The diaphragm is the main respiratory muscle but is also considered to contribute significantly to trunk stability and posture due to its unique anatomy and coordination with other trunk muscles. Rehabilitative Ultrasound Imaging (RUSI) technique provides a safe, and cost-effective method for assessing diaphragm muscle function. RUSI is employed in musculoskeletal physiotherapy to assess diaphragm thickness or mobility in trunk conditions such as lumbopelvic pain, as a method to investigate rehabilitation effectiveness or as a biofeedback tool for muscle training. Although, the reliability of RUSI measurements for diaphragm thickness and excursion has been previously investigated in healthy populations, individuals with pelvic girdle pain and athletes with lumbopelvic pain, its reliability on postpartum populations has never been researched. Therefore, this study aims to assess the reliability of diaphragm thickness and excursion measurements in healthy women, including nulliparous and parous individuals of varying ages, utilizing RUSI.

This reliability study adheres to Ethical considerations and the Helsinki Declaration and is approved by the Research Ethics Committee (R.E.C.) of the University of Patras. The study incorporates a robust quality assurance plan, according to the Internal Review Board of the Research Ethics Committee (R.E.C.) of the University of Patras, including data checks to assess data validation and registry procedures performed by the Data Monitoring Committee of R.E.C. of the University of Patras, as well as site monitoring and auditing, to maintain protocol adherence and data collection integrity. The accuracy, completeness, and representativeness of registry data is assured by the research team, comparing the data to preliminary data published in similar reliability studies. A comprehensive data dictionary is established among the research team, containing detailed variable descriptions, coding information, and normal ranges, ensuring consistency and clarity in data collection.

RUSI is performed by two trained physiotherapists (with more than 5 years of clinical experience in musculoskeletal physiotherapy). The physiotherapists received one-day training from a consultant radiologist, specializing in musculoskeletal sonographic imaging, followed by several hours of independent and joint practice between the radiologist and physiotherapists. A convenience sample of adult women coming from around the broader university campus area is invited to participate in the study, via advertisements and university-mediated means (emails, e-platform university announcements etc.). Informed consent is obtained from all participants. Participants undergo an ultrasound assessment in the semi-recumbent position with the Versana Active™ ultrasound system. Anatomical guide points are marked by a surgical skin marker for accuracy, and the 2 trained physiotherapists perform the assessments according to Boussuges et al. (2020) recommendations. Diaphragm thickness is measured using a high-resolution linear transducer in B-mode at the end of a full inspiration and full expiration. Diaphragm excursion is assessed using a high-resolution curvilinear transducer in M-mode during tidal volume diaphragmatic breathing. A two-minute resting interval follows each measurement. During the resting intervals, calipers are placed on the captured images according to literature recommendations and measurements are recorded and saved. Intra-tester and inter-tester reliability are evaluated by taking 3 consecutive measurements per parameter. Operators and subjects are blinded for all measurements taken, by covering measurement tables at the edge of the ultrasound screen. An independent researcher blinded to the subject identity (the identity of the subject on the ultrasound screen is concealed by the operator at the end of the assessment) and the examination procedure is responsible for the extraction, and documentation of all variables in a spreadsheet software.

All patient data are stored anonymously/coded in a secure cloud platform accessed only by the research team members. Imputation methods will be applied to address situations where variables are missing, uninterpretable, or inconsistent. Any adverse events will be addressed and documented, while medical consultation will be accessible during patient assessment hours. Any modifications to the study protocol, data collection methods, or other aspects of the study will be reviewed thoroughly among researchers and the Internal Review Board of the R.E.C., and the study protocol as well as study registration will be adjusted accordingly. Statistical analysis of the data will be performed using SPSS software (version 28.0; SPSS Inc, Chicago, IL). Reliability will be calculated using intraclass correlation coefficient (ICC) assessed with a two-way random effect model (ICC2,1) and 95% confidence intervals.