Concurrent validity and interrater reliability of a new smartphone application to assess 3D active cervical range of motion in patients with neck pain

Background: There is a lack of valid, reliable, and feasible instruments for measuring planar active cervical range of motion (aCROM) and associated 3D coupling motions in patients with neck pain. Smartphones have advanced sensors and appear to be suitable for these measurements. Objectives: To estimate the concurrent validity and interrater reliability of a new iPhone application for assessing planar aCROM and associated 3D coupling motions in patients with neck pain, using an electromagnetic tracking device as a reference test. Design: Cross-sectional study. Methods: Two samples of neck pain patients were recruited; 30 patients for the validity study and 26 patients for the reliability study. Validity was estimated using intraclass correlation coefficients (ICCs), and by calculating 95% limits of agreement (LoA). To estimate interrater reliability, ICCs were calculated. Cervical 3D coupling motions were analyzed by calculating the cross-correlation coefficients and ratio between the main motions and coupled motions for both instruments. Results: ICCs for concurrent validity and interrater reliability ranged from 0.90 to 0.99. The width of the 95% LoA ranged from about 5 degrees for right lateral bending to 11 degrees for total rotation. No significant differences were found between both devices for associated coupling motion analysis. Conclusions: The iPhone application appears to be a useful discriminative tool for the measurement of planar aCROM and associated coupling motions in patients with neck pain. It fulfills the need for a valid, reliable, and feasible instrument in clinical practice and research. Therapists and researchers should consider measurement error when interpreting scores.