BACKGROUND: It is difficult to evaluate the transversus abdominis (TrA) and internal oblique (IO) due to their dual role in both trunk control and breathing.

OBJECTIVES: To investigate whether TrA and IO thickness as measured by ultrasound differs across the respiratory cycle in upright standing.

DESIGN: Observational study.

METHODS: Thickness of TrA and IO was measured with ultrasound in 67 subjects in upright standing. Measures were performed 3 times and by 2 assessors, at the end of relaxed expiration, at the end of a full inspiration, and at the end of full expiration. Differences were assessed by ANOVA. Intra- and inter-rater reliability (of a single measure and the average of 3 measures) were assessed by intra-class correlation (ICC).

RESULTS: Thickness of the TrA and IO was higher at full expiration than at the end of relaxed expiration (p < 0.001), and in turn compared to at full inspiration (p < 0.001). Intra-rater reliability was excellent at all respiratory phases (ICC 0.76-0.87). Whereas inter-rater reliability for a single measure was only fair to good for TrA (ICC 0.52-0.71) and good to excellent for IO (ICC 0.61-0.78), the inter-rater reliability of the average was excellent at all respiratory phases (ICC 0.75-0.90).

CONCLUSIONS: Thickness of TrA and IO increases when lung volume decreases. The intra- and inter-rater reliability of an average measure were excellent at the end of relaxed expiration, full inspiration and full expiration. This provides new opportunities to evaluate the deep abdominal muscles, and their role in respiration, in a physiotherapeutic setting.