Any number of factors can make your running shoes wear out faster. Even though the bottom of the shoes may appear to have plenty of tread, the mid-sole material can still be broken down. So how do you know when it's time to chuck out the old and replace your running shoes?
If you viewed the mid-sole material under a microscope, you would see hundreds of tiny air pockets that look a lot like a sponge. This material is a polymer called EVA, or Ethylene Vinyl Acetate. When the foot strikes this material, the tiny air pockets collapse just like a sponge reacts when it's squeezed. This action provides your foot with shock absorption.
Three Common Causes of Worn-Out Running Shoes
A runner's weight: The heavier the runner, the more likely the cushioning part of the shoe located at the mid-sole is going to break down.
Lack of rotation: Your shoes might last longer if you're rotating them with other pairs. The tiny air pockets within the EVA have more time to recover or bounce back from being compressed when they are rotated. Studies have shown that rotating two pairs of shoes will help them last about the same amount of time as three pairs that are not rotated. By wearing a different pair of shoes every other day, you can get a 50 percent improvement on the wear of that mid-sole section.
Shoe strike: Some runners scuff or drag their heels when their feet strike. This can cause the material on the bottom of the shoes to wear out more quickly. The rate of wear will depend on the amount of scuffing and the type of material used on the shoe's outer sole.
Most shoe manufacturers use carbon rubber on the outer sole of the shoe. This material is usually black and is very durable. Some manufacturers use blown rubber. Blown rubber looks very much like carbon rubber, but it's softer and wears faster. The upside to blown rubber is that it's lighter weight and makes the shoe feel softer then carbon rubber. So the trade-off between the two types of rubber is comfort versus durability.
When to Replace Your Running Shoes
One way to tell it's time to replace your running shoes is to test the rigidity of the mid-sole material. To do this, grab the heel counter—the round stiff part of the heel—then take your thumb and push in on the cushioning part towards the bottom of the shoe.
When a shoe is new, this material will feel very rigid. However, when the shoe starts to age, the mid-sole material softens. This is when it's time to time to think about replacing the shoe. It's important to get the feel of the shoe when it's new to have an idea of how rigid this material should be.
The mid-sole material softens because the air cells in the EVA are collapsing and staying compressed, which means it no longer has much shock absorption. The shoe may no longer be suited for running, but it can still be used for walking around or mowing the lawn. Judging a shoe's usefulness by just looking at the wear on the black outer sole is not necessarily a good indicator of a shoe's true wear.
If you're a lightweight runner, you may be able to get 500 miles out of a pair of shoes. But if you weigh more than 200 pounds, you'll probably only get 250 to 300 miles before it's time to replace your running shoes. In the end, it all comes down to the physics of weight crushing down on the shoe and breaking the mid-sole down. Once cushioning breaks down, you can increase your chances of getting injured. So keep those shoes new.