You know the guy who is running downhill and meeting running backs in the hole with bone-jarring hits? The one staring down the quarterback and making hits on receivers that cause snot bubbles? That's your middle linebacker.
Few can think of this position and not remember the greats to play it, like Dick Butkus, Ray Nitschke, Jack Lambert and Mike Singletary. These guys helped build the tradition of the middle linebacker being the most physical guy on the field. The game has changed and evolved some over the years with the importance of speed and having inside linebackers who can play sideline to sideline.
Despite the emphasis being put on speed, many of the top players at this position still know how to bring it and set a tone.