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SAN ANSELMO, CA - MARCH 14: Class participants warm up with a bar bell during a CrossFit workout at Ross Valley CrossFit on March 14, 2014 in San Anselmo, California. CrossFit, a high intensity workout regimen that is a constantly varied mix of aerobic exercise, gymnastics and Olympic weight lifting, is one of the fastest growing fitness programs in the world. The grueling cult-like core strength and conditioning program is popular with firefighters, police officers, members of the military and professional athletes. Since its inception in 2000, the number of CrossFit affiliates, or "boxes" has skyrocketed to over 8,500 worldwide with more opening every year. Justin Sullivan/Getty Images/AFP