App-Based Feedback for Rehabilitation Exercise Correction in Patients With Knee or Hip Osteoarthritis: Prospective Cohort Study - Johanna Theresia Biebl; Marzena Rykala; Maximilian Strobel; Pawandeep Kaur Bollinger; Bernhard Ulm; Eduard Kraft; Stephan Huber; Andreas Lorenz (Journal of Medical Internet Research, 2021)