**4. Conclusions**

This study implemented the Madgwick filter-based joint angle measurement algorithm to construct a wireless wearable sensor system for simplified joint angle measurement. The design and development of a 3D rigid body included the attachment of three IMU sensors to the body's arms. The proposed algorithm was tested against a gold standard EG system and also compared with the outcomes of WitMotion software (WM). The comparison confirms that the proposed algorithm performed better for joint angle measurement, which is necessary for clinical acceptance. The suggested approach successfully predicted the elbow joint angle with a maximum root mean square error (RMSE) of 0.46◦. The joint angles were also determined using external acceleration, and the RMSE was 0.996◦. All the RMSE values were within the range of acceptable joint angles appropriate for medical applications. Still, there are some critical issues and possibilities for future investigation. In this analysis, a rigid body was used to assess how efficiently the proposed algorithm performed. Therefore, in future work, the measurement in real-world scenarios (with the human body upper limb joint angle) will be used to assess the efficacy. For quantitative risk assessment of the human upper limb joints, we will incorporate a real-time feedback system with this proposed system. A focus may be given to semi-permanent tissue implantation with a remote sensor for accurate measurement, and in this way, researchers will have round-the-clock total body measurements. Finally, a comprehensive wearable system based on IMU will be developed to monitor all upper body joints at home.

**Author Contributions:** Conceptualization, M.M.R., K.B.G. and N.A.A.A.; Methodology, M.M.R. and K.B.G.; Software, M.M.R. and K.B.G.; Validation, M.M.R., K.B.G. and N.A.A.A.; Formal analysis, M.M.R., K.B.G. and N.A.A.A.; Investigation, M.M.R. and K.B.G.; Resources, M.M.R.; Data curation, M.M.R.; Writing—original draft, M.M.R.; Writing—review & editing, M.M.R., K.B.G., N.A.A.A., A.H. and Y.H.W.; Visualization, M.M.R., K.B.G. and N.A.A.A.; Supervision, K.B.G.; Project administration, K.B.G.; Funding acquisition, K.B.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by "Ministry of Higher Education under Fundamental Research Grant Scheme, grant number FRGS/1/2020/TK0/UKM/02/14" and "Research University Grant, grant number GUP-2018-048".

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
