# A Kinematic Model of the Shoulder Complex Obtained from a Wearable Detection System
## 5 Conclusions  
Due to the complex coupled motion of the shoulder mechanism, the design of the guiding movement rules of rehabilitation robots generally lacks specific motion coupling information between the GH joint center and human elevation angle to describe the shoulder complex. To obtain the specific motion coupling information, the first step is to establish a mechanism model of the shoulder complex. However, to model the movements of the shoulder complex, the joint coupling motion relationship between the GH joint and the shoulder girdle during humeral elevation as well as their differences during humeral elevation in different planes should be considered.  
In this work, considering the whole coupled motion of the shoulder complex for a given pointing position, a mechanism model of the shoulder complex, which is a generalized GH joint with a floating center, was presented. The GH joint center displacement variable quantity relative to the thorax coordinate system is equal to the overall motion of the shoulder girdle, and the coupled motion can clearly describe the characteristics of the shoulder girdle and GH joint. Then, a wearable detection system was designed, and the experiment was set up with data processed. This study also established a kinematic model of the shoulder complex in the sagittal and coronal planes and evaluated the differences in GH joint motion during humeral elevation in the sagittal and coronal planes.  
The experimental results presenting the GH joint motion information during humerus raising and lowering movements in the sagittal and coronal planes were presented. Going a step further, the motion coupling relationship between the GH joint center displacement variable quantity and humeral elevation angle in the sagittal plane and coronal planes was obtained. This is also consistent with the results of other researchers [21; 25]. However, it should be highlighted that due to the effect of the deviation of the skin from the bone target during movement of the arm, wear deviation, and the GH joint being modeled as a simplified spherical joint, the kinematic model can only be applied to provide basic data for the design of the shoulder exoskeleton mechanism, shoulder function simulation, and ergonomics, while it cannot be used in joint replacement, prosthesis design, and other areas that need accurate kinematic information [22]. The exact coupled motion information between the GH joint and shoulder girdle using a pin inserted into the sternum, clavicle, scapula, and humerus will be addressed in our future work.  
Nevertheless, this study provides a new method to obtain the kinematic model of the shoulder complex obtained from a wearable detection system in an accessible workspace, and the results provide basic data for the design of the shoulder exoskeleton mechanism and have practical significance for the daily human movement science.  
Formal analysis, C.Z.; Funding acquisition, J.L. and M.D.; Methodology, J.L., C.Z. and M.D.; Validation, J.L. and Q.C.; Writing--original draft, C.Z.; Writing--review and editing, M.D. and Q.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded in part by the National Natural Science Foundation of China, grant numbers 51675008 and 61903011, in part by the Beijing Natural Science Foundation, grant numbers 3171001, 171.20019, and 3204036, and in part by Beijing Postdoctoral Research Foundation, grant number Q6001002201901.  
The authors would like to acknowledge the National Research Center for Rehabilitation Technical Aids, Beijing, China, for the meaningful guidance on the development of the kinematic model.  
The authors declare no conflict of interest.