You may think you’ve got one good foot and one bad foot. On the good foot you land pretty flat and your weight moves right through the middle towards the toes as your body passes over it.
On the bad foot you land on the outside of the foot and your weight moves across the sole of your foot in a curving way, shifting inwards as you come towards your toes.
Thinking of your feet this way probably affects your running form, causing you to try to persuade the “bad” foot to be more like the “good” foot so you’ll have less pain and/or fewer problems.
There are two things wrong with this view. The first is that you can’t have one bad foot and one good foot because they’re attached to each other through your pelvis and thus affect each other. If one foot goes wrong, so to speak, the other also has to go wrong but often in the opposite manner. Furthermore, the impetus for either foot changing often originates elsewhere in the body to begin with and will affect both feet simultaneously.
In other words, you either have two “bad” feet or two “good” feet, not one of each.
The second problem with this view is that the flatter foot, with your weight moving more centrally and in a straight line, would never be a healthy scenario for running. Two good feet would resemble a more moderate version of the foot you thought was “bad,” the one that lands with your weight on the outside, then moving forward and ultimately inward.
A foot that feels very stable, like the flatter foot in this scenario I’m describing, is a foot that may not itself hurt but is connected to a world of other problems and pain elsewhere in your body. The outside of your midfoot is made for weightbearing, whereas the inside edge of your foot is your arch – obviously not meant to touch the ground! Your weight should land where it can be supported, on the outside. And then as you pass over your foot your weight shifts towards your toes and inwards, towards the big toe, which provides a final bit of propulsive spring to the ground as you roll off it. This curved movement is facilitated by the movement of the pelvis and the upper spine and works best when you have healthy trunk mechanics.
If you have the scenario I describe and as a result have pain in the “bad” foot or leg, you can work on that limb with stretching, strengthening, and therapy forever with very little result because it’s not free to move differently in running till the other foot changes. Instead, work on the apparently healthy so-called “good” side, improving your mobility in your waist, hip joint and ankle. This will allow your two feet to stop being polar opposites and both shift towards a happy medium. Very happy indeed, in fact, and much more likely to be healthy and comfortable.