Hurt My Back. Leg Training Advice?

Two weeks ago, when I showed up at the gym for my squat workout, I could feel my lower back was tight, even before my first set. I squatted anyway and completed my workout (top set 20 reps with 235) as well as Romanian deadlifts and calf raises. The next day my back was tight and certain movements were painful, and this took about 5 days to recover.

The pain seems to be related to hips/tailbone/lower back (not mid back). Some movements don’t seem painful at all (I can pick things up off the floor, no problem. Bending with back parallel to floor, no problem). I get warning twinges doing things like rolling over in bed (shoulder/hip torsion) and sitting up in the morning with legs over the bed (hip flexors/abs).

Over the course of the week, I nursed my back, but stayed on schedule in the gym. When it came time to squat last week, I felt fine (lingering stiffness from previous week but not bad at all) and I warmed up properly and achieved my top set goal (245 x20).

But on my warmup set of Romanian DL, my lower back seized up (like a warning) at the very top of the movement, towards the end of my set - a sharp jolt of warning pain. I ended the workout. Recuperation seems slower this week, I’m still stiff and sore, and movements feel restricted. It’s Sunday night already, and I’m supposed to train legs again on Tuesday already.

Here’s my basic question: What should I do to train legs now? I obviously need to stop injuring myself, but I also would like to maintain my strength if not continue working towards my end-of-year goal (315x20 rep squat). I’m at a loss for a leg training strategy.

Background: I’m 49 years old and work a desk job, with a fairly long history of recreational bodybuilding. No major gym injuries over 30 years. About 2 years ago, a new gym opened in my area which really got me serious about training again, but I’m a mess: badly sprained ankle 15 years ago was never rehabbed, weak abs (always trained last and always shortchanged), poor flexibility and mobility.

My hips, hamstrings and piriformis are tight, my hip flexors are tight, my shoulders are tight, I’m tight all over. A year ago I couldn’t squat with correct form, but I have been working fairly hard on rehab/prehab with foam rolling, stretching, mobility drills (step-overs and duck unders almost every single training session) and getting my glutes to work properly again, and I’ve been gradually getting my squat together (it used to be my best lift, now it’s lagging).

I hadn’t solved all my prehab issues, but I honestly thought my prehab strategy was working, and everything seemed to be going right. I’m at a loss because I didn’t seem to injure myself during a set of squats - the tightness came before the first set on week one, and after squats were over on week two.

I’d guess it’s a combination of poor core strength, poor flexibility/mobility, but I’m not sure where during the workout the injury is actually occurring, or why now… For a month prior, I’d been training with 225 x 20 (even did 25 reps once) as a ‘consolidation’ or just to make sure that I had really achieved that weight, with no problems.

So I honestly don’t even know where I went wrong… I obviously have multiple structural weaknesses but I’ve been trying to address them. I spent months doing one legged split squats and one legged RDL just trying to get this point, and now something has given way.

Sorry this is so long, but I didn’t want to leave out anything that might be important. Any helpful comments appreciated. I just started reading this Rehab forum, and I’m impressed. I know I need to train abs more diligently, and I can do more mobility work and stretching (yikes, it’s already a 30 minute process, but I guess I need even more). How do I train legs now?