## 4 Features and Issues of Existing Applications
### Objective, Application Domain and Test Environment  
Figure 5 classifies the objectives of existing human-focused DT applications for OSH into three categories proposed by the authors: 'Risk Assessment', 'Real-time Accident/Injury Prevention', and 'Monitoring'. 'Risk Assessment' is the objective of about two thirds of existing solutions. These solutions have in common that they are designed to assess the potential risks within workplaces or operations. Typically, the major concerns of these applications are postural risks that can cause musculoskeletal disorder and injury [14; 17; 24; 25; 26; 27; 31; 32; 39; 40; 41; 42]. In contrast, some of those applications aimed to assess other types of risk, such as human worker fatigue [15; 33] and collision risks [8; 36]. The assessment results of the applications for 'Risk Assessment' can be used to redesign workplaces or manual operations. 'Real-time Accident/Injury Prevention' is the second most popular objective in Figure 5. The applications for this objective primarily concern a mechanical type of risk, such as a collision between human workers and robots [4; 28; 29; 30; 34; 37; 38]. [PERSON] et al. [35] also considered human workers' fatigue. To prevent accidents and injuries caused by a collision between human workers and robots in real-time, these applications monitor both human workers and robots and sometimes generate commands for the safe control of the robots. For instance, this DT application can instruct robots to stop, adjust their speed, or move away from human workers when a collision risk is identified [29; 37]. In addition, [PERSON] et al. [13] proposed a DT application that supports 'Real-time Monitoring', which means that information about human workers and their context is provided in real-time. Figure 6 shows the number of relevant publications based on target industry (i.e., manufacturing, general, warehouse, and construction). The analysis shows that manufacturing is the most popular application domain for human-focused DT for OSH. Clearly, human-focused DTs for OSH are based on Industry 4.0 technologies, and a smart factory is a key part of the agenda of the Industry 4.0 era. Thus, the manufacturing industry is the most important application domain for such technologies. Similarly, this is also the case for human-focused DT for OSH [4; 8; 17; 24; 27; 29; 32; 33; 34; 35; 37; 38; 40; 41; 42]. The term 'General' in Figure 6 indicates that the associated applications can be applied to a wide range of industries [15; 26; 30; 39]. Warehouse [13; 14; 25] and construction [31; 36] are the third and the fourth most popular target industries, respectively, in Figure 6. In other words, they are a less popular application domain than the manufacturing industry. Figure 5: Main objectives of existing applications. Figure 6: Target industries of existing applications. Nevertheless, they are also promising application domains for human-focused DT for OSH in that both are traditionally labor-intensive industries. Still, state-of-the-art technologies have recently been actively introduced. In Figure 7, the tasks performed by human workers in existing applications are classified into four categories (i.e., assembly, manual material handling (MMH), ad hoc motion, and others). The majority of the applications were designed to create and utilize DT for human workers performing assembly tasks that involve combining a number of parts to obtain an end product [4; 8; 17; 24; 27; 28; 31; 32; 33; 34; 35; 38; 39; 40; 41; 42]. Most of these applications target manufacturing applications. The objective of an MMH task is not to make a product but to move an object from one place to another via common actions such as lifting, picking, and placing. Both assembly and MMH tasks contain a series of actions. However, the latter generally has a smaller number of actions and a shorter cycle time. In addition, the repetitive nature of MMH tasks poses ergonomic risks that can cause musculoskeletal disorders and injuries [12; 40].