I'm a medium speed sprinter with 10 years of experience. As usual, I ordered shoes that were a full size larger than my foot size. On the first few runs I developed a blister over my heel where the shoe was rubbing against me above the toe line. I bought taller socks and I still have blisters. Finally, I laced the boot up to the top eyelets and pulled the laces tighter than before. That fixed it! Now the shoes are comfortable. She does a great job of keeping the momentum going. The soles of the shoes curve gently, like the legs of a rocking chair, so I roll from one step to the next with less impact than in other shoes.