How Pro Football Is Declining: A ChartMarcos J. Alonso12/26/13 11:59amFiled to: ScienceFiled to: ScienceScienceSportsEditSend to EditorsPromoteShare to KinjaToggle Conversation toolsGo to permalinkThomas B. Shea/Getty ImagesYour browser does not support HTML5 video tag.Click here to view original GIF
