Well as coincidence would have it I too have encountered a failure of a day haha. Week 2 squat strength day and I blew out my back on the second warmup set... Not even a work set, a god damn warm up at about 60% 1RM.
In my case I suspect it's because this past week I have been doing something at work that falls very outside my usual job description and has me crawling around underneath heavy equipment disassembling by turning very hard bolts and moving very heavy stuff all day long. Parts probably weigh 50-60 lbs each and it's impossible to move them with proper form as space is so limited.
I've been eating a ton and sleeping more than usual and my workouts during the week were solid, including week 2 deadlift day on friday! Although I was tired and it took me over 80 minutes. Today was obviously a complete write-off though, but I'm rather surprised my back held up for the deadlift day just fine but not today. Maybe because I was sitting on the couch all day yesterday and today.
I have felt like my squat form has been solid given that my back never gives me problems afterwards, so I'm also a bit surprised about that too. I guess I leaned a bit too far forward which is what I've been trying to fix with the paused squats. I've switched to low bar position for 915 and I might be trying consciously to allow myself to lean forward more than low bar allows, or perhaps even with a low bar position my body prefers to be upright.
Anyway, after that warmup set where I cranked my lower back I walked around for 10-15 minutes, did some bodyweight squats, bumped it back down to my first warmup weight and threw a belt on. Back felt pretty good then. Bumped up the weight to the same where I hurt myself and cranked out a few reps. Using the belt I felt no pain in my back. Went up to my work weight of 80% and did a couple sets of 3. I couldn't go on though, something about hurting my back took all my energy out--I was exhausted at this point and felt extremely weak.
Not sure if I should...
-try this squat workout again in a couple days and move the whole program back
-skip it, go on to high pull day next
-start using a belt more? It felt really good to brace against something. Made me feel the movement more in my abs and lebs.
Aside from this my results have been great. I didn't think the mPA and I3G were doing much but now... 1.5 weeks after starting them I've gained 7 pounds and have more ab definition. The volume work feels great too. I would never have thought I'd be able to survive so much volume with squats and deadlifts but I am loving it rather.
Really bummed about this workout but there are lots of good things to look at too.