The Influence of the Measurement Position of a Smartphone with an Inclination Application on the Accuracy of Joint Range of Motion Measurements

Abstract

Goniometers, commonly used to measure joint range of motion, are increasingly being replaced by smartphones in the field of sports. However, the position, orientation, and fixation methods for smartphone placement and the applications used have not been validated. This study aimed to determine the validity of measured angles using a smartphone application based on its position and orientation. We measured thirty different angles by using two cardboard sheets, each containing fifteen randomly drawn lines. The cardboard pieces were placed on a horizontal desk and a vertical wall perpendicular to the floor, and a smartphone was placed in six orientations to measure each angle. A protractor was used for validation.

In the Horizontal-Vertical position (HVP), with the smartphone placed on its short side perpendicular to the desk, measurements were unclear and unusable. Fixation errors were observed on the smartphone’s Horizontal-Supine position (HSP) (p=0.01), placed parallel to the desk, and proportional errors were observed on the Horizontal-Lateral recumbent position (HLP) (p=0.034), measured with the long side placed perpendicularly to the desk. Although measurement at the HVP was unreliable, others were accurate. Future verification with new smartphones with improved and updated gyro and accelerometer sensors is needed.