Examination of the correlation between the knee extension angles in the sitting and supine positions measured using ImageJ software: a prospective cross-sectional study

[Purpose] We aimed to examine the correlation between the knee extension angles in the sitting and supine positions measured using ImageJ software. [Participants and Methods] A total of 50 legs in 25 healthy participants (17 males and 8 females) were included in our study. The knee extension angle was measured in the sitting and supine positions with the participants actively and maximally extending their knee joint on one side. The participants were photographed from the side with their knees centered in the image. Thereafter, the photographs were imported into the ImageJ image processing software to calculate the knee extension angles. [Results] The mean values of the knee extension angles in the sitting and supine positions were 131.5 ± 11.2° and 132.1 ± 12.2°, respectively, with a correlation coefficient of 0.85. No systematic errors were observed, and the minimal detectable change was 12.9°. [Conclusion] The knee extension angle in the sitting position showed a strong correlation with that in the supine position, with no systematic errors observed. Therefore, measurement of the knee extension angle in the sitting position can be an alternative to its measurement in the supine position.