Why does running downhill cause more pain than running uphill?

I've heard many runners suggest that running downhill is harder on the legs than running uphill. But why? I almost always run faster when I'm going downhill. Running uphill is much more exhausting, which must mean the muscles are working harder, right? If I work my muscles harder, then that should cause more pain, right?

If I do an especially difficult weightlifting session, my muscles ache like the dickens the next day. In an easier workout, not so much. So working muscles harder, like on uphill segments, should be more painful. Yet that's not my experience in actual runs. If I run a few steep downhill segments early in a long run, I can often feel the soreness in my legs by the end of the run.

Biologist P.Z. Myers has a great post that might explain why. He volunteered to serve as a guinea-pig in a student's demonstration. The student had him do an easy weight-training session, spotting him on the lifts but letting him lower the weights on his own. The result: Tremendous pain!

The next morning, I tried to get up. Aaaaiaiaiaeeeaaargh. His experiment had been spectacularly successful, and I could barely move. Let me tell you, brushing my teeth that day was the most exquisite agony — just raising my hand to my mouth was bad enough, but wiggling my arm gently once I got it there? Forget about it.

As Myers explains, when you flex your muscles, all the muscle fibers work together. But when you allow your muscles to unflex in a controlled fashion, the fibers work differently. Individual muscle fibers can either contract or relax; there is no "inbetween." So in order to lower the weight in a controlled fashion, just some of the fibers contract — enough so that the weight doesn't plummet rapidly to the ground - or worse, crash down on some tender part of your body. The effect is very hard on your muscles; they try to contract, but you're allowing the force of the weight to stretch them out anyways.

The top-left image is a section of muscle before exercise; the bottom-left is just after exercise involving controlled lowering of a weight, and the bottom-right is one day later. Only after a couple weeks do the muscles start looking normal again (top-right).

I think this applies to running downhill as well: When you're running downhill, your body is like a weight that you're lowering over and over again with each step. Your muscles are being slowly shredded. So that's my hypothesis about why running downhill makes your legs so much more sore than running uphill.

If you're interested, details about today's workout are below.

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Today I needed to run 15 miles — that's my longest midweek workout yet. Normally on Wednesday I just do 9 miles — 1.3 miles into town, the 6.3 mile DART loop, and 1.3 miles back. I wanted some company, so I did all that, meeting Jeff for a nice, easy loop, then returning home. At that point, I still needed 6 more miles. So I did another DART loop in reverse (the loop actually passes right by my house). I would have been happy today running 9-minute miles, but since Jeff was just doing the 6.3 miles, we took the first loop quite a bit faster — roughly an 8:20 pace. After getting back home and fueling up, I was back down to a 9-minute pace, but I wanted to finish strong, so I gradually built up speed towards the end. My last 4 splits were 8:47, 8:37, 8:35, 8:22. Overall, an 8:41 pace. A good, strong midweek workout.