Validity and reliability of a smartphone application for knee posture quantification and the effects of external markers on the precision of this measure

Abstract Background Technological resources, such as smartphones can contribute to the quantitative assessment of posture. Purposes Test the validity and reliability of using a postural assessment application to quantify the frontal plane knee posture in orthostatism and to test the influence of the use of external markers on the precision of this measure. Design Methodological study. Methods The knee frontal plane posture of 30 volunteers were analyzed by two independent examiners. The photographs were taken with different external marker arrangements. The photographs were analyzed at two moments using the Kinovea software and PhysioCode Posture (PCP) application. Reliability was analyzed using the intraclass correlation coefficient (ICC) between measures with each instrument conducted at two moments with a 7-day interval. Concurrent validity of PCP with Kinovea measure was analyzed using Pearson's correlation coefficient. Standard error measurement (SEM), minimum detectable change (MDC) and Bland Altman plots were analyzed. Results PCP demonstrated excellent intra-rater [ICC = 0.92 (95% confidence interval [CI] 0.90–0.93)] and inter-rater [ICC = 0.88 (95%CI 0.85–0.90)] reliability. Concurrent validity analysis showed excellent agreement between PCP and Kinovea software (r = 0.88). The use of markers, independent of positioning, did not influence the measurement properties of measures with both softwares. The SEM was inferior to 1.2°, and the MDC was below 2.85°. No systematic errors were observed in the Bland Altman graphs. Conclusions The use of PCP application to measure knee posture was valid and demonstrated excellent intra- and inter-rater reliability levels. The use of external markers did not influence the measurement.