College athletics, have had two back surgeries, and eventual training with a physical therapist who worked with Olympians. Suggestion: Lots of core work; i.e. sit-ups, if done wrong can be counter productive for those of us with bad backs. Your lower back must STAY in a safe position while doing these excercises or you are burning down a house to kill a fly. (Wish I had known this earlier when I began to have back issues)
To find the safe postion: Lay on your back on the floor with knees up. Arch your lower back to extreme (lumbar away from the floor). THEN press lumbar FLAT to the floor- feel those two EXTREMES- total arch to flat. THEN sightly arch your lumbar to MIDWAY between those two extremes. Feel it, get use to that, lock your lumbar in that position. That is the safe position.
SO- if you do a sit-up (or any excercise, lift weights, take things out of a car trunk, etc) you must keep your lower back in that safe position. It takes deliberate concentration. You will find that if you come up too far your lower back will want to push into the floor- Don't sit-up that far! The only way I can keep my back in this position is half a crunch. For lifting things out of a car trunk I have to put my knee up on the bumper. The whole point is to always keep you back in the safe position.
If your back was healthy you could sit-up with lumbar flat to the floor, allow it to move, etc. but NOT for a damaged back-which you now have for the rest of your life.
This technique has brought me great strength and activity without continued damage. ANY activity I do; shooting, yard work, lifting, climbing, sitting for long stretches, etc. I keep my back in this position- if I do- I have seldom, if ever have problems.
Good Luck
Dan