## 1 Introduction  
Due to the increasing attention to health and well-being in the workplace, the interest in support systems, such as exoskeletons and power tools, for workers in these situations is growing. Such systems can be used in physically demanding work situations and thus reduce the user's physical stress at the workplace [1; 2; 3; 4]. For the development of such systems, the interaction with the user is of great importance, as these systems are in direct force and information transmission with the human [5; 6]. However, studies with subjects and prototypes or test benches [7; 8; 9; 10] are still the most commonly used methods for analyzing the effects of the user-product interactions in detail. Due to the extensive interactions of support systems with humans, special attention should be paid to these and the support systems should be developed in a more proactive user-centered way. [PERSON] et al. [11] present an approach for the consideration of physiological and psychological aspects of the user-product interaction. In this way, digital human models can be used for virtual evaluation and validation in addition to approaches that map the subjective factors in product development. Depending on the use case, different kinds of digital human models can be used in the product development process. Anthropometric digital human models can map user groups and are typically used in ergonomics assessment for reachability and visibility analyses [12; 13]. Due to the representation of the human as a multibody system, musculoskeletal human models are suited for biomechanical analyses. In these models, structures such as bones, tendons and muscles are modelled with particular anatomical accuracy. By using this kind of digital human model, the influence of movements and forces on the human body can be analyzed by using biomechanical parameters such as the muscle activation or joint reaction forces. In addition to the use of these models for medical applications such as gait analysis [14; 15], they are also increasingly being used in product development [16; 17]. For example, [PERSON] et al. [18] use musculoskeletal human models for parametric simulations to evaluate different product designs of a saw handle. Furthermore, [PERSON] et al. [19] present an approach for using digital human models in combination with virtual reality in order to support the design of ergonomic workstations. Moreover, there are already initial approaches for using musculoskeletal human models for the user-centered product development of support systems [20; 21; 22]. However, simplifications are often made in these approaches. For example, the exoskeleton's support is simplified as a constant moment acting on a joint [23; 24] or as a pure force acting on the bone [25; 26]. Thus, principal evaluations due to design concepts or ideas of the support systems can be conducted. [PERSON] et al. [27] show that the influences of user interactions with an exoskeleton and power tool can be mapped by this and possible effects of design changes on the user can be depicted. Nevertheless, for a more holistic view and a better assessment of design impacts, co-simulation models should be used. On the one hand, this allows for a detailed representation of the user and the products in individual models, whereby simplifications of the individual virtual models can be avoided. On the other hand, interactions between the human and the technical systems can be simulated as realistically as possible. As a result, a co-simulation model can help to represent and quantify the human-machine interactions and their effects on the human body by means of suitable parameters. Based on these parameters, design options for the technical systems, such as the modification of object geometries, can be derived and the user-centered product development process can be supported already in the early phases of product development.