Triathlons, such as the famously arduous Ironman Triathlon, and "extreme" mountain biking-hair-raising events held over exceedingly dangerous terrain-are prime examples of the new "lifestyle sports" that have grown in recent years from oddball pursuits, practiced by a handful of characters, into multi-million-dollar industries. In Why Would Anyone Do That? sociologist Stephen C. Poulson offers a fascinating exploration of these new and physically...