Fifty-eight patients completed the 1-year follow up in a randomized study comparing the McKenzie method vs. motor control exercises for the treatment of chronic low back pain. The primary outcome of interest was the change in trunk muscle thickness of the transversus abdominis, obliquus internus and obliquus externus muscles. Secondary outcomes of interest included the patient specific functional scale (PSFS), the global perceived effect (GPE) and pain on a Visual Analog Scale (VAS). No significant differences in all outcomes were observed between the two treatment groups.