Reliability and validity of lower limb joint range of motion measurements using a smartphone

This study aimed to examine the reliability and validity of using a smartphone to measure the multi-joint range of motion of the lower limbs. We measured the straight leg raise angle, ankle dorsiflexion angle, and hip internal rotation angle in each of the 40 lower extremities of 20 healthy adults. Measurements were compared between a conventional method using a goniometer and a smartphone application method. The intraclass correlation coefficient (ICC) was used to evaluate the reliability of each smartphone measurement, and Bland–Altman analysis was used to examine measurement errors. The criterion-related validity of the two methods was also examined. Intra-rater reliability (ICC 0.668–0.939) was substantial to almost perfect, with no systematic errors found for all items, and the standard errors of measurement were acceptable. Inter-rater reliability (ICC 0.701–0.936) was also substantial to almost perfect, but the straight leg raise angle and hip internal rotation angle showed fixation errors. For these two measurements, with more than one examiner, the limit of agreement of error needs to be considered. No systematic errors were found in the ankle dorsiflexion angle, and the standard error of measurement was within the acceptable range. A moderate to strong correlation (r = 0.626–0.915) was found between the conventional and smartphone methods, demonstrating good criterion-related validity. However, in the ankle dorsiflexion angle measurements, the reliability and validity were shown to be lower than the other two items. This suggested the necessity of changing the measurement conditions in order to use the ankle dorsiflexion angle in clinical practice.