval, but if you want to align your return to running with meniscus recovery, day 4 is the earliest point at which the knee, on average, looked the way it did before the start. Walking, easy cycling and swimming are reasonable alternatives on those days.
Body weight is the most obvious lever. Body mass index is one of the three independent factors, and in the Ostrava study it was obesity, not running, that was associated with cartilage changes.
Load on the inner part of the knee can be reduced through technique. Raising cadence by 5–10% lowers impact load on the knee — how to increase your step rate without getting hurt.
High volume is not a reason to run less, but it is a reason to take the taper and the week after the race seriously.
Pain along the inner side of the joint, swelling or the knee “locking” after day 4 is not normal and is a reason to see an orthopaedic specialist.

The bottom line

Ultrasound in 100 marathon runners: medial meniscus extrusion immediately after the finish — +38% (from 1.14 to 1.57 mm).
The shift lasts three days and returns to baseline from day 4.
Knees react more strongly in runners with a higher load on the inner part of the joint, a higher BMI and a greater monthly volume.
The clinical significance of the temporary shift is unknown. Another 2026 study found no structural or functional cartilage problems in recreational runners — unlike people with obesity.

Sources: Dobashi S., Iwasaki K., Tanaka S., Hishimura R., Matsuoka M., Ebata T., Sato D., Onodera T., Kondo E., Iwasaki N. “Marathon Running Transiently Causes Medial Meniscus Extrusion: Recovery Time Course and Factors Associated With Extrusion”. Scandinavian Journal of Medicine & Science in Sports, 2026;36(9):e70366. DOI: 10.1111/sms.70366 · Ishii Y., Hashizume T., Okamoto S. et al. “Cumulative knee adduction moment during jogging causes temporary medial meniscus extrusion in healthy volunteers”. Journal of Medical Ultrasonics, 2023;50(2):229–236. DOI: 10.1007/s10396-023-01288-w · Slovák L., Plešek J., Šustková L. et al. “Knee Cartilage Composition, Morphology, and Function in Asymptomatic Recreational Runners, Non-Runners, and Obese Adults: A Matched 4HAIE MRI Study”. Medicine & Science in Sports & Exercise, 2026. DOI: 10.1249/MSS.0000000000004118