Follow J Moyer on Twitter @JMoyerFB and his YouTube channel, Skill Films. Walter Payton, Barry Sanders, Emmitt Smith, and Adrian Peterson make it clear; running back is not a one-size, or one-skillset, fits-all position in the NFL.  Saquon Barkley is the embodiment of explosion. Derrick Henry wins with a freakish size-speed combination. Dalvin Cook thrives…