Have had worsening low back pain for several months and now have leg weakness. finally saw md and had mri to rule out herniation. report looks totally normal. what can be cause? muscular? have started in pt?

Leg weakness in both legs or just one? If both legs are weak, the cause could be either neurologic or muscular problem. In this case, you may need to see a neurologist. Back pain and leg weakness due to radiculopathy (herniated disc) is usually affecting one leg only and this is almost ruled out with negative MRI.

Might consider that pain generator could include sacroiliac joints, piriformis syndrome, or facet impingement. Weakness might be due to reconditioning, but also peripheral neuropathy, or a peripheral sciatica issue. An EMG study might be helpful in evaluating what is occurring here. Muscle enzymes should be tested. A neurologist can assess these different possibilities. Physiatry can also help.