Intra- and inter-rater reliability of ultrasound assessment of rectus femoris muscle thickness by physiotherapists: influence of operator experience

Background Rehabilitative ultrasound imaging is increasingly used in physiotherapy as an objective method for muscle assessment. However, its reliability across different levels of operator experience requires further clarification. Objective To evaluate the intra- and inter-rater reliability of rectus femoris thickness measurements among expert and novice physiotherapists. Methods Four physiotherapists (two experts, two novices) assessed rectus femoris thickness in healthy individuals. Reliability was evaluated using intraclass correlation coefficients (ICC) and limits of agreement (LoA). The standard error of measurement (SEM) and minimal detectable change (MDC) were calculated to quantify measurement precision. Results Both expert raters demonstrated good-to-excellent intra-rater reliability (Rater A: ICC2,1 0.84–0.98; Rater B: ICC2,1 0.81–0.98). Novice raters showed good-to-excellent (Rater C: ICC2,1 0.80–0.98) and moderate-to-excellent reliability (Rater D: ICC2,1 0.73–0.97). The pooled SEM was 0.48 mm, with no clinically relevant differences between raters. The MDC was 1.34 mm. Inter-rater reliability was moderate-to-good for single measurements and good-to-excellent for average measurements, with similar patterns across expertise groups. LoA ranged from −3.13 to 3.05 mm for experts and from −3.25 to 2.09 mm for novices. Conclusions Ultrasound assessment of rectus femoris thickness demonstrates high intra-rater reliability among physiotherapists, supporting its use in clinical practice. Our preliminary findings suggest that novice raters may approach expert-level reliability following a short training protocol. Inter-rater reliability was acceptable and improved when averaging multiple measurements. For longitudinal monitoring, assessments are likely to be more consistent when performed by the same operator; when this is not feasible, averaging repeated measurements may help reduce inter-operator variability.