Running has plenty of heath benefits and can help you get physically fit, but from shin splints to blisters, the sport can also throw your body for a loop. Ultra-marathoners—a special breed of runner not content with stopping at a 26.2-mile footrace—face health concerns that are different from a typical person. A new study suggests that among other effects, these taxing races may temporarily impact their brains.

Intrigued by the physical benefits and detriments of these ultra-races, researchers recently studied runners in the Trans-Europe Footrace, a course that takes 64 days and covers 2,788 miles from southern Italy to Norway. They found the runners endured some surprising changes in the runners bodies, reports Jessica Hamzelou for New Scientist.

Uwe Schütz at the University Hospital of Ulm in Germany and his colleagues followed 44 runners, and used an MRI machine to scan the athletes joints, limbs and organs. They reported their findings last week at the annual meeting for the Radiological Society of North America.

After about 1,553 miles, the researchers noted that the cartilage that normally cushions joints showed signs of breaking down. But after that point, the runners started building their cartilage again, even without rest.

That wasn’t the only change. 13 runners from the group that volunteered for additional study all lost brain volume during the race. As potentially alarming as that seems, they gained it back by eight months after the race, the researcher reported in BMC Sports Science, Medicine & Rehabilitation. Several years ago, a similar study pinned the volume changes at an average of six percent.

The brain changes might have come from under-stimulation of the brain, Hamzelou writes. For 64 days, these runners looked at the road in front of them and concentrated on going forward. "It is hard to explain what’s going on," Schütz told New Scientist.

In general, exercise has many benefits for the brain. But perhaps it isn't surprising that such extreme exercise comes with some extreme-sounding body changes.