It has been almost four decades since Jim Fixx wrote The Complete Book of Running, which many credit with starting a fitness revolution in the U.S., and it's still going strong. According to data gathered by Fitness USA, a nonprofit organization, the number of people who finished a running event more than doubled from 2005 to 2013. Fitness is more than running, of course. In 2014 people in the U.S. spent $63.5 billion on sporting equipment, an increase of 32 percent over the previous decade.
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