The concurrent validity and reliability of the Angulus android application for measuring rear foot angle (RFA) and medial longitudinal arch angle (MLAA)

Abstract

INTRODUCTION: Methods for a detailed assessment of foot structure are essential for the physical examination of all patients. The medial longitudinal arch angle (MLAA) and rear foot angle (RFA) are two of the most widely used measures and are typically measured with a universal goniometer (UG). But the reproducibility of results among examiners remains a concern. Smartphone-based applications such as Angulus provide automated, image-based angle measurements and may prove to be a practical alternative. MATERIALS & METHODS: The study aims to find the reproducibility of results and the concurrent validity of the Angulus app. A total of 144 subjects aged 18 to 25 years were invited to participate in the study by word of mouth at the Maharana Pratap Sports College Outpatient Department (OPD) and Sardar Bhagwan Singh University, Dehradun. After screening participants according to selection criteria, 100 subjects recruited via convenience sampling were evenly distributed by gender, and the study was conducted over four months. Three assessors measured RFA and MLAA bilaterally for all participants using a goniometer and the Angulus app, as assigned following a standardized procedure. To check the concurrent validity and intra-rater reliability, three readings were taken by assessor 1 on alternate days or at different times on the same day. To measure inter-rater reliability, both the 2nd and 3rd assessors took their readings independently on different days, using the Angulus app only. The data were checked, and the normality of RFA and MLAA measurements was assessed using the Shapiro–Wilk test. Because the data were approximately normal, Pearson’s correlation coefficient was used to assess validity and reliability. A Bland-Altman analysis assessed agreement between the universal goniometer and the Angulus application.  RESULTS: A total of 100 participants completed the study, and no adverse events were noted. The first assessor recorded three readings for RFA and MLAA, and the mean of these three readings was calculated for further analysis. The readings of the goniometer were correlated with the readings of the Angulus app by calculating Karl Pearson’s correlation coefficient to assess the concurrent validity of the Angulus app, which yielded values of (r = 0.73 & 0.84), respectively, considered good. Assessor 2^nd and 3^rd calculated RFA and MLAA values for all the subjects using the Angulus app only. These readings were used to calculate the Inter-rater reliability of the Angulus app. ICC values were 0.71 for RFA and 0.88 for MLAA. The highest value of RFA and MLAA readings taken by assessor 1 using the Angulus app were used to calculate the Intra-rater reliability of the Angulus app, which came out to be excellent, ranging from 0.97 for RFA and 0.96 for MLAA. CONCLUSION: The Angulus application demonstrates significant concurrent validity and reliability and has the potential to effectively replace manual methods for RFA and MLAA measurements. CATEGORY: Biomechanics, Foot Measurements, Joint Measurements, Digital Health, Healthcare Technology, Sports Physiotherapy, Physical Medicine and Rehabilitation.