JD is so paranoid about downhill running and injury risk, he recommends treadmill for hill intervals, so you dont have to run downhill at all. at least in 2nd edition. but personally, i've never had any problems running downhill. does seem like there would be more impact force, esp w a pack.
Funny ... just read that section in 3rd ed last night .
. Ive always liked downhill s and going fast ... ran the last hill this morning
On the hasgrass about as fast as i could go