Back problems, any tips?

fliss4

5yrs ago I tore the ligaments in my lower back and ever since then it's been a weak spot. A couple of times a year it *goes* and I'm in awful pain and have limited mobility for anywhere between 4days and 3 weeks. Have had physio and I do my stretches, run 4-5 times a week but need to do some more core strengthening that's not going to aggrevate my back. Can anyone recommend some exercises??