## 4 Discussion  
Repetitive muscle activity around the shoulder for over-the-head tasks is the main source of fatigue and resulting discomfort for workers. Exoskeletons are one of the most promising devices that prevent WMSDs [23]. Exoskeletons have also been introduced to reduce the physical strain on workers [24]. It has been shown, in detail, that occupational exoskeletons can significantly reduce the muscle activity for these tasks. Passive exoskeletons with lightweight designs also avoid the drawbacks of active exoskeletons and a trade-off between benefits and costs can be achieved with almost no problem if the psychological barriers can be removed for workers. Passive exoskeletons also enable the load to be evenly distributed across the shoulder joints [25]. In this study, a very lightweight passive exoskeleton was presented and the efficiency of the design was measured via muscle activities for over-the-head and forearm-stretched tasks. Compared to the readily available exoskeletons known to the authors, the original design in this work provides more freedom when wearing an exoskeleton, especially during breaks, because its support mechanism is set to be activated after a 120-degree flexion and released for an 89-degree extension back. In addition, no work known to the authors has shown efficacy for tasks where the forearm is completely stretched with a tool held still (Task 2). The muscle activity measurements for the two main tasks prove that the exoskeleton helped to reduce discomfort around the shoulder, because the deltoid muscles' activity decreased significantly. These deltoid muscles are mainly responsible for performing tasks and WMSDs can be prevented or delayed thanks to reduced muscle activity. The design presented an adjustable resistance for the arm rest and this also provides extra comfort, because the muscle history of workers may vary significantly. A reduction in the all the main muscles was shown, except for a negligible increase in the BB activity in Task 2 and a slight increase in the LT activity. However, it can be said that the metabolic cost will be significantly reduced because of less overall muscle activity. [PERSON] and [PERSON] [26] claimed that the available passive exoskeleton designs help in reducing metabolic costs, especially for overhead tasks, in their extensive review. The design presented in this work supports their findings; however, the original design with an adjustable mechanism presented in Figure 4 is expected to lead to even more reduced metabolic costs. They also criticized the work in the literature focusing on overhead tasks without providing data proving a reduction in the back or core muscles. LD and LT muscle activities can be considered as sources of potential back pain created by an exoskeleton. The muscle activity for LD is very small compared to other muscles and there was no increase for both tasks when the exoskeleton was on; however, there was an increase in LT, and this increase was more prominent for Task 2. Any contraction velocity change in these muscles was not expected due to wearing the exoskeleton; therefore, additional costs can be evaluated based only on muscle force. Due to reductions in the main muscle activities around the shoulder, there was an obvious comfort provided by the exoskeleton for the over-the-head occupational task. The increase in the LT was very small compared to the decrease in the deltoid muscles; however, a worker with present back issues may feel uncomfortable due to the exoskeleton after prolonged use. An investigation into the effects of an exoskeleton on the muscles has been performed under two static tasks for 1 min. Overhead tasks vary and task times could take more or less than 1 min according to the task. Thus, further investigations should be performed for more or less than 1 min. Muscle fatigue develops with time, so different time period scenarios would be interesting to take into account, in order to evaluate such exoskeletons in real conditions.