## 3 Search Results
### Demographics of Relevant Research Papers  
Figure 2 illustrates the number of relevant research papers by year, where the following observations can be made: First, no research papers on human-focused DT for OSH had been published before 2013, which indicates that both concepts of DT and its technological enablers were immature in the early 2010s, since the vision of Industry 4.0 was suggested in 2013 [12, 16]. Second, most of the relevant research papers (22 out of 25) were published in the last four years, while only three papers were published from 2013 to 2018. Increasing interest is being paid to human-focused DT for OSH, and a more comprehensive range of such applications has become feasible owing to recent modern information and communication technologies (ICTs). Moreover, this trend is similar to those reported in recent surveys on DT [43, 44]. Figure 3 shows the geographical distribution of the research papers, where the nationality of a research paper is determined according to the location of the first author's affiliations. Only three continents, Europe, Asia, and North America, have one or more rel \begin{table}
\begin{tabular}{c c} \hline \hline
**No.** & **Research Paper** \\ \hline
17 & [PERSON] et al. (2021) A digital twin-driven human–robot collaborative assembly approach in the wake of COVID-19 [38] \\ \hline
18 & [PERSON] et al. (2021) Digital twin-driven human robot collaboration using a digital human [39] \\ \hline
19 & [PERSON] et al. (2021) IoT and digital twin enabled smart tracking for safety management [13] \\ \hline
20 & [PERSON] et al. (2022) WEM-platform: A real-time platform for full-body ergonomic assessment and feedback in manufacturing and logistics systems [40] \\ \hline
21 & [PERSON] et al. (2022) An integrated mixed reality system for safety-aware human–robot collaboration using deep learning and digital twin generation [4] \\ \hline
22 & [PERSON] et al. (2022) Development and validation of guidelines for safety in human–robot collaborative assembly systems [8] \\ \hline
23 & [PERSON] et al. (2022) Manufacturing operator ergonomics: A conceptual digital twin approach to detect biochemical fatigue [15] \\ \hline
24 & [PERSON] et al. (2022) A methodological framework to integrate motion capture system and virtual reality for assembly system 4.0 workplace design [41] \\ \hline
25 & [PERSON] et al. (2023) Digital ergonomics: An evaluation framework for the ergonomic risk assessment of heterogeneous workers [42] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 2: Number of research papers per year. event research papers, which implies that human-focused DT for OSH is still an emerging research topic. Among the three continents, human-focused DT for OSH is most actively studied in Europe, and 60% of the research papers (15 out of 25) were authored by first authors from Europe. Publications from Asia and North America account for 28% and 12% of the relevant research papers, respectively. In the WoS database, a journal can be associated with one or more research areas, and Figure 4 shows the five most frequent research areas of the journals that published relevant research papers. Note that a single research paper can be associated with two or more research areas. Computer science is the most frequent research area for publications about human-focused DT for OSH. To implement a DT application, several modern software and hardware technologies must be applied and integrated, and such technologies are typically studied by researchers from the field of computer science [16]. Computer science is essential in implementing human-focused DT for OSH. Industrial engineering is the second most frequent research area. Indeed, OSH has been one of the most important research topics for ergonomics, a sub-area of industrial engineering. Thus, industrial engineering seems to provide a theoretical basis and practical tools for human-focused DT for OSH [18].