**5. Conclusions**

Activity tracker-based continuous shoulder activity assessment in patients with a complex proximal humerus fracture treated with a locking plate was feasible and revealed that the mean value of the shoulder angle had up to two times differences between individuals but hardly increased during the first 6 postoperative weeks for most patients. Up to 10-fold differences in the daily shoulder elevation events between patients could be seen. There was a considerable difference in the number of shoulder elevation events > 10◦ between patients of both hospitals, which may be due to different rehabilitation protocols. Event counts above a functionally demanding threshold seemed to be the most sensitive digital mobility parameter monitoring post-traumatic recovery and may streamline wearable sensor data analysis in future studies as well as establish comparability between trials. These observations require confirmation by future studies including a larger cohort. When applied to a larger group of patients, the presented methods could be used in future studies to objectively and functionally evaluate the effect of postoperative activity on the outcomes of proximal humerus fracture fixation and to assess patient compliance. The resulting data could serve as the basis for developing improved and potentially personalized rehabilitation protocols and guidance for the patient.

**Author Contributions:** Conceptualization: V.C.P., B.G., S.N. and P.V.; investigation: M.H., A.R., J.B., F.S., C.H. and S.N.; resources: M.H., A.R., J.B., F.S., C.H. and S.N.; methodology: M.R., V.C.P., B.G. and P.V.; software: M.R.; validation: M.R. and P.V.; formal analysis: M.R. and P.V.; data curation: M.R. and P.V.; writing—original draft preparation: M.H., A.R. and P.V.; writing—review and editing: M.R., J.B., F.S., V.C.P., B.G., C.H., R.A. and S.N.; visualization: M.R. and P.V.; supervision: C.H., R.A., S.N. and P.V.; project administration: V.C.P. and P.V.; funding acquisition: P.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by AO Foundation via the AOTRAUMA Network, grant number AR 2018/01.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committees of University Hospitals Leuven (protocol code S62376, date of approval: 24 April 2019) and Medical University Innsbruck (protocol code 1281/2018, date of approval: 24 April 2019).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank Jérôme Schlatter for his assistance with the illustrations.

**Conflicts of Interest:** The authors declare no conflict of interest. The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.
