I Had Boer Back Surgery With Bone Fusion. It Is

I had boer back surgery with bone fusion. It is 3 months now I am healing well. My doctor asked me to do lower back isometric exercises to strengthen my muscles. I use a back brace when I have to go to work and also a osteogenesis stimulator for 4 hours each day to help the bones fuse together. This machine is a spinal stem and puts some rays in me each day for 4 hours.
I want some isometric exercise to do to help me heal and get strong in my muscles. How can you help me?