Having the ability to run downhill fast does not happen by chance. The same rules of efficient running apply whether you're on the open road or a rocky trail, uphill or downhill.
In simple terms, when the foot hits the ground with a certain amount of force, the better the body is at scooping that energy back up as the foot leaves the ground and not letting it "leak out", the more efficient, fast and powerful your running can be.
When running efficiently downhill, gravity is your friend. If the body is unable to handle the momentum, it will be forced to put on the brakes to maintain control down the hill. This means a runner will miss out on the advantage of gravity, and will actually have to use some of their energy to fight the force that could be helping them downhill!
There are a few factors that come into play during efficient downhill running, both on and off-road. It's not about different mechanics or a different style of running than when on flat ground. Think about it as maintaining proper running mechanics and keeping your legs under you as long as you can at the highest speed you can handle.
In the case of extreme trail running, it is also important not only to have the leg speed, but also the coordination and quick reaction time needed to either utilize and/or avoid obstacles along the way.
To do all of this requires proper running mechanics, key muscles activated, stability through the joints and strong, elastic muscles.
1. Proper Running Mechanics
As in all running, the foundation of running efficiently downhill relies upon maintaining tall posture and a strong circular motion of the legs underneath the body. In its simplest terms, this means lifting the knees out in front, foot striking directly beneath the body and then pulling the heel back around to start again.
In general, I see many people running lower-leg dominant instead of from their hips, with minimal knee raise. This alone would greatly reduce the ability to keep up with momentum while running downhill and certainly make it necessary to put on the brakes very early on.
2. Activating the Abs and Glutes
In any type of movement, the abdominals and the glutes are important to both force production and overall control of the body. Activating and utilizing them properly within your running mechanics gives the body much greater control over the legs. It also minimizes the impact on the quadriceps and knees that so often take most of the beating during downhill running.
Having that control is pivotal to taking advantage of gravity and keeping control over the momentum, especially while avoiding obstacles when on the trail. Be sure to integrate core strength and stability work along with glute activation exercises in your training.
3. Joint Stability
Your ability to maintain stability through your spine, hips, knees and ankles during each step is crucial to controlling you body's direction and forward speed as you hit the ground. Having strong muscles surrounding the joints is key to creating this stability.
Your joint muscles can be strengthened with single- and double-leg strength and balance exercises that challenge both linear and lateral movements. Especially for extreme trail running, being able to remain stable during quick changes in direction is not only great for preventing injury, but helps with quickness and agility.
4. Elasticity
Elasticity is one of the most overlooked elements of endurance performance, and I would consider it like icing on the cake for downhill running.
"Think of your body as a pogo stick. The metal framework is analogous to your pillar and the springs are like your muscles. We want our bodies to be able to store and release energy powerfully, just like that pogo stick. It is like your body's shocks and springs." writes Mark Verstegen in the book Core Performance.
You can also think of elasticity as how fast your body is capable of changing the direction of force. In tennis it might be a lateral move to get back across the court. In running it would be how quickly you can get your foot off the ground once it hits, ideally while scooping up all the force you hit with and bringing it all with you into your next stride.
It is pivotal to being able to keep your legs under you, as mentioned earlier. You can train the body to be elastic with explosive exercises, like jumps and bounds, and specific running drills. Opt for fewer reps and make perfect form a priority.