Running up hills is great for your strength and endurance, and it's inevitable in some parts of the world (especially if you run on trails). But downhills can result in some of the worst next-day soreness if you haven't trained for them.
Photo by Peter Mooney.
That's because downhill running uses eccentric muscle contractions — you're using your muscles to lower yourself down the hill. (This is the same idea as a negative pull-up, where you lower yourself slowly down from the bar.) You can train your downhill-running muscles by carefully scheduling downhill running repeats: start with a few and work up to more.
Good form on the downhills helps, too: stay upright, and keep your feet moving fast. This sounds scary at first, but is actually easier on your body. Instead of slowing yourself down by landing on your heel every step, you're just letting your momentum carry you forward. Practice this technique, and you'll fly past the crowd next time you're in a race that takes a downhill turn.
Tips for Downhill Running [Runner's World]