## 2 Materials and Methods
### Concept of the Co-Simulation Model
#### 2.2.2 Simulation of the Human-Machine Interaction in the Co-Simulation Model  
Based on the general adjustments of the human model and the models of the technical systems, the co-simulation, depicted in Figure 6, can be conducted. Therefore, interfaces between the human model and the technical systems are defined. The human's motion forms the base for the co-simulation. The motion could be recorded with motion capture methods either based on inertial measurement unit (IMU) [40; 41] or optical tracking methods [39; 42], which can be transferred to the human model in OpenSim. For the implementation of the co-simulation model, the use case of working at and above head height is chosen. For dynamic simulation of human-exoskeleton interactions, an interface is created between the human and the exoskeleton model. The interface defines which motions and forces are exchanged between the two submodels, as well as their coordinate systems. The interaction between the human and the exoskeleton model is performed in four steps in the co-simulation model: First, the human motion data required by the exoskeleton are imported from the human model into the exoskeleton model in the form of marker positions and joint angles of the human. Second, the exoskeleton model simulates the behavior of the exoskeleton in response to human motion and the interaction forces at the physical interfaces between the exoskeleton and the human body. Third, the simulated  
Figure 6: Structure of the co-simulation model. Building the co-simulation by modelling the interactions between the technical systems and the human model as the base for biomechanical simulations and analyses. interaction forces are exported to the human model. Finally, the biomechanical effects of the interaction forces on the human body are exported to the musculoskeletal human model. Using the shoulder exoskeleton model mentioned above, three interface points are defined in the human model. These are at the back of the pelvis and at the humerus of each arm, respectively, to the lap belt and armrests as physical interfaces between the exoskeleton and the human body. The marker positions of each humerus are required to simulate the movements of the exoskeleton arms. The resulting interaction forces at the armrests and lap belt are applied to the humerus and pelvis, respectively. Human-Power Tool Interface  
The interface between the human and the power tool is defined by a reference point located on the palm of the dominant hand of the user. The power tool model determines the load (forces and torques on the human) at this reference point using experimental data from the study described by [PERSON] et al. [43] as input for the interaction simulation. The model integrates the motion data recorded by the Vicon motion capture system, the ground reaction forces (GRF), and the battery current of the cordless screwdriver. The user's applied push force is calculated from the GRF, while the screwdriver's battery current is used to estimate the process torque applied to the screwdriver during the screw-in process using the correlation model [43]. The parameters of the power tool can be adjusted to evaluate different tool design options during the co-simulation, as shown in Figure 4. The resultant simulated vectorial interaction forces and torques at the reference point are then fed into the human model as input for the musculoskeletal simulation. A comprehensive model description can be found in [PERSON] et al. [38]. In the framework of the musculoskeletal analyses, the influence on the human's motion caused by the working application as well as the influence of support systems can be detected and quantified. The biomechanical analysis not only enables a representation of the relief of the human body caused by the support systems, but also illustrates any additional load on the user. This can be quantified by biomechanical parameters such as muscle activation or joint reaction forces.