Objective: The purpose of this study was to determine the intraday and interday variability and systematic change over the day of active cervical range of motion (aCROM) measurements in asymptomatic persons using a clinically applicable measurement device.

Methods: A prospective observational study was performed. Sixteen adults (8 men and 8 women, median age 51 years) without neck pain in the last 3 months were recruited in 2 physiotherapy practices. Active cervical range of motion was estimated using the Apple iPhone application "3D Range of Motion." Measurements were performed 3 times a day for 7 days and spread over a period of 3 weeks. Mean values of aCROM were calculated. Intraday and interday variability was estimated by calculating limits of agreement.

Results: The limits of agreement for intraday variability ranged from ±12.1° for left rotation to ±15.5° for total rotation. For interday variability, the limits of agreement ranged from ±14.2° for right rotation to ±20.1° for total rotation. No systematic change over the day was found.

Conclusion: This study showed substantial intraday and interday variability of aCROM measurements in asymptomatic people. No trend toward an increased or decreased aCROM was observed during the course of the day. When interpreting aCROM values, clinicians should consider the degree of variation in aCROM measurements over time.