Where Does a Shoe Bend When You Walk?
When you walk, your heel lifts and your weight rolls forward onto the front of your foot. Your toes bend upward at the joints where they meet the rest of the foot, the "ball" of the foot. A lab study that tracked the foot with cameras and force plates found that this joint keeps bending as you push off, and that its axis sits a little ahead of the line you would draw between the big and little toe knuckles, by an average of about 16% of the foot's length [1]. A shoe-testing lab in Tokyo describes what this means for the shoe: in its account, walking bends the ball area of the shoe sharply over a narrow strip, with every step [2]. So the "valley" is probably not random. It is drawn by your own walking.
Why Does Bent Leather Wrinkle Instead of Bending Smoothly?
Leather is made of tangled protein fibers, mostly collagen. A hide has a tightly packed outer layer, called the grain, on top of a looser layer underneath [3]. In most leather shoes the tight layer faces outward. One shoe-care writer explains the creasing this way: the leather upper is held between the toe cap, the instep and the foot below, so when the foot bends, the dense outer layer has nowhere to go and folds into creases [3]. This is one person's explanation, not a lab result, but it matches what you can see. A simple way to picture it is bending a thick sheet of card: the inside of the bend bunches up.
The same writer suggests that folding breaks up the fibers a little at the fold, which makes that spot softer, so later bends tend to happen there again. That would explain why a crease, once made, tends to come back in the same place [3]. The writer also says the first few steps in a new pair decide a lot about where the lines end up, and that a shoe that fits closely has less loose leather to fold [3].
How Do Labs Bend a Shoe Thousands of Times?
Companies that make shoes want to know how long a shoe will last before it cracks. The Tokyo Leather Technology Center describes a "flex test" based on a method from SATRA, a UK footwear research group. A machine clamps the toe end of a whole shoe and bends the back part, adjusted so the shoe bends where it would when worn. The bending angle is set by shoe type, from roughly 10° for some women's styles up to 45° for men's and boys' lace-up shoes. The machine bends the shoe at a set rate of 140 times a minute (give or take 10), and the test stops at a maximum of 500,000 bends [4].
That upper limit is large. By my own arithmetic, 500,000 bends at 140 a minute is about 60 hours of non-stop flexing, around two and a half days. Testers check the shoe along the way and write down the bend count when something goes wrong. They look at the outside of the shoe, not the inside [4].
When Does a Crease Turn Into a Crack?
A few creases are normal. The Tokyo lab says that a worn sole and creases in the upper are unavoidable, but cracking early in the shoe's life is a problem [2]. Their example is a pair of men's shoes made of goat leather that had been finished stiff to look firm. The lab estimated that sweat made it stiffer still, and it cracked early [2]. Stiff material cannot fold gently, so the fold becomes a break.
Rubber rain boots show the opposite. The lab says that they generally do not crack, and gives as reasons that rubber is flexible, bends smoothly and resists bending little. It also thinks the loose fit of such boots probably plays a part [2]. Softness alone is not a guarantee, though: the same report describes velvety fabric and stretch nylon uppers that cracked within a short time, probably because the folds piled up in one sharp spot [2].
How Can You Look at Shoe Creases Yourself?
With an adult's permission, pick up a pair of shoes that has been worn for a while. Find the deepest creases and compare them to where your toes bend inside the shoe. Then look at a pair of rubber boots or a canvas sneaker and see whether the lines look the same. Compare a shoe that fits snugly with one that is a little big. You are only looking, so there is no need to bend the shoes hard. If you want to read more, search for "shoe flex test" or "SATRA PM92" and see how shoe makers describe it.