Have you heard of Vertical Oscillation before? It's the amount of up & down bounce your body creates when running. In general, faster runners have less vertical oscillation, in the 8.6cm or under range, as per Garmin's metrics, so it seems more efficient runs demonstrate less bounce. Form junkies hope that angling your torso forward can change the general trajectory of your effort from an up & down bounce to a forward push, increasing your running efficiency. "Running is a controlled fall forward" is a motto I've heard often as well. Running uphill seems to lower vertical oscillation, and can be proof that angling & pushing forward seems to do the trick for reducing bounce.
Another reason to care about your bounce is in order reduce injury risk. Running has been shown to increase the force of impact up to 4-7x that of walking. While I would argue that accommodating to increases in force across your joints is actually healthy for you, a slow progression of this force increase is best, just as with weight training. The higher you bounce up, the greater the potential to land hard. If this happens frequently without adequate force absorption through muscle ability, tendons & joints will begin to complain. As you become a more efficient runner, increasing your cadence and landing on your mid-foot or forefoot seem to naturally improve, thereby improving your oscillation.
So how do you reduce your Vertical Oscillation?
1- Angle your torso forward, but not by rounding your shoulders! Try to lean forward from your hips and notice how your weight shifts to your mid foot or toes. Try to feel as though you are able to control a fall forward.
2- Run on an incline: This seems to engage the torso lean without most people's awareness. Best done on a treadmill- While uphill running does the same job as an incline treadmill, outdoor hills also need you to run DOWN the hill which can actually increase ground reaction forces.
3- If running on a treadmill, lower your music & pay attention to how LOUDLY you are landing. This can be the most obvious way to notice your bounce/ground reaction force. Try to soften the sound, either by reducing your speed just slightly or by trying tips #1 or 2 above.
4- Work on Eccentrics in the gym. Eccentrics are the slow lowering/landings after you push against resistance. Landing softly after high jumps, Lunges with an "up fast/down slow" emphasis, Slow step backs from a high step are a few suggestions that should work well.
5- Pay attention to cadence: Smaller steps when running makes a softer landing with less bounce. In the gym, building in alternating leg work into your program is a good idea when focusing on faster cadence. Toe taps to a step, lateral steps across a step & back, alternating lunges, Alternating lunges jumps are a few exercises that fit this principle.
A note: Vertical oscillation can increase as you attempt to run faster. But controlling for your bounce & impact is one way to become comfortable at this speed, progress endurance in the new speed range & reduce both acute and chronic injury risk when you are a running athlete.
It's nice when you can see how improving your health, reducing injury risk and becoming a better athlete all can work together. Happy running!