## 2 Materials and Methods  
### Subjects  
Persons with NSCLBP were recruited by local advertisement. Subjects with NSCLBP, between 25-60 years of age and able to speak and understand Dutch, were included. NSCLBP was defined as pain localized below the costal margin and above the inferior gluteal folds not attributable to a recognizable, known specific pathology (e.g., infection, tumor, osteoporosis, fracture, structural deformity, radicular syndrome or cauda equina syndrome) and persisting for at least 12 weeks. Subjects were excluded if they underwent invasive surgery of the lumbar spine, had co-morbidities (e.g., paresis and/or sensory disturbances by neurological causes, diabetes mellitus, rheumatoid arthritis), had a musculoskeletal disorder other than NSCLBP that could affect the correct execution of the therapy program, were pregnant, had ongoing compensation claims and/or work disability >6 months, had followed an exercise program in the past 3 months or were not able to attend regular therapy appointments. All interested subjects were invited for an intake session. They were informed about all the aspects of the study, in- and exclusion criteria were evaluated and a study-specific screening form concerning red flags for low back pain rehabilitation was filled out under the supervision of the study physician. All subjects gave their informed consent for inclusion before they participated in the study. This randomized controlled trial is part of a larger project that evaluates the effects of training intensity and training mode in NSCLBP rehabilitation, which was registered at clinicaltrials.gov (accessed on 6 April 2021) as NCT02786316. The study was performed at the REVAL Rehabilitation Research Center of Hasselt University (Diepenbeek, Belgium). In the larger project, 147 persons with NSCLBP were screened for eligibility between October 2016 and March 2019. Of these, 41 did not meet the inclusion criteria and six were not able to start the therapy program due to conflicting job schedules or commuting problems and were not further included. As such, 100 participants were included, of which 38 were randomly assigned to one of the two intervention groups evaluated in this analysis. A comprehensive research design flowchart is displayed in Figure 1. This project was approved by the Ethical Committee of Hasselt University and Jessa Hospital Hasselt under protocol name 15.142/REVA15.14 and complies with the Declaration of Helsinki. Figure 1: CONSORT flowchart of the research design. MIT: moderate intensity training; HIT: high intensity training; ES: erector spine; MF: multifidus. Randomization and blinding  
Participants were randomized into an experimental group and a control group by sealed, opaque, sequentially numbered envelopes containing the allocated group for the participant. These envelopes were picked by an independent research assistant, not involved in the study, to ensure concealment of allocation. The experimental group performed a HIT program, while the control group performed a moderate intensity training (MIT) program, comparable to usual care [23; 35]. Blinding of the participants and caregivers was not possible due to the nature of the exercise therapy. To limit the performance bias of the participants, the study was explained as \"a comparison between different modes of exercise therapy in which equal progression could be expected in all groups.\"  
Interventions  
Participants of both groups followed a 12-week training program (2/w, 1.5 h/session). The 24 individual training sessions were organized at REVAL (Hasselt University, Diepenbeek, Belgium) under supervision of a physiotherapist of the research team. A standardized follow-up of all participants was assured by using a manual with protocols, explanation of the exercises and definitions of progression. Both groups performed a protocol consisting of cardiorespiratory training, general resistance training and core muscle training according to a protocol of [PERSON] et al., 2019 [24].