How do you wear out something that’s still there? I can understand a bald tire or a hole in a sock. But the foam in my running shoes hasn’t gone anywhere.

I watched the Apollo 13 movie on the treadmill this week. After catastrophe, NASA solved a series of nearly impossible problems to get the astronauts home. But one question remained: had the command module’s heat shield survived the explosion intact?

Apollo 13 hit Earth’s atmosphere at nearly 25,000 mph. Protecting the bottom of the capsule was Avcoat, a mixture of resin, silica fibers, and microscopic hollow spheres packed into a fiberglass honeycomb. As it heated, the resin chemically decomposed. Gases escaped, an insulating char formed, and some material eroded away, carrying heat with it. Engineers call this ablation. The shield protected the astronauts by sacrificing part of itself.

Is that what happens when running shoes wear out? Are we slowly leaving the foam behind us?

After hundreds of miles, it turns out that most of the foam is still in the shoe. What’s changed is inside.

A running shoe’s midsole is a polymer foam filled with tiny gas pockets. Every footstrike compresses that structure. Lift your foot, and it rebounds. That’s the cushioning and spring you feel in a new pair. But the structure doesn’t rebound perfectly forever. After hundreds of thousands of loading cycles, cell walls can buckle or rupture. The foam becomes permanently compressed, gradually losing cushioning and rebound. 

Apollo’s heat shield lost material to do its job. Your shoes accumulate internal damage while doing theirs. That makes retiring running shoes surprisingly difficult. The upper looks good. The laces work. You remember what they cost, and you can’t point to anything obviously broken. But the part you bought them for is under your feet. You don’t have to wear a hole through it to wear it out.

How many miles do you typically get from your running shoes? What tells you it’s time for a new pair?

Happy running!