## 5 Conclusions  
This paper presents a methodological framework, based on Digital Twin, aimed to assess the ergonomic performance in a manufacturing production scenario. Implementing motion data, collected during the working activities, in a virtual 3D scenario allows performing the ergonomic screening of the investigated workstation. In this way it is possible to evaluate the desired risk indexes and figure out eventual production issues.  
A case study regarding a simple assembly task has been conducted in a laboratory environment to demonstrate the effectiveness of the proposed framework. Data related to working postures have been collected by a wearable inertial motion tracking system for 60 consecutive working cycles and transferred to the Digital Twin.  
In this way it has been possible to easily evaluate four risk indexes related to working postures, exerted forces, material manual handling and repetitive actions, sources of biomechanical overload.  
The first simulation figured out issues related to exerted forces and material manual handling, whose values overcame the upper limit of the low risk area. Hence, workstation layout and equipment changes have been proposed and a further time-based simulation has been run to test the solutions. The subsequently ergonomic assessment showed a significant reduction of the risk indexes.  
It is fundamental to underline that the traditional ergonomic screenings, carried out in an observational way, are time consuming procedures which require several hours of work. The framework herein proposed allows drastically reducing the evaluation times as well as making the assessment objective and repeatable.  
In summary, three important key elements can be pointed out in the present study:  
* it has been demonstrated the possibility to assess ergonomics through an automated approach by implementing experimental data, collected by wearable sensors, in a simulation environment, creating the DT of a real manual workstation;
* this approach allows evaluating ergonomics in a faster and more accurate way than manual analysis. In fact, the subjective judgments of analysts are avoided, and the assessment becomes objective based on the used ergonomic indexes;
* the workstations can be continuously monitored, assessing the ergonomic indexes whenever necessary (e.g., in case of change of production volume). Moreover, ergonomists and engineers can identify critical situations, based on real data, and proposing solutions (such as the workstation layout change) to reduce risk indexes. Hence, new solutions may be verified by means of numerical simulation before their implementation.  
The procedure described in this paper offers an improvement over current ergonomic screening techniques, but a bigger advantage will come from the use of devices capable of transferring data in real time, providing immediate ergonomic analyses.  
Conceptualization, A.G., M.C., M.F., S.G.; methodology, A.G., M.C.; software, A.G., M.C.; formal analysis, A.G., M.C., M.F., S.G.; investigation, A.G., M.C.; data curation, A.G., M.C.; writing--original draft preparation, A.G., M.C., S.G.; writing--review and editing, M.F., S.G.; visualization, A.G., M.C.; supervision, M.F., S.G. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
This work was supported by the University of Campania _[PERSON]_ under SCISSOR Project--V:alere program 2019.  
The authors declare no conflict of interest.