Objective To evaluate the reliability and validity of a standardized ultrasound examination protocol for measuring vastus lateralis muscle size. Design Prospective cohort study. Subjects Sixteen staff members of the university hospital of Heidelberg. Methods Muscle thickness, cross-sectional area and subcutaneous adipose tissue thickness were measurxsed at 3 standardized sites on the right and left vastus lateralis muscle. Ultrasound measurements were collected by 2 independent investigators on 2 different days and compared with magnetic resonance imaging measurements. Results Intraclass correlation coefficients (ICC) for intra- and inter-rater reliability showed very good closeness of agreement for all parameters (ICC = 0.929–0.994, p < 0.001). Muscle thickness and subcutaneous adipose tissue thickness ultrasound and magnetic resonance imaging measurements revealed good to very good closeness of agreement (ICC = 0.835–0.969, p < 0.001), whereas cross-sectional area showed only average closeness of agreement (ICC = 0.727, p < 0.001). A strong predictive positive correlation for ultrasound and magnetic resonance imaging-based measurements of cross-sectional area was found (R ² = 0.793, p < 0.001). Conclusion By standardization of an examination protocol, quantitative vastus lateralis muscle ultrasound proved to be a reliable method for assessing vastus lateralis muscle size. Furthermore, this protocol is valid for measuring muscle thickness and subcutaneous adipose tissue thickness, although there seems to be a systematic underestimation of cross-sectional area depending on subcutaneous adipose tissue thickness. LAY ABSTRACT The quantification of muscle mass is crucial for diagnosis and monitoring of muscle atrophy caused by inactivity or in the context of diseases, and could also be used for monitoring the development of muscle hypertrophy during resistance training. Application of a standardized examination protocol for quantitative vastus lateralis muscle ultrasound proved to be a reliable method for assessing vastus lateralis muscle size in people with varying body weight and body composition, respectively. In this regard, it could be considered an alternative examination tool to time- and cost-consuming gold-standard magnetic resonance imaging (MRI), which is seldom readily available in clinical practice. However, compared with MRI measurements, muscle cross-sectional area measured using ultrasound seems to be systematically underestimated, depending on subcutaneous adipose tissue thickness.

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