That's not "mountain biking," those are constructed jumps and obstacles, spaced so the riders can land perfectly. It's a dirt track in the woods. I must have mountain-biked a thousand miles in the Appalachians...and those were the old school trails you're talking about FK. Even if you had run one half a dozen times, you had no idea what was coming up. We had a trail we'd run called "suicide machine." It kept eroding, so if you ran it a month ago, you may not be aware of the new 5 foot drop around the next corner, all the while traveling at 15-20 mph. We had this dude on our team named Crazy Geoff. He was ejected from his saddle at speed on that trail, and the only thing that survived was him and his rear wheel/tire...it even split his helmet in half, bent his front wheel and fork, and bend his frame. We couldn't believe his neck wasn't broken.