Soles actually. I've been studying the wear patterns on the soles of my shoes. I can see the evolution of my running form in the soles of my shoes.
New Balance 622.
I started off by running in the shoes I had. My running form was completely awful. I knew nothing about running and these shoes completely shielded my feet from any feedback I would get from slamming my heels into the ground. They are worn from the outside of the across to the big toe area. My toes stick out to the sides when I walk (duck feet) and when I ran in these shoes I did the same thing. Not only did I heel strike, I also zig-zagged somewhat when running.
Vibram Five Fingers Bikila: The majority of the wear was on the outside edge of the forefoot, with some minimal wear at the heel, and another large area of the wear on the sides of the toe units.
Wait, what? On the sides of the toes? How does that happen?
after discovering this wear on the sides of the toes and the outside of the heel I started really trying to see what my feet were doing. I was landing on the forefoot, with my duckfootedness still happening, as my heel lowered, my foot would rotate into a forward pointing position causing the sideways wear on my toes and the sideways wear at the heel.
Running in this shoe overlapped with running in the Bikilas and had a similar wear pattern.
My new Bikilas also feature the sideways wear at the toes, but lack the sideways wear at the heel. I think my foot is coming to the forward position more quickly than before so that by the time the heel lands the rotation is completed.
Time will tell what my new Xero shoes will tell me with their wear pattern, but they are already tattling on me quite a bit. I get feedback faster from them thanks to the construction. There are more details on that here.
What do your wear patterns tell you?