Last year I was injured, and I'm about 96% certain it was because of a lot of downhill running.
It's tempting, isn't it? You might be running along, or perhaps UP a hill, your stocky, bootylicious self, and you see the downhill and you say to yourself, "WARP FIVE, MR. SULU!". Chances are, if you are not a bird-person, and if you haven't (gently) trained running a lot of downhills, you might be cutting your race short and you will suffer.
If the finish is down at the bottom, fine. But if not, if you have a couple more miles to go after that long downhill (I'm talking here of marathons, 10ks, et cetera, not necessarily 5ks, although I'm not ruling them out.) say, a couple more miles or flat or--shudder--another hills, baby, you are DONE.
Last year I did several long downhills on pavement. I did the New Mexico marathon, which has a 6-mile downhill between about mile 15 and 21.
At right, this is an old profile from a training ride, but the area between miles 1 and 13 are the first half of the NM Marathon. -->
At this marathon, I abandoned my works-for-me plan and answered the siren call of gravity, hauling ass down that hill.
I laid off, and then did the Colorado Relay 2 weeks later. My splits were short, about 4 to 6 miles, but all downhill - my choice. The last one dropped in altitude over 900 feet over 3 miles. OWWW. But I was okay, I could jog, a little. X-rays were negative. Diagnosis: shin splints.
Three days later, I went for a quick (2.5 mile run) to the local aquatic center, swam, and then headed home. 100 feet into a very steep downhill on blacktop (running full tilt, of course) I suddenly could not run any more. The theory was that at that point I'd developed a stress fracture. I didn't run on pavement for over 8 weeks, although I could do some light trail running. The rest is history. I finally feel like I'm recovered.
Ahead of time, a fellow ultra runner, Don, warned me about the downhill. Don't go flying back down that hill, he said. You're not done yet.
There was a woman just ahead of me, who seemed invested in staying ahead of me. She is a bit trimmer than I, and she kept looking back at me. Because she did that, well, I had to beat her.
You understand, right?
She was good on the flats, but challenged by the uphills - I passed her going up that long 2.5 mile climb. I've started doing this little hopping run--tiny steps-up hills that is faster than hiking, but I could only just now do it because I had to develop quads and calves. But anyway. So I passed her, and said as nicely as I could, "boy, I'll sure be glad when this hill ends, won't you?" as she walked dejectedly up the hill, hands on hips, taking long, huge steps*.
As tempting as it was, I'd been thinking about New Mexico marathon lately, and last year's injuries, and Scott's warning, and I continued my gentle jog down the slope.
When we got to the bottom, she was done. DONE. as in, she could barely move. She'd shocked the crap out of her shins and quads, and now barely shuffling on the rollers.
*Big mistake. When going uphill, change your stride to smaller steps, else you'll burn your adductors out.