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Background: Therapeutic exercise is a central component in the management of many common conditions. It is imperative, therefore, that clinicians monitor and correct patient performance to facilitate the use of proper form both in the clinic and during home exercise programs. Although clinicians are trained to prescribe exercise and analyze form, there are many subtleties that may be missed by relying on visual assessment. This study investigated the accuracy and precision of a novel, exercise-training smartphone application (app), running on an iPhone® (Apple, Cupertino, CA) 4 and using its LIS331DLH accelerometer to dynamically measure and record movement during exercise. Materials and Methods: The iPhone, running the app, was mounted to the movement arm of a Biodex™ isokinetic dynamometer System 4 (Biodex Corp., Shirley, NY). Angle and time measurements taken by the app were compared with the dynamometer (gold standard) while rotating at 30°/s, 60°/s, 90°/s, 120°/s, and 150°/s. Accuracy was assessed using limits of agreement and fast Fourier transform analyses. Precision was assessed using the coefficient of variation. Results: The mean difference between the app and the Biodex recordings was less than 1°/s for all test velocities. The coefficient of variation was less than 3% at velocities from 30°/s to 120°/s and less than 7% at 150°/s. Conclusions: The app was highly accurate and precise. The validation of apps designed for motion tracking is a vital prerequisite to clinical implementation. The app described in this article is clinically identical to the Biodex dynamometer in its ability to accurately and precisely read angular movement over time. [ABSTRACT FROM AUTHOR]