# Lessons Learned from Investigating Robotics-Based, Human-like Testing of an Upper-Body Exoskeleton
## 1 Introduction  
Exoskeletons, wearable mechanical systems designed to enhance human physical capabilities [1], have seen substantial growth in various domains over the past few years. The global exoskeleton market is expected to grow significantly, reaching 14.67 billion USD in 2030 from 0.9 billion USD in 2022, reflecting a compound annual growth rate of 42.2% according to Fortune Business Insights [2]. The healthcare and manufacturing sectors are the primary drivers of this growth [2]. In healthcare, exoskeletons aid individuals with physical disabilities by reestablishing motor functions and supporting rehabilitation [3], for instance [4, 5]. In manufacturing, they assist in physically demanding tasks, enhance endurance, and reduce the risk of physical stress [6].  
Despite significant efforts in developing exoskeleton devices [7], there remains a need for more standardized testing [8]. Currently, testing procedures predominantly occur within the development cycle, tailored to the specific characteristics of individual exoskeletondesigns [4, 9]. However, systematic testing is crucial, specifically due to the close human-machine interaction and various wearing effects [10, 11]. Additionally, systematic testing could contribute to benchmarking exoskeletons against each other, offering insights for improvement and supporting iterative development.  
User studies, while essential, suffer drawbacks for systematic testing. Conducting studies requires a significant number of participants, making them time-consuming and reliant on expert knowledge [12]. Test-retest reliability criteria are challenging to meet [13]. Technical measurements within user studies, like measuring contact forces, present challenges with special equipment requirements [14] and potential side effects. Specifically, installing the required sensor at the contact location can cause inaccurate results or change the original behavior of the test subject.  
In this context, a promising approach evidently is robotics-based testing that mimics human body dimensions and motion behaviors, inspired by test benches in automotive or aviation. This technical evaluation enhances comparability between different systems or designs, addressing the variation in test setups such as test samples, tasks, execution, and environments [15, 16]. User feedback offers essential insights into the exoskeleton's performance with regard to different measures like comfort, usability and human acceptance. These qualities are hard to measure objectively, which is why user studies are conducted in many areas of human-machine interactions, for instance in [17, 18, 19]. Robotics-based testing rather offers a complementary tool to address the limitations inherent in user studies and, in combination, offers a more comprehensive testing toolkit. In the literature, the first approaches utilizing humanoid robots as test platforms have been documented, e.g., the humanoid robot \"PETMAN\" for testing chemical clothing [20]. Regarding exoskeletons, several approaches [21, 22, 23, 24, 25] primarily focus on back-support systems, with shoulder support systems yet to be thoroughly explored.