Objectives: To investigate the effectiveness of instrument-based mobilization compared with manual mobilization in patients with non-specific low back pain (NLBP), in terms of spinal muscle activation, decrease in pain, and improvement in range of motion (ROM).

Design: Randomized clinical trial, double blind. Participants and interventions: A total of 66 participants with localized NLBP will be divided into 2 groups matched for sex, age, and body mass index. The intervention group will receive central postero-anterior mobilization with a force of 150 N through physiotherapy instrument mobilization at the level of pain in the lumbar spine. The control group will receive central postero-anterior grade III mobilization through the pisiform grip method at the level of pain in the lumbar spine.

Main outcome measures: Measurements will be taken before the intervention, after the 1st session, after the 6th session, and at 4 weeks follow-up. Rehabilitative ultrasound imaging will be done to identify lumbar multifidus (LM) activation through the measurement of muscle thickness at the level of L5-L4 vertebrae, during rest and lower-extremity movement. Each participant will self-rate the pain intensity in the lumbar spine according to the Numeric Pain Rating Scale (NPRS). The pressure-pain threshold (PPT) will be measured using an algometer. Lumbar ROM will be measured using a dual inclinometer method in flexion and extension.

The results will be compared between the intervention and control groups through mixed analysis of variance for LM thickness, lumbar ROM, PPT, and NPRS.

Inclusion Criteria:

• age 20-59 years (commonly affected age range), BMI 18.5-30 kg/m2 (normal to overweight), localized NLBP, pain duration <12 months, and pain score ≥3/10 on the NPRS.

Exclusion Criteria:

• history of spinal surgery, significant previous back trauma, spinal or lower-extremity deformities, LBP associated with peripheral neurological symptoms (paraesthesia, anaesthesia, signs of nerve root compression), neurological diseases, systemic diseases (diabetes mellitus, rheumatoid arthritis), and any contraindication to manual therapy (spinal instability, healing fracture, dislocations, pregnancy in women).