This happens to me on the elliptical. It generally means that I am leaning to far forward and putting too much weight on the ball of my foot. I try to focus on keeping my feet as flat as possible and pushing with my heels, rather than being up on the ball of my feet. I also try to make sure I am standing up straight, not leaning forward (when I'm doing it right, it actually feels like I'm leaning backwards slightly). All of this helps, but I still don't have it down perfectly because I still experience some numbness. Almost always in my left foot, which makes sense because that foot just does not want to stay flat.
If you are working out in a gym, you could just limit yourself to 15 minutes on the stair master and try a different piece of cardio equipment for the last ten minutes. That's certainly my strategy on some days.
- Barbara