I’ve had back surgery twice now and wrote the following up for someone who was asking about training after sciatica.
While it doesn’t sound like you have it, it’s a worthwhile read:
The biggest thing is 1) making sure you have the right fit, and 2) working mobility
They go hand in hand, and as mobility increases you should have your fit reassessed. Even since I wrote the original post i’ve been able to change some things due to increased mobility and better muscle activation.
Back pain is rarely due to the back itself. If your back is actually messed up you’re likely going to have leg issues (ie sciatica). Back pain is generally the result of muscle activation issues which causes some to tire before others and then all of that pulling into the muscles in your back which then fatigue and give you the “sore back” feeling. So when @TLRozzle was saying that they get pain hammering on CX bike I am very sure that’s muscle activation (likely glutes) not happening properly. Glutes tire, they in turn then start pulling on lower back muscles to pick up the slack…bam sore back. CX can do this because big power and slower cadence combined really can tire out your glutes quick if fit is not accurate.