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Running shoes wear out – but it's not obvious when they do. By the time most running shoes' soles show significant wear, it's long past the time to replace them. Here's why: The midsole – the shock absorbing layer of a shoe – of most running shoes is made either partly or entirely of a dense foam called EVA (ethylene vinyl acetate). Each time your foot lands, it compresses the EVA - which absorbs much of the shock - and then the EVA re-expands when you take your weight off the shoe. You know how Homer Simpson's couch has a depression where he sits? Well, that's what happens to EVA over time: It loses the ability to bounce back. You don't notice it, because it happens very gradually. But every footfall reduces the foam's effectiveness. By the time you've used a pair of shoes for 300 – 500 miles, the foam has lost enough “bounce” that the shoes should be replaced. Typically, lightweight runners can go closer to the 500 mile mark, while heavy runners will have to replace their running shoes at closer to 300 miles. Also, if you run outdoors in all weather vs. indoors on a treadmill, your shoes will wear out more quickly.