Could it be that marathon training and racing are actually good for our knees?

A myth-toppling new study of novice, middle-aged runners suggests that the answer is a qualified yes. The study finds that taking up distance running rebuilds the health of certain essential components of middle-aged knees, even if the joints start off somewhat tattered and worn.

But the results also contain a caution. Marathon mileage could erode one vulnerable area within the knee, the study finds, if runners are not careful.

Those of us who run can almost be assured of hearing from concerned friends and relatives that we are ruining our knees. This notion is so entrenched that it discourages some people from trying the sport and worries those of us who continue.

But little of the available research about knees in motion supports that idea. Most past experiments indicate that running generally is not harmful for healthy knees. In one much-cited study, elderly runners developed knee arthritis at lower rates than sedentary people. And in another, more recent study, young people’s knees grew less inflamed after a run than after a long stretch of sitting.