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PURPOSE Transversus abdominis has been shown to be dysfunctional in low back pain patients. Specific exercises involving contraction of core muscles of the spine have been shown to be effective in treating low back pain. Although it is widely claimed that pilates exercise develops the core muscles surrounding the spine, including transversus abdominis, there is little research to support this. OBJECTIVE To investigate the effect of a pilates exercise programme on transversus abdominis thickness in subjects with a history of low back pain. To investigate the reliability of ultrasound measurement of transversus abdominis thickness. STUDY DESIGN Test retest design, with subjects recruited via convenience sampling. METHOD Intra-tester reliability was investigated by measuring transversus abdominis thickness with B-mode ultrasound in eight subjects with a history of low back pain on two separate days. A further twenty-two subjects were recruited for a six week pilates matwork exercise programme with measurements of transversus abdominis thickness taken pre and post intervention. RESULTS The intra-tester reliability was found to be high in supine lying for transversus abdominis measurements taken one week apart (ICC = 0.92, 95% CI 0.60 to 0.99). There was a large and varied effect of the pilates intervention on the change in thickness of transversus abdominis (effect size = 1.27, CI -2.7 to 5.5). History of respiratory dysfunction was found to be very highly correlated with a decrease in transversus abdominis thickness measured at the end of expiration (r=0.7, CI 0.3 to 0.9). SUMMARY The ultrasound methodology was found to be reliable in measuring transversus abdominis thickness. It is not known why there was a wide variation observed in terms of magnitude and direction of change of transversus abdominis thickness after the pilates intervention. It is thought that changes in transversus abdominis thickness measured by ultrasound at the end of expiration (functional residual capacity) were related to respiratory function status. This relationship may have been modified by the six week beginner pilates exercise programme in subjects with a history of respiratory dysfunction. It is postulated that this relationship changed by reducing the contraction of transversus abdominis at the end of expiration. No definitive conclusions can be made, however, due to the small sample size in this preliminary study.