Inter-Rater Reliability of Ultra Sound Based Rectus Abdominis Thickness Following Blood Flow Restriction Training

A retrospective methodological reliability study design will be employed to evaluate the inter-rater reliability of ultrasound-based measurements of rectus abdominis muscle thickness following a standardized blood flow restriction (BFR) training protocol. The study will utilize previously collected B-mode ultrasound images obtained from healthy young adult male participants who completed the original BFR training protocol. The existing dataset will be reviewed according to predefined eligibility criteria, and only ultrasound images of sufficient quality for identification and measurement of the rectus abdominis muscle will be included in the analysis.

The original BFR training protocol involved low-load resistance exercise performed under blood flow restriction. During the intervention, pneumatic cuffs were applied to the limbs, and participants performed a standardized squat exercise protocol at a low percentage of one-repetition maximum with prescribed repetitions and rest intervals. Ultrasound images of the rectus abdominis obtained according to the standardized imaging protocol will be analyzed retrospectively. The rectus abdominis was selected because it is a superficial abdominal muscle that can be visualized using B-mode ultrasonography and has relatively well-defined anatomical boundaries, allowing its thickness to be assessed using a standardized measurement procedure. Furthermore, the rectus abdominis contributes to trunk stabilization during functional and resistance exercises.

Ultrasound images will be analyzed using ImageJ software. Two independent raters will perform the measurements of rectus abdominis muscle thickness using the same standardized measurement protocol. The raters will independently identify the anatomical boundaries of the rectus abdominis and obtain linear thickness measurements from the ultrasound images. The use of a single standardized image-analysis platform and identical measurement procedures for both raters will minimize variability related to software and measurement techniques. The primary outcome will be the rectus abdominis muscle thickness measured in millimeters (mm).

Inter-rater reliability will be assessed using the Intraclass Correlation Coefficient (ICC) with a two-way random-effects model for absolute agreement and single measurements [ICC (2,1)], reported with the corresponding 95% confidence interval. ICC values below 0.50 will be interpreted as poor reliability, values from 0.50 to 0.75 as moderate reliability, values from 0.75 to 0.90 as good reliability, and values above 0.90 as excellent reliability. Absolute reliability and measurement error will additionally be assessed using the Standard Error of Measurement (SEM) and Minimal Detectable Change at the 95% confidence level (MDC95), where appropriate. Bland-Altman analysis will be performed to evaluate systematic differences and limits of agreement between the two raters. All statistical analyses will be conducted using SPSS, with the level of statistical significance set at p < 0.05.

If the ultrasound-based rectus abdominis thickness measurement demonstrates good to excellent inter-rater reliability, the findings will support the reproducibility of the standardized measurement protocol and its application in future research involving abdominal muscle morphology. The study may provide a methodological foundation for future investigations examining changes in abdominal muscle morphology following BFR and other exercise interventions, with potential relevance to sports physiotherapy, rehabilitation, and exercise science.