Buning back pain

I been having problems with my back since last year. I had mri's and all are normal I get burning in lower back near buttock and upper back between shoulder and spine mostly on left side told by a few physicians I need to build up my muscles. My question is what kind of exercises I should do and how long does it take to see improvement. Would kickboxing classes help?
could I possibly have muscle imbalances I have been to 3 different physical therapist and it seems that no one has giving me an exercise program that is consistence in helping me to improve.

Strange choice

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kick boxing

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If the MRI's are all normal and the nerve tests then perhaps general exercise that moves your spine and stimulates the smaller stabilizer muscles would be good. Get moving: swimming, yoga, tennis, pilates, gyrotonic are all good ideas to strengthen. It's much better to choose activity to address this rather than specific 'exercises'.

The kick boxing class I want to take is for conditioning exercise would that be helpful.