## 1 Introduction  
In 2015, the average age of individuals engaged in agricultural tasks in the Republic of China (ROC) was 57.1 yr. [1], which indicated that the ROC was suffering from an aging agrarian population. According to the statistical information of the ROC in 2013, orchard farmers aged between 25-44 yr. and 45-64 yr. accounted for 11.14% and 64.85% of all orchard farmers, respectively [2]. In recent years, the Council of Agriculture of the ROC has vigorously encouraged young people to engage in agricultural tasks, owing to which, the average age of the agricultural technology group in Kaohsiung has decreased to 41 yr. [3]. Nevertheless, orchard farming-related tasks, such as fruit thinning and pesticide spraying, require considerable muscle movement. During these arm-lifting movements, orchard farmers must repetitively lift their arms, leading to considerable physical exertion. Specifically, such repetitive motions of lifting the upper limbs generate significant body fatigue and reduce efficiency, especially in the case of elderly orchard farmers. Work-related musculoskeletal disorders (WMSD) are painful conditions of the muscles, tendons, and neuralgia. WMSD is a common occupational injury caused by the overuse of a muscle without proper rest. In general, nearly all tasks require the use of arms and hands, and thus, WMSD of the upper limbs in the modern workplace must be addressed [4]. Furthermore, several ergonomic risk factors of WMSDs exist in agricultural tasks, particularly from overwork and non-neutral postures [5,6]. A report demonstrated that 93% of the occupational injuries of agricultural workers were related to WMSD. Carpal tunnel syndrome accounted for one-third of the WMSD cases, and the shoulder was the second most affected body part [7]. In recent years, automation and robot technology have been widely implemented, but the amount of workload has not significantly decreased. The load levels continue to be relatively high in the construction, agriculture, and manufacturing domains [8]. Even during low-intensity physical activities, muscle fatigue can occur, which can damage muscle function. The continuous contraction of muscles can restrict blood circulation and further increase muscle fatigue [9]. In particular, the repetitive lifting movements of the arms, which are required in orchard farming activities, can increase the risk of developing WMSD [10; 11]. Therefore, research on the use of exoskeletons to help alleviate muscle fatigue has attracted much attention. In general, exoskeletons can be divided into active or passive systems. Active systems consist of one or more actuators to enhance the strength of the wearer. In contrast, passive systems employ gas springs or other materials as a power source that stores and releases energy to assist the wearer in performing physical activities [12; 13]. With the development of various exoskeletons, the passive upper-limb exoskeleton (PULE) has emerged as a promising preventive measure to decrease upper-limb fatigue caused by repetitive lifting movements. In this regard, the PULE is composed of two parts, namely, the wearable passive power driving device and the support mechanism suit. The driving device enhances and assists human movement to reduce the physical exertion of the wearer, thereby reducing the wearer's risk of developing WSMD [12; 14]. Existing research on PULEs is mostly focused on industrial tasks such as precision tasks and repeated manual material handling tasks in car factories. In these tasks, the PULE can effectively decrease the stress level of the wearer in lifting his/her arms. It has been demonstrated that, on average, the use of a PULE increased work performance by 30% and reduced the fatigue of the wearer [15]. In load lifting/lowering and box unstacking/stacking tasks, the use of a PULE reduced anterior deltoid (AD) muscle activity by 54% and 73%, respectively [16].