I think that too much is made out of the SILB or Mike (depending on scheme, but for ease of discourse, let's assume there isn't a Will and a Mike, just a S/WILB) here. I have experience with both positions, but in my scheme the strongside inside postion was the Mike. Due to this, the position required checks and tremendous awareness pre-snap. It was very time consuming from a film study and SWOT standpoint, but was quite easy to play from the on field standpoint. The 34 run defense is predicated upon gap management and flow.
Let's look at a very basic example of an offtackle run with a generic 26 handoff. For those that don't know the nomenclature, the 26 means the halfback runs the ball through the six hole, which is the right side hole between the tackle and the tight end. The SILB is the playside defender. In a generic "30 basic" call, the SILB is responsible for the C gap. This is the gap between the tackle and the TE. The SILB reads the offtackle run from the blocking scheme, usually by reading the guard trying to seal him, or the TE cracking if the z on the jet also cracks the playside OLB. The read is three steps to the football, engage and shed if it is a guard, stack and drive into the hole if it is the TE. In order to execute this assignment, the backer needs exceptional hip strength, decent quad and lower back strength, and to a certain degree; good front delt, tricep and upper pec strength. It is also critical to speak about the importance of burst. In sealing the run plays, the player needs a great burst to the hole. It is critical to the play that the linebacker arrive at optimum momentum in order to hold the LOS and maintain gap integrity. Driving an opposing player back is not about brute strength. It is a function of hip explosion, momentum, and technique. Compare this to the flow to the football, covering recievers out of the backfield, covering the TE, and downfield pursuit requirements of the Will. The Will is a far more athletic position than the Mike, and obviously, it has many more responsibilities.
Before the geekout gets too far, I think that it is necessary to speak on the Mike's difficulty from a playing standpoint. Physically, you need only hips, acceleration and technique. These are all acquired in the weight room, with a moderate degree of athleticism required to build upon. The actual urgency to fill the position is frankly like the urgency to fill the need at fullback. Yes, it is an integral starting role in the defense, but it is hardly a position that the scheme is predicated upon.
