After initial contact along the outside margin, the foot everts and rolls inward (what we commonly refer to as pronation). The amount of intial supination, the speed with which the foot pronates, and the amount that it pronates are largely what differ between runners.
To illustrate the initial contact point, I want to direct you to three great videos from Daniel Lieberman’s lab at Harvard. These videos are part of the supplementary information from his 2010 study in Nature and provide underfoot pressure tracings from barefoot runners exhibiting forefoot, midfoot, and rearfoot strikes. Watch how pressure migrates from outside to inside in all three – in no case is there any pressure under the ball behind the big toe at initial contact. I’ve had some trouble getting the videos to play in my browser, so if that doesn’t work you can right-click and save them to watch them on your computer.
Video 1: Pressure Tracing – Forefoot Strike
Video 2: Pressure Tracing - Midfoot Strike
Video 3: Pressure Tracing – Heel Strike
What you will also notice is that in all three videos the center of pressure marker passes right under the second metatarsal head toward the big tow at toe-off. Ever wonder why second metatarsal stress fractures are the most common type of foot fracture in runners? That’s why – it’s due to bending in late stance, not initial impact since the second met isn’t even touching the ground at contact!
So next time you hear someone call themselves a supinator, ask them exactly what they mean by that. My guess is most people don’t even know and were simply told that by someone else who watched them run. You can direct them to these videos because just as pronation is a completely normal part of the gait cycle, so is supination.
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