Running Form Can & Should Change
How your legs move also changes based on speed, terrain, shoes, fatigue, how far you are into a run, etc etc. A study of barefoot Kenyans noted that their at habitual easy pace 3/4th of them were rearfoot strikers. But when they sped up their footstrike shifted forward! In the Leiberman study noted in Born to Run that helped start the "everyone should midfoot strike" craze, the Kenyans who ran barefoot with a forefoot landing where running at a sub 5:00-mile pace! Of course they were tending to forefoot land!