Around 500 A.D., the Byzantine monastery of St. Stephen, just outside Jerusalem, was a bustling refuge for up to 10,000 monks. Custom dictated that when a monk died, his body was put in a crypt beneath the monastery. When the body had decomposed, the bones went into a repository. Susan Sheridan, an anthropologist at Notre Dame, has studied some 6,000 of these bones and found that the monks were a fairly robust group. They were the healthiest population I’ve ever studied, says Sheridan, except in one respect--almost all the monks seem to have had arthritic knees. Many of their kneecaps’ edges (above right) were worn smooth and shiny as a result of rubbing directly against their thighbones. Sheridan saw the characteristic roughening of arthritis at the points where muscles used in kneeling attach to the bone. Historical records show that the monks spent an impressive amount of time kneeling; praying at midnight, sunrise, twice during the day, at sunset, and again at night. One monk wrote about his nightly practice of descending 18 steps into a holy cave and making 100 genuflections on each step. If you think about what that’s doing to your legs as you come up and down that hard pavement, says Sheridan, you can do some damage.