That is certainly true, although cycling is easier on the knees than walking or running, IMHO. I can go nearly any distance level or up without hurting my knees, but downhill is the worst. In fact, I was reading about the new craze of skiing the fourteeners, where extreme athletes climb the mountains in Colorado above 14,000 ft in elevation in the winter and then ski down. A new book has just been published on the subject. It's something I would like to try, but I instead want to cross-country ski on downhill trails, rather than attempt the couloirs, which require steely nerves, thighs of steel, and great skill.