Examination of the intrarater reliability of ultrasound measurements of the thickness of the lumbar and lateral abdominal muscles in the prone position.

[Purpose] This study aimed to examine the intrarater reliability when measuring the thickness of the lumbar and lateral abdominal muscles using ultrasound with the participants in the prone position. [Participants and Methods] The participants were 10 healthy adult males without chronic low back pain. The muscle thicknesses of the lumbar multifidus, erector spinae, obliquus externus abdominis, obliquus internus abdominis, and transversus abdominal muscles were measured using ultrasound with the participants in the prone position. [Results] The intraclass correlation coefficients of the within-day and between-day intrarater reliability measurements were 31.1-34.1 mm (lumbar multifidus), 32.0-33.5 mm (erector spinae), 7.4-8.0 mm (obliquus externus abdominis), 9.4-10.4 mm (obliquus internus abdominis), and 2.9-3.4 mm (transversus abdominal). The standard error of measurement and 95% confidence interval of minimal detectable change of the within- and between-day measurements were 0.1-1.2 mm and 0.3-3.4 mm, respectively, for each muscle. [Conclusion] The reliability of measuring the lumbar and lateral abdominal muscles in the prone position using ultrasound was confirmed. It was suggested that measurements during muscle activity and extremity movement are possible when participants are in the prone position.