The researchers asked these men and women about their knees. At this point, the marathon was still six months distant, and their joints were those of middle-aged adults and not yet those of runners. All of the soon-to-be marathoners responded that the joints were in good shape, with no creaks or pains.
The researchers then gathered the volunteers at a university facility and scanned everyone’s knees, using a sophisticated, high-resolution type of M.R.I. that reveals even minor damage in the joint’s tissues.
A few months later, the men and women began the same, four-month marathon-training program. Eventually, 71 of them finished the race, in an average time of 5 hours and 20 minutes. Two weeks later, they returned to the lab to have their knees re-scanned.
The scientists then compared their hundreds of before-and-after scans and turned up some surprises. For one, while the participants had reported at the start of the study that their knees felt fine, many, in fact, harbored damage. About half of the pre-training knees contained frayed or torn cartilage, and others showed lesions in the joint’s bone marrow. Similar patterns of tears and lesions can signal incipient bone erosion and arthritis, the researchers knew.
The post-race scans held other, new and unexpected results. “I expected to see additional damage” in runners’ knees, Dr. Hart said.
Instead, many of the existing bone-marrow lesions had shrunk, as had some of the damage in the runners’ cartilage. At the same time, some racers had developed new tears and strains in the cartilage and other tissues at the front of their knees, around the kneecap, a part of the joint known to be stressed during running. That area also, though, tends to be less prone to arthritis than other portions of the knee.
Over all, “the main weight-bearing knee compartments showed beneficial effects from the marathon,” says Dr. Horga, meaning that, in general, the knees were healthier.