Strengthening and Targeted Rehabilitation for Optimal Neuromuscular Gains for chronic BACK pain (STRONG-BACK): protocol for a randomised controlled trial in participants with primary nociceptive pain drivers.

The trial will enroll 106 participants with chronic low back pain and a predominant nociceptive pain profile, randomized 1:1 to two exercise interventions.

• Participants will be recruited through primary and secondary care.
• Randomization ratio is 1:1 to either MC+ILEX or free-weight resistance training.
• Sample selection uses the Pain and Disability Drivers Management Model (PDDM) and the Lumbar Spine Instability Questionnaire (LSIQ) to identify a homogenous nociceptive pain profile subgroup.
• The targeted care approach is intended to identify participants more likely to respond to the interventions.

Both intervention arms will consist of 48 exercise sessions delivered over 16 weeks.

• Arm 1 involves motor control plus isolated lumbar extension exercises (MC+ILEX).
• Arm 2 involves free-weight resistance training.
• The duration of 16 weeks was chosen to reflect evidence that effective exercise interventions should be of sufficient duration.
• The trial is registered under ISRCTN14864451.

The primary outcome is LBP-related disability at 16 weeks, measured by the Oswestry Disability Index.

• Disability will be assessed at the 16-week endpoint.
• Linear mixed-effects models will be used to assess primary and secondary outcomes.
• The primary aim is to compare the effectiveness of MC+ILEX versus free-weight resistance training in reducing LBP-related disability.

Secondary outcomes encompass multifidus muscle composition and size, lumbar and gluteal muscle strength, hip range of motion, pain, physical and mental function, satisfaction and recovery, health-related quality of life, sleep quality, and physical activity levels.

• Changes in multifidus composition will be examined as a potential mediator of disability improvements.
• Regression analyses will explore whether baseline LSIQ scores moderate treatment effects on multifidus composition and other outcomes.
• Sex/gender will also be explored as a moderator of treatment response.
• Baseline LSIQ scores will be examined as a moderator of treatment response.

A mixed-methods design is incorporated, with a qualitative component involving semistructured interviews in a subsample of participants before and after the intervention.

• Qualitative interviews will explore illness perceptions, illness mindsets, perceptions of exercise therapy, and experiences and satisfaction with the two exercise interventions.
• Reflexive thematic analysis will be used to analyse qualitative data.
• The qualitative component aims to investigate participants' perceptions and experiences of exercise therapy.

Exercise therapy is described as the most recommended treatment for chronic low back pain but with evidence supporting only modest effects, attributed to heterogeneity of patient presentations.

• Systematic reviews emphasise that effective exercise interventions should be patient centred, target paraspinal muscle health, and be of sufficient duration.
• Evidence suggests that matching individuals to the most appropriate exercise type could improve outcomes.
• The trial design addresses these gaps using a targeted care approach to identify a more homogenous sample.