BACKGROUND: Based on the kinesiopathologic model, the Movement System Impairment (MSI) classification of LBP has shown that repetitive movements could contribute to pathoanatomic tissue changes. However, these changes have not been evaluated in different MSI classification subgroups of patients with LBP. OBJECTIVE: This study compared the grades of fatty infiltration as one of the muscle's pathologic changes in the lower lumbar paraspinal and psoas muscles between the two subgroups of patients diagnosed with MSI syndromes having opposite movement direction impairments. DESIGN: Observational cross-sectional study. METHOD: Forty-five participants with chronic LBP were enrolled in the study, with 23 patients in the lumbar flexion-rotation (FlexRot) subgroup and 22 in the lumbar extension-rotation (ExtRot) subgroup of MSI. Magnetic resonance imaging (MRI) and the Goutallier Classification System (GCS) were used for fatty grading of lumbar paraspinal and psoas muscles. After the reliability of this grading scale was evaluated, the results were compared between the two subgroups. RESULTS: The Mann-Whitney U Test showed significantly higher fat infiltration of lower lumbar multifidus and erector spinae muscles in the lumbar ExtRot subgroup, with no significant difference between the two subgroups in terms of psoas muscles (P ≤ 0.05). Inter-rater reliability of GCS was acceptable to excellent, and intra-rater reliability was good to excellent. CONCLUSION: The fatty infiltration grade of lumbar paraspinal muscles in L4-L5 and L5-S1 levels are significantly different between the two LBP subgroups of MSI that have two opposite movement direction impairments. The lumbar paraspinal muscles, which contribute to extension, have a higher grade of fat in the ExtRot subgroup, whose symptoms are aggravated by lumbar extension.