Objectives: The study aims to investigate the relationship between abdominal muscle activity and the cross-sectional area (CSA) of the lumbar muscles and assess their role in the functional assessment of patients with chronic non-specific low back pain (CNSLBP). Methods: 142 patients with CNSLBP were included in this study. Disability levels were evaluated with the Roland-Morris Low Back Pain and Disability Questionnaire. The functional assessments of the participants were evaluated with a 6-minute walk test. Abdominal muscle activity was measured using a pressure biofeedback unit. The CSA of the bilateral multifidus, erector spinae, and psoas muscles were measured T2-weighted MRI images at the L2-L5 levels. Results: Significant correlations were found between the abdominal muscle activity during the posterior pelvic tilt movement and the CSA of the erector spinae muscle at the L4 and L5 levels, and the psoas muscle at the L2-L5 levels (correlation coefficient range from 0.32 to 0.48). Abdominal muscle activity yielded a significant additional contribution to the variance on the functional assessment (R2 change=0.101). Conclusions: The relationship of abdominal muscle activity with lumbar muscles and the contribution of muscle activities to functional assessment should be considered in the management of patients with CNSLBP.