Anyone who has run a race with a lot of downhill (hello, Boston marathon) knows that running downhill can actually be more taxing than the grind up the hill. The inner quad (i.e. the vastus medialis, the muscle that is HUGE in cyclists) takes the brunt of downhill running, but the hips, ankles and low back are also affected. The eccentric contractions when running downhill are fighting the stretch from gravity; force on the legs is coming from two directions. Downhill running, whether it is extended amounts or just a quick burst down a steep hill, can cause lasting muscle damage.
The key to running downhill quickly without long term repercussions requires good technique and lots of practice. Most people brake when they run down a hill by landing too far back on their heel causing a sitting-like position. Instead, approach the hill without fear and relax and let gravity help you. Always keep your foot plant directly under your center of mass and do not over stride. Glide smoothly down the hill with a shorter stride and higher cadence (get your cadence close to 100, which is a better way to run in general). That will shorten the length of your quadriceps muscles and minimize stress and potential muscle damage.
Incorporate hill repeats into your training, but mix it up. Choose a day where you use the uphill as “recovery” and work the downhill; this is an opportunity to execute good technique and allow you to run faster than normal. If you are running on a “rolling” course, this is also a chance to practice running well downhill. Of course, ease into this type of regimen, with just a few repeats, keep the speed restrained and start with a moderate grade, such as 2%. Over time, as your legs get used to the extra pounding, you can increase not only the length of the repeat but the grade and your pace. Let the downhills become your forte and a place to gain “free” time. Better downhill technique will translate into better overall running performance.
You can ready your body for the rigors of downhill running by doing some specific exercises in the gym or at home. Lunges, with good technique and very light dumb bells, are excellent for stability and strength building. Watch yourself in the mirror to make sure your hips stay level. Plyometric drills, such as box jumps, hops, and bounding are also important for any runner. Check out this step up exercise to help you with some downhill running: https://sprongo.com/video/917674/share