Resistance training in addition to multidisciplinary rehabilitation for patients with chronic pain in the low back: Study protocol oleh Vegard Moe Iversen, Ottar Vasseljen, Paul Jarle Mork, Ida Reitan Berthelsen, Janne-Birgitte Bloch Børke, Gro Falkener Berheussen, Anne Therese Tveter, Øyvind Salvesen, Marius Steiro Fimland