The culprit

I was on the treadmill last week and my foot started to hurt. I had only been on the treadmill for 10 minutes so it caught me off guard. I took my shoe off to see if there was a rock or something, but I didn't see anything. I decided to switch to the elliptical (less friction) and finished my cardio. When I got home and took off my shoes I found this



and then I figured out what was the cause of all this pain (I took a picture, but can't find it). There was a hole about the size of a nickel that had worn through the inserts of my shoe. I concluded that it had been there for a while, but because I had been on the bike and elliptical, I hadn't noticed it.

So my guess is that a rock did get in there and I had been wearing it down until it got lose and made this nice little hole. The inserts were supposed to last through 2 pairs of shoes. Needless to say, I had to get these replaced. All is healed and feeling better!a