Now I knew this, but I didn't expect it to be so dramatic. I've been doing my cv stuff on machines for ages to spare my knees/achilles - speedmarch up a treadmill, steppers, bike. On none of these things do you do a negative contraction, as you would coming downhill. If you don't train negatives it will really knacker you if you have to -I knew that. Last week I thought I'd run a tower block - something I used to do. Jogged over, up 25 floors with no real problems, thought "That's not bad", went to get the lift down and met a bloke coming out who said, "I wouldn't use that; it's just on the point of breaking down". A female had died a few days before after getting stuck and forcing the doors, so I thought, best not risk it, so I bimbled down the stairs. F'kin hell - half way down I could feel my thighs starting to cramp. Significant, although not severe pain. I got to the bottom and ran back up - no problem. Down again, and the same thing, just the start of cramp kicking in. I would never have believed that I could provoke cramp coming down ten floors in a tower block, especially if I had the fitness to blatt up the stairs no problem. So if you don't train eccentrics you might have problems. That said, I've went and ran stairs again, and the problem seems a lot better even after one session. The only indoor thing, low impact, I can think of to train eccentrically is putting the treadmill below 0% and running down that.