RAY HARDING: At heel strike, the foot needs to absorb the shock of impact.
So it pronates -- it rolls in.
As the weight goes forward, the other leg is swinging around to get in front.
The foot is recovering back and the arch is coming up.
The foot is now becoming a more rigid, stable structure.
And when you push off, you've got an efficient lift.
RAY HARDING: When you're walking, your feet are pronating too much, and they stay pronated for too long.
What happens is that this part of your foot rolls in here, and it should recover back out again.
But it doesn't -- it stays in, for too long.
SHELLY HORTON: So, although my foot type is normal, my old runners weren't doing me any favours.
Exercising in poorly-fitting shoes is a common cause of injuries.
RAY HARDING: Supposing you had an unstable foot that pronated excessively and you chose an excessively cushioned shoe that's not suitable for that foot type.
The foot is going to roll even more.
So that could create excessive rotation through the knee, or even your back.
You would cause muscle-overuse injuries, because the muscles are trying to stabilise your foot.
So they're going to work overtime.