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Background: The reliability studies are limited to support ultrasound usage during dynamic conditions; for example, unstable sitting position. Objective: This study aims to examine the reliability of ultrasound measurements of the lumbar multifidus and transversus abdominis during lying and unstable sitting positions in individuals with chronic low back pain (CLBP) and asymptomatic individuals considering abnormal lumbar lordosis. Material and Methods: In this observational study, intrarater within-day and between-day reliability of muscle thickness and contraction ratio of the lumbar multifidus and transversus abdominis muscles were assessed using ultrasound imaging. In total, 40 participants (27 with CLBP, 13 asymptomatic individuals) with abnormal lumbar lordosis were recruited. The degree of lumbar lordosis has been measured by a flexible ruler. The muscle thickness was assessed at lying and sitting on a gym ball for both muscles in three sessions. Results: Both groups had well to high ICCs of thickness measurement and contraction ratio in the transversus abdominis and lumbar multifidus muscles during both static (ICC=0.71-0.99) and semi-dynamic conditions (ICC=0.73-0.98). The standard error of measurements and minimal detectable changes were rather small in both groups. Conclusion: Ultrasound imaging is a highly reliable method to assess muscle thicknesses and contraction ratio of the transversus abdominis and lumbar multifidus during different conditions, even in patients with CLBP and abnormal lumbar lordosis. [ABSTRACT FROM AUTHOR]