Completing a set of therapeutic exercises correctly and regularly is essential for early and safe recovery from knee reconstructive surgery. We propose EXTRA, a platform for monitoring and improving the quality of therapeutic exercises that the patients need to perform during the recovery. The data acquisition framework obtains observations of the knee movement using an embedded flex sensor in a hinged knee brace. A connected Android application transmits the data to the database real-time. We evaluated the accuracy of the knee joint angle measurement and the usability in a study with young healthy adults. The mean absolute error of the knee angle measurements was 13.31 degrees. The high score of the usability questionnaire indicates that EXTRA provides a user-friendly interface to motivate therapeutic exercises. EXTRA maintains high standards in accurately monitoring and interpreting the exercises. Our proposed platform has the potential to provide effective guidelines and improve the rehabilitation process for orthopedic professionals and patients.