The Dolphins are doubling down on running back additions in the wake of Alexander Mattison's season-ending neck injury.

The Miami Dolphins signed two free-agent running backs following the injury to Alexander Mattison, who is out for the season.

With Alexander Mattison done for the season, the Dolphins are adding some running back depth. Read PFR for more.

The Dolphins signed Boone to a contract Monday. With Alexander Mattison (neck) being placed on injured reserve, the Dolphins bolstered their backfield Monday by adding Boone and Aaron Shampklin. Boone ...