Stressing the Relevance of Differentiating between Systematic and Random Measurement Errors in Ultrasound Muscle Thickness Diagnostics.

Background: The majority of studies that explore changes in musculature following resistance training interventions or examine atrophy due to immobilization or sarcopenia use ultrasound imaging. While most studies assume acceptable to excellent reliability, there seems to be unawareness of the existing absolute measurement errors. As early as 1998, methodological research addressed a collective unawareness of the random measurement error and its practical indications. Referring to available methodological approaches, within this work, we point out the limited value of focusing on relative, correlation-based reliability indices for the interpretability in scientific research but also for clinical application by assessing 1,512 muscle thickness values from more than 400 ultrasound images. To account for intra- and inter-day repeatability, data were collected on two consecutive days within four testing sessions. Commonly-stated reliability values (ICC, CV, SEM and MDC) were calculated, while evidence-based agreement analyses were applied to provide the accompanied systematic and random measurement error. Results: While ICCs in the range of 0.832 to 0.998 are in accordance with the available literature, the mean absolute percentage error ranges from 1.34 to 20.38% and the mean systematic bias from 0.78 to 4.01 mm (all p ≤ 0.013), depending on the measurement time points chosen for data processing. Conclusions: In accordance with prior literature, a more cautious interpretation of relative reliability values should be based on included systematic and random absolute measurement scattering. Lastly, this paper discusses the rationale for including different measurement error statistics when determining the validity of pre-post changes, thus, accounting for the certainty of evidence. Key Points: • While reliability of a testing protocol is most often determined via relative reliability indices such as the intraclass correlation coefficient, further reliability values such as the systematic bias and the random error have been described as valuable for results interpretation. • This study used the most frequently employed procedure to determine muscle hypertrophy (ultrasound) as an example linking relative reliability values to absolute measurement errors under special consideration of appropriate calculation models using three scenarios (best case, worst case, stability). • Overall, 504 ultrasound images were examined showing excellent relative reliability, but the corresponding measurement errors suggest that caution must be exercised when interpreting pre-post settings in cases where the measurement error exceeds the expected changes. [ABSTRACT FROM AUTHOR]