People affected by the frozen shoulder syndrome show limited shoulder mobility which is often accompanied by pain. The frozen shoulder syndrome often lasts from months to years, and mostly affects people in the age group of 40 to 70 years. The frozen shoulder syndrome severely reduces the quality of life and the ability to work. A common treatment method is physiotherapy. Patients are referred to a physiotherapist, who selects specific exercises adapted for the specific patient. Physiotherapy requires patient compliance, time, and effort. Correct exercise performance and compliance are the main issues in physiotherapy. A smartphone app could support patients by providing detailed exercise instructions and motivation through exercise logging, as is common for fitness and sport. In this work, such an app for frozen shoulder syndrome, the ShoulderApp, is evaluated in two user studies. The main contribution is that the user studies were conducted in an ambulatory assessment setting, which allows to draw conclusions about real-world usage, usability and user acceptance. The app was regularly used and study participants were satisfied. Additionally, we researched the usability and usage of interactive 3D and multi-modal exercise instructions, motivational aspects, exercise correctness and the interplay of physiotherapy and app usage. Measurements of shoulder mobility are the key assessment tool for the state and progress of the frozen shoulder syndrome. A smartphone sensor-based measurement tool, which only required a simple band in addition to the smartphone, was developed and evaluated. Interventions with the ShoulderApp were evaluated in a three-week short-term intervention and an 18-week midterm evaluation with 5 patients each. For the evaluation of the results, we used standardized questionnaires, SUS, TAM-2, and USE. In addition, semi-structured interviews and automatic logging of user-interactions in the app were included as the outcome measurements. Overall, the results for both the short-term and mid-term user studies showed that the ShoulderApp could support physiotherapy for frozen shoulder patients. The positive results of the studies show the potential of a generalization of the ShoulderApp concept to the large group of musculoskeletal disorders such as lower back pain and knee injuries.