Do running shoes loosen up?
No, running shoes don’t loosen up. But they do deform around the foot. And when they do that, it means that those shoes never did fit.
A new pair of shoes should fit properly, right out of the box. If a pair doesn’t fit, you have two choices. You can wear them until they deform around your foot. Or you can put them back and find a pair that fits. The second choice is the right choice.
Your entire foot should be supported by the shoe’s midsole. That means that no part of the foot should protrude over the side of the midsole. When that happens, the shoe’s upper has to hold-up the protruding part of the foot—kind of like a hammock. But the upper isn’t meant to keep parts of the foot suspended in the air. The upper is meant to keep the foot securely on the midsole. When used like a hammock, the upper will be stretched out of its original shape. Once deformed, the upper can no longer be expected to perform as it was designed to.
Do your running shoes fit?
There are two tests.
Test #1: Take off your shoes and look closely at the upper of your running shoes. Look closely at the front of the shoe, where the toes are. If you see bulges, holes, or broken material, then the shoe is deformed.
Test #2: Pull out the sock-liner. If you can see your foot’s imprint, then no part of the imprint should be cut-off at the edges of the sock liner. If the imprint is cut-off, then part of the foot is hanging over the edge of the midsole.
In addition to length and width, a foot has depth. And some feet are extraordinarily deep. There are cases where a shoe is not deep enough for a foot, even though it gives adequate length and width. In those situations, it’s best to use different lacing techniques to accommodate the foot. A good running shoe store will know the different ways to lace a shoe.
Buy running shoes that fit. Don’t deform the shoes around your feet!