## 1 Introduction  
Overhead work is one of the greatest risk factors for work-related musculoskeletal disorders (WMSD) [1]. Sixty percent of workers in the EU with health problems have reported WMSD as their most serious health problem [2]. In Germany, 22.8% of the total sick days at work are caused by \"musculoskeletal and connective tissue disorders\" [3]. An exoskeleton is a potential solution to reduce the physical load on workers that affects the stress on, for example, the shoulder region [4, 5]. The most commercialized shoulder exoskeletons are passive, for example, Paexo shoulder (Ottobock, Duderstadt, Germany), ShoulderX (SuitX, Emeryville, CA, USA), and Airframe(tm) (Levitate Technologies, San Diego, CA, USA). Passive systems use mechanical energy and have no need for an additional powered actuator, energy source, or electronic components, which demand more space and weight. Active systems (e.g., shoulder exoskeleton S700 (exolQ, Hamburg, Germany)), on the other hand, have the potential to regulate the support level during the use itself, adapting to the requirement of the manual task and individual wishes. Many studies have shown the effect of load reduction on workers with an exoskeleton [6, 7, 8]. [PERSON] et al. showed a possible reduction in shoulder muscle activity of up to 58.2% with the Lucy exoskeleton during overhead activities [9; 10]. [PERSON] and [PERSON] simulated an overhead drilling procedure using an arm support exoskeleton and were able to demonstrate a reduction in the physical demand on the upper extremity [5]. Further simulative approaches have shown the potential reduction in the user's muscle activity through the implementation of different support concepts [11].  
For the evaluation of exoskeletons, methodological approaches have been proposed using both objective (e.g., muscle activity) and subjective (e.g., perceived exertion) measurements to demonstrate the reduction in the physical demands of the user [12; 13]. These approaches are particularly suitable for validating exoskeleton designs when ensuring that the same workload is applied to the user. Exoskeleton testing setups are usually designed in such a way by assuming the participants have the same workload under different test conditions through the standardized task definition [14]. Many studies have investigated the influence of the support force on the user's movement [15]. However, the use of exoskeletons can also lead to a different behavior of the user, which can result in different interaction forces or process forces of the task (e.g., the push force of screw-in task) [14]. The process force of the test task is hardly measured by exoskeleton evaluation. Furthermore, earlier research indicates that the support requirement of a task depends on its workload and the relevant movement [16]. Thus, the optimization of an exoskeleton is always application-related.  
To identify the optimization potentials of an active shoulder exoskeleton for selected overhead tasks with a cordless screwdriver, the influence of two different support levels (50% support, and 100% support) were investigated in this paper. Two screw-in tasks with different workloads were replicated from the industrial manual works. The user's physical strain and the human-exoskeleton interaction force were measured and analyzed in three key motion phases (lifting arm, screw, and lowering arm) of the tasks. Additionally, the push forces of the tasks were also recorded to monitor the workload. Based on the evaluation, an adjustment in the support level related to the key motion phases and a switching time detection approach are proposed.