The next time you see a Dolphins running back take a handoff, burst through a hole, use his quick feet to juke a linebacker and then speed his way to a 12-yard gain, double check the jersey number. “I never tagged myself as a speed guy,” the 5-foot-11, 220-pound Mattison said, “but it’s fun to get out there and kind of push the boundaries a little bit, push my limits, and see how much I can flourish in an offense like this.” This show of speed from Mattison is low-key big news. Mind you, the Dolphins were ninth in the league in run defense last year.