March 23, 2007, Newsletter Issue #46: Know When to Replace Your Shoes
Tip of the Week
By the time your women's running shoes show signs of significant wear, it's long past the time you should replace them. Critical wear on running shoes is largely invisible. Here's why:
A dense foam called EVA (ethylene vinyl acetate) makes up all or part of the midsole – the shock absorbing layer of a shoe – of most running shoes. Each time your foot lands, it compresses the EVA - which absorbs much of the landing's shock - and then the EVA re-expands when you take your weight off the shoe.
Each time your foot lands, the EVA bounces back an infinitesimally smaller amount than the time before. Eventually, it loses much of its ability to bounce back – and absorb shock. You don't notice it happening, 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 500 miles before replacing their shoes. Relatively heavy runners, though, should replace them closer to 300 miles.
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