Might want to start SLDL's from just the height of the 45 plates if your interested in more power and development, with the prime focus being on the lower back. Bend the knees to releive some of the excess stress on that area and the small of the back (spine). Keeping them stiff during the exercise could cause more harm than good when heavy weight is used. Have the back straight at all times and the shoulders inline with the back as you lift. The lower back is a very powerful short range muscle group. The capacity to work into very heavy weight is possible in a rather short period of time invested in that muscle group. Can also work in a power rack (or set of boxes, stands, etc) where the bar would only be moving 3 or 4 inches, at the most, to a standing position. Any SLDL version can build a foundation of core strength if the exercise is preformed correctly and a favorable stretching/warm-up scheme is done before approaching heavy lifts. The GoodMorning exercise also falls in the class of a muscle mass and strength builder. Be sure to work the abs just as strongly and heavy as the lower back. This insures balance to the pivot point of the lower bacK & ab wall. If you have excessive soreness or feel that the lower back really is out of whack the next day than you may want to check your form when doing SLDL's.
Most every football coach has a different slant on training. My coaches in college always included forms of SLDL's, GMornings & regular DL's in training sessions. Also did power cleans & HI-pulls which can develop the lower back muscles strongly. When approached with logic and correct style, the SLDL is an outstanding movement for the whole chain of rear muscles from the ankles all the way up to the neck
