I am unclear as to the definition of a “posterior ray fusion” but will assume that this is a posterolateral fusion of L4-5. X-ray of the fusion can be definitive as to fusion status but more commonly may not really define what the fusion status is. I hope you had flexion/extension films along with the standard two films and they were all performed standing up.
You might consider an examination with a spine expert who can also go over the films. MRIs are very helpful to determine disorders and with your note of standing improving pain, that could be associated with a disc herniation.
Most pain generated in the back is not muscular in origin. It might feel like the muscles are the cause but typically muscles react to a disorder and are not primary pain generators.
Dr. Corenman
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.