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Purpose: The present study investigated group differences between the thickness changes of the transverse abdominis (TrA), internal oblique (IO), and external oblique (EO) muscles, during performance of the isometric supine chest raise and the supine double leg-straight leg raise tests in women with and without low back pain (LBP). Method: Twenty women with LBP and 20 women without LBP participated in this case-control study. The thickness of the right TrA, IO, and EO muscles was measured using B-mode ultrasound (US) at rest, immediately at the beginning of performing the tests and when participants in both groups self-reported fatigue. The percentage of change in thickness of the abdominal muscles from rest to the initiation and fatigue stages of both tests was measured. Results: The results indicate a statistically significant difference in the pattern of changes in deep (TrA) and superficial (EO) abdominal muscles thickness at the fatigue stage of the supine double straight-leg raise (SDSLR) test between groups (P < 0.05). There was no significant difference between groups for the abdominal muscles thicknesses change at rest or during stages of the clinical isometric endurance tests (P > 0.05). Conclusions: An altered activation pattern in the deep (TrA) and superficial (EO) muscles of LBP participants during fatigue stage of the SDSLR test as compared to controls indicates motor control dysfunction in the LBP group. Comparison of the activity of TrA and EO muscles during fatigue stage of SDSLR test can be used to assess alterations in motor control of abdominal muscles. [ABSTRACT FROM AUTHOR]