Background:Ankle joint range of motion is a frequently assessed measure used by health care clinicians who manage lower limb pathologies to identify ankle equinus and/or other joint motion concerns that may negatively impact on function. The purpose of this study was to assess a new iPhone application (the level function of the 'Measure application'), for measuring the weightbearing ankle lunge test in a healthy adult population (reliability) and measuring known angles (validity) when compared to a digital inclinometer. Methods:To determine intra-rater reliability, inter-rater reliability and concurrent validity, 168 measures were conducted on 21 participants. Participants were preconditioned prior to assessment, and two experienced raters measured ankle dorsiflexion range of motion in the knee extended and knee flexed positions of the weight bearing lunge test, using an iPhone level function (of the Measure application) and a digital inclinometer in a randomised order, over two timepoints. Concurrent validity was also determined by comparison of measures of the two devices at known surface angles (0 and 15 degrees) in multiple planes. Reliability and validity were determined with intraclass correlation coefficients, concurrent validity was explored with the Bland Altman plot and an intraclass correlation coefficient. The Standard Error of the Mean and the minimal detectable change were also explored. Results:The intra-rater reliability using the iPhone and inter-rater reliability using the digital inclinometer, in the knee extended position, were ICC 0.85 respectively, indicating good reliability. All other intra-rater reliability and inter-rater reliability for both devices and both leg positions were over ICC 0.90, indicating excellent reliability. Concurrent validity between the two devices on a flat and known angle surface were ICC 1.0 (Limits of Agreement - 1.0 to 0.61), indicating excellent validity, with good validity demonstrated by a Bland Altman plot of all measures in all positions (ICC of 0.84 (Limits of agreement = - 4.51 to 6.49)). Conclusion:The use of the iPhone level measure, within the Measurement App has demonstrated to be an easy and reliable measurement tool to determine ankle joint dorsiflexion during the weightbearing lunge test in healthy adults.