This study explores the selection criteria used in trials of treatments for non-sepcific low back pain. 168 studies were included. Two inclusion themes were identified: anatomical area, and symptoms and signs. Exclusion criteria comprised 21 themes, the most common being: previous or scheduled surgery, pregnancy, malignancy, trauma and psychological conditions.

You can read the full paper at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5898037/