Predictors of change in trunk muscle strength for patients with chronic low back pain randomized to lumbar fusion or cognitive intervention and exercises

Objective. We have previously reported in two randomized controlled trials that cognitive intervention and exercises more effectively improved isokinetic trunk muscle strength than lumbar fusion and postoperative rehabilitation in patients with chronic low back pain. The aim of the present study was to predict changes in muscle strength as regard to changes in pain, function, fear-avoidance beliefs (self-rated questionnaires), changes in cross-sectional area and density of the back muscles and treatment. Methods. We assessed 1) isokinetic trunk muscle strength; 2) cross-sectional area and density of back muscles by computer-tomography (CT); and 3) pain, function, fear-avoidance beliefs, and self- efficacy for pain in the included patients at baseline and 1-year follow-up. Multiple regression analysis was performed with change in muscle strength as dependent variables, and changes in area/density and self-rated symptoms and treatment as independent variables. Results. The correlations between the change in muscle strength and change in both cross-sectional area and density were weak (r < 0.1) and nonsignificant and these variables were not included in the multiple regression analysis. The change in pain, change in fear-avoidance beliefs, change in self-efficacy for pain and treatment explained 46% of the change in muscle strength, with change in pain and treatment as significant predictors. Conclusion. Our results emphasize the central role of pain and treatment for the improvements in muscle strength in patients with chronic low back pain.