Today on my ride of 41 miles I did some standing, now I'll tell you something, I DID NOT think that would cause the burn it caused in my legs! I really thought that while sitting, and peddling hard I was working my legs... No sir,,, standing is what really makes 'em burn. The last 5 miles had three sets of decent hills. More like "Valleys, where you see the road go distinctly down ahead, and back up. I made a "Pact" with myself that I would pour it on down the hill,, and then keep in it, and stand trying to maintain 18+ going up those hills. My legs were jelly at the end of the ride.... I didn't maintain 18 either,, but stayed over 15 on them.