Evolving offenses have forced defenses to change over the years. One change in particular is the physical traits and roles of inside linebackers. It wasn't long ago that inside linebackers were run-stoppers first and foremost. Today's modern offenses still necessitate the need for run-stopping linebackers, but now also require more athleticism to counter spread offenses. Generally speaking, they may sacrifice size relative to linebackers ten years ago, but today many linebackers post forty times comparable to receivers and running backs - not just tight ends. This position requires elite athletes more than ever.
Given that linebackers are now required to contend with spread offenses, and are defined just as much - if not more - by speed than run-stopping power, it's important to reiterate the symbiotic relationship between that position and defensive linemen.