Misi was drafted by the Dolphins in the second round of the 2010 NFL Draft and he stepped in right away, starting 11 games in his rookie year. Last season, Misi started 14 games, recording 65 tackles, 3.5 sacks and two forced fumbles.
The extension comes as Misi was entering the final year of his rookie contract. Misi's biggest strength is in run defense, and he normally gets subbed out on third downs due to his lack of coverage skills. For now, he'll continue to serve as a two-down 4-3 OLB.
Also, this likely means the team does not see Dion Jordan as a guy who should be spending significant time at 4-3 OLB on running downs, at least for now, given that we now have 3 starting linebackers under contract for several years, which limits the number of snaps Jordan could spend as an LB. Jordan is likely being groomed to become a 3-down DE.