The solution: Make sure that you don't lunge forward with your feet. This is especially important when running downhill. Focus on landing mid-sole, with your foot directly underneath your body with every step. A short, low arm swing is the key to keeping your stride short and close to the ground. Try to keep your steps light and quick, as if you're stepping on hot coals.
More: What Part of My Foot Should I Land On When Running?
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