Exercise Intensity Matters in Chronic Nonspecific Low Back Pain Rehabilitation, Introduction Exercise therapy (ET) is advocated as a treatment for chronic nonspecific low back pain (CNSLBP). However, therapy effect sizes remain low. In other chronic disorders, training at higher intensity has resulted in greater improvements on both general health related and disease specific outcomes compared to lower intensity ET. Possibly, high intensity training also improves effect sizes in CNSLBP. Objective To compare the effects of a high intensity ET program with a similar moderate intensity ET program on disability, pain, function, exercise capacity, and abdominal/back muscle strength in persons with CNSLBP. Methods In a randomized controlled trial, persons with CNSLBP performed a 12-week ET program (24 sessions, 1.5hours/session, 2x/week) at high (HIT) or moderate intensity (MIT). Questionnaires to assess disability (Modified Oswestry Index (MODI)), pain intensity (Numeric Pain Rating Scale (NPRS)), and function (Patient Specific Functioning Scale (PSFS)), a cardiopulmonary exercise test to assess exercise capacity (VO2max, cycling time), and a maximum isometric muscle strength test to assess abdominal/back muscle strength (maximum muscle torque) were administered at baseline and after the training program. Results Thirty-eight participants (HIT: n=19, MIT: n=19) were included (mean age: 44.1y, SD=9.8, 12 males). Groups did not differ at baseline. Between group differences (p<0.01) in favor of HIT were found for MODI, VO2max, and cycling time. Within group improvements (p<0.01) were found in both groups on MODI (HIT:-64%, MIT:-33%), NPRS (HIT:-56%, MIT:-39%), PSFS (HIT:+37%, MIT:+39%), VO2max (HIT:+14, MIT:+4%), cycling time (HIT:+18%, MIT:+13%), and back muscle strength (HIT:+10%, MIT:+14%). Conclusion HIT proved to be a feasible, well tolerated, and effective therapy modality in CNSLBP. Moreover, it shows greater improvements on disability and exercise capacity than a similar ET performed at moderate intensity. Corresponding author: Jonas Verbrugghe, Hasselt University, Faculty of Rehabilitation Sciences, REVAL, Gebouw A, Agoralaan 5, 3590, Diepenbeek, Belgium, Tel: +003211269239, jonas.verbrugghe@uhasselt.be This project is funded by the UHasselt research fund BOF New initiatives (project number R-5211). This funding source had no role in the design of this study and will not have any role during its execution, analyses, interpretation of the data, or decision to submit results. The authors would like to thank all the persons with CNSLBP that participated in this study. Conflict of Interest. The authors report no conflict of interest. Accepted for Publication: 24 June 2019 © 2019 American College of Sports Medicine