Nothing’s sadder than a runner who can’t run. At any one time, there are a lot of sad runners. Some studies say that 50 percent of runners every year get injured enough so they can’t run, while others say it’s as much as 80 percent.

I was sad and desperate the day I walked into the Santa Monica, California, office of Forster Physical Therapy, the go-to fix-it shop for West L.A.’s runners, cyclists, and adventure racers. It was April 1999, 2 weeks before the Boston Marathon. I’d planned to run it, had purchased a plane ticket months before, but had a problem: I’d only done a total of 20 miles (32 km) in the previous 4 months because ...

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