Finally getting time to post to this.
This problem is related to the other issue you reported with excessive arching when attempting to contract your glutes. Sorry, in interests of not representing myself as qualified to make a medical assessment, insert a “probably” all through that statement and everything that follows.
Flat feet appear to be about your feet, but for most people and I suspect you’re one of them, the issue is with your ability to control your hips correctly.
Kelly talks about your problem in this video http://www.mobilitywod.com/2013/05/the-flat-feet-solution/ but you need a subscription to MWOD Pro to see it. It’s titled “Pro Episode #3 The Flat Feet Solution”, here http://www.mobilitywod.com/episodes/
As far as free videos, Kit Laughlin has a good one here https://www.youtube.com/watch?v=XfAJa0yNliM that doesn’t solve the problem in an identical way to Kelly, but does talk about the relationshiop between hip dysfunction and flat feet.
Now as to how to address that excessive arching thing.
First off, stand up.
Second, never sit down again.
Sitting kills your ability to get your glutes to fire correctly. When you sit and slouch a bit, the pressure is on exactly the bit of your butt that I was getting you to grab in the other post and that paralyses it. Do it for years and your body learns to use other muscles to compensate for that paralysed muscle group and you have a hard job retraining your body to move correctly.
I’ve been working at it for several years now and I’m getting there, but it’s still very hit and miss.
There is a technique called NeuroKinetic Therapy that is designed to address this type of compensatory movement pattern. It sounds promising but I don’t know if it’s always as effective as their promotional material suggests. There isn’t anyone near me offering it or I’d try and report back.
You’re looking for exercise and movements that get your glutes to fire. Air squats to a box, use as high a box as you need to maintain perfect form are good. Banded good mornings are good, so are monster walks and variants. But they all rely on you being able to fire the muscle you’re targeting. If you’ve rendered that muscle unable to fire under conscious control, they aren’t an instant fix.
I’ll ask Kelly the question and see if he picks it up amongst the zillions he gets asked and puts up a video about it.