When people talk about a stress fracture in a runner, they usually picture a metatarsal or the shin. That is how most people search for it, too: “stress fracture in the foot”, “metatarsal stress fracture”, “tibial stress fracture”. A new Japanese study shows that in high-level runners the picture can be different — and the most common site is where you would least expect it.

What the researchers did

Narusawa and colleagues from Nippon Medical School Hospital (Tokyo), together with Aoyama Gakuin University, published their study in International Orthopaedics (September 2026). They retrospectively reviewed the medical records of one elite university distance-running team from April 2022 to October 2025.

They looked for all MRI-confirmed bone stress injuries of the pelvis and legs and calculated the rate per 100 athlete-years — that is, how many injuries occur per 100 runners over a year of training.

The starting point was the authors' hypothesis: with the mass switch to maximalist shoes with thick soles, the distribution of fractures in runners might have shifted. The study did not test this hypothesis directly — footwear was not compared.

What they found

92 athletes, a total of 172 athlete-years.
51 bone stress injuries — 29.7 per 100 athlete-years. Roughly: about 30 such injuries for every 100 runners per season (one athlete could have had several).
The femur was the most common site: 21 cases, 41%.
Proximal injuries (pelvis and femur) — 18.6 per 100 athlete-years; distal (shin and foot) — 11.0. The “top” broke more often than the “bottom”.
There were significantly fewer injuries in winter than in spring.
On MRI, all grade 4 (the most severe) femoral injuries showed a characteristic sign — longitudinal bone marrow oedema running along the bone.

How it compares

An American study by Wolff and colleagues (American Journal of Sports Medicine, 2025) followed 221 runners from two NCAA university programmes for seven years. The rate was almost the same — 32 stress fractures per 100 athlete-years. In women it was 45, in men 20. Tibial and femoral shaft fractures peaked during competition periods, while metatarsal fractures stayed steady throughout the year.

In other words, a high rate of stress fractures in university distance runners is not a Japanese peculiarity. What is distinctive about the Japanese data is where the bones break.

Why the femur is more dangerous than the foot

A metatarsal stress fracture is unpleasant but usually predictable: a specific spot on the foot hurts and is easy to find with your fingers. The femur and pelvis are different:

The pain is diffuse: in the groin, deep in the thigh, in the buttock. It is easy to mistake for a “tight” adductor or hip flexor.
You cannot feel the bone — it lies deep under the muscles.
The femoral neck is a high-risk site. A fracture you keep running on can displace, and that means surgery and a long time away from the sport.

That is why the Japanese authors emphasise early recognition: their conclusion is that the data “may help to suspect proximal injuries earlier” in runners.

Limitations

One team, elite level, young athletes. The rates cannot be transferred directly to recreational runners over 40.
A retrospective review of records: anything that never reached a doctor did not make it into the statistics.
The published abstract gives no breakdown by sex, and links with footwear, training volume and nutrition were not tested.
The seasonality may reflect the Japanese calendar: university teams start their track season in spring.

How to apply this

Pain in the groin or deep in the thigh that gets worse when running and does not go away within a few days is a reason for an MRI, not a massage. Especially if it hurts at night or when walking.
A simple test at home: hop on one leg. If hopping on the painful side causes pain in the groin or thigh, stop running and see a doctor. It is not a diagnosis, but it is a strong signal.
X-rays often show nothing in the first weeks. MRI is the standard for bone stress injuries.
Be careful with sharp increases in load at the start of the season. In the US data, injuries peaked during the competitive phases; in the Japanese data, there were more injuries in spring than in winter.
Do not forget about energy. A chronic calorie deficit is one of the main risk factors for stress fractures; we covered this in detail in our article on RED-S and runners' bones.
Changing shoes is not a death sentence. The maximalist-shoe hypothesis has not been tested yet. But if you are switching to a new model, do it gradually — just like rotating your running shoes.

The bottom line

In an elite Japanese university team — 29.7 stress fractures per 100 athletes per year.
41% of them were in the femur; the pelvis and femur broke more often than the shin and foot (18.6 versus 11.0 per 100 athlete-years).
There were more injuries in spring than in winter.
Pain in the groin or deep in the thigh in a runner is a reason to rule out a stress fracture with MRI, not to treat a “muscle”.

Sources: Narusawa Y., Tasaki A., Takashima M., Mashimo S., Hara S., Hirao M. “Investigation of lower limb stress fractures in university long-distance runners: characteristics of pelvic and femoral stress fractures”. International Orthopaedics, 2026. DOI: 10.1007/s00264-026-07037-1 · Wolff A., Kurina L.M., Sainani K.L., Tenforde A.S., Nattiv A., Fredericson M. “A Descriptive Analysis of the Seasonal Patterns of Bone Stress Injury Incidence in Division I Collegiate Distance Runners”. American Journal of Sports Medicine, 2025;53(3):708–716. DOI: 10.1177/03635465241307231