Reliability and validity of a smartphone goniometer application for measuring the quadriceps angle in healthy women: a cross-sectional study

Abstract

Background/Aims The quadriceps angle, which depicts the quadriceps muscle's mechanical effect on the knee, is frequently used as an assessment metric when diagnosing a variety of knee-related issues. Smartphone goniometer applications have clinical assessment capabilities that can be used instead of the universal goniometer. The aim of this study was to analyse the criterion validity and inter- and intra-rater reliability of a smartphone goniometer application for measuring the quadriceps angle. Methods A total of 50 typically healthy women were included in this study. The quadriceps angle was evaluated with the universal goniometer as the gold standard and the Angulus smartphone goniometer application on each participant. The intraclass correlation coefficient and 95% confidence intervals were used to analyse reliability, while the standard error of the mean was used to calculate precision. Bland-Altman plots and 95% limits of agreement were used to establish the levels of agreement between the smartphone goniometer application and the universal goniometer. The Pearson correlation coefficient was used to quantify the smartphone goniometer application criterion validity in comparison to the universal goniometer. Results The smartphone goniometer application had good intra-rater reliability (intraclass correlation coefficient 0.99; 95% confidence interval 0.96–1) and inter-rater reliability (intraclass correlation coefficient 0.98; 95% confidence interval 0.94–1). A strong and significant correlation was found between the universal goniometer and the smartphone goniometer application, showing good criterion validity (r=0.99; 95% confidence interval 0.98–0.99; P=0.006). Conclusions The use of the smartphone goniometer application to measure the quadriceps angle was valid and demonstrated excellent intra- and inter-rater reliability levels. Implications for practice The smartphone goniometer application can be used in clinical practice as a valid and reliable quadriceps angle measurement tool.