Yeah, I've run into lower back issues occasionally (I think we all get there at some point).I usually just switch to a different squat variation (front, olympic, belt, etc). If you wanted to try a grinder, there are (for fun) methods you could try. I mentioned Tabata above. I've never met anyone that doesn't come out of that ready to face plant. I've got a video on the site going through it (<a href="http://www.bodybuilding.com/fun/video-christmas-day-squat-till-you-drop-a-thon.html).">http://www.bodybuilding.com/fun/video-christmas-day-squat-till-you-drop-a-thon.html).</a> Your a more advanced guy, a lot stronger with a lot more muscle than most. Also, while for advanced bodybuilding you can make a great argument for slow, controlled reps, my reps are always done explosively for maximum motor unit recruitment. If you are fighting back issues, I would recommend a lot of accessory work involving hip extension (hip thrusts, pull throughs, RDL, good mornings, etc). Include single leg training, and I heavily recommend belt squats.