Against the run, the 3-4 scheme uses three big, two-gap defensive linemen playing nose tackle and the two end positions. These linemen are asked to occupy blockers up front, forcing the outside linebackers to seal off the edges. This leaves two inside linebackers chasing down the run instead of one, which helps explain why smaller 4-3 middle linebackers Zach Thomas of the Dolphins and London Fletcher of the Bills wear down during the season.