## 4 Discussion
### Muscle Activity  
The purpose of the PULE is to reduce the upper limb muscle tension of the person wearing the PULE. With a decrease in muscle tension, fatigue is expected to be lessened, reducing the risk of developing WMSD in the upper limb [17]. The PULE demonstrated a decrease in the AD muscle activity of 17.64% and 19.86% for groups A and B, respectively, for which the experiments were conducted in the laboratory. Similarly, the PULE demonstrated a decrease in the AD muscle activity of 37.67% and 39.57% for groups C and D, respectively, for which the experiments were conducted in the orchard. This decrease in the AD muscle activity indicates that the PULE can effectively reduce the muscle tension experienced by orchard farmers. The PULE effect observed in the orchard farming simulation was significantly lower than that of the actual orchard farming operation. This  
Figure 8: Linear regression for the torque of the PULE.  
Figure 7: Mean LPP scale of the upper limb using the PULE for: (**a**) fruit thinning and pesticide spraying simulations for groups A and B; (**b**) fruit thinning of the jujube tree and pesticide spraying of the pomelo tree for groups C and D.  
phenomenon is likely due to the fact that the actual operation actions include internal rotation, abduction, and external rotation, for which the PULE could also decrease the LD and PD muscle activities.  
The %MVC of the AD for groups A and B during pesticide spraying was higher than that during fruit thinning, indicating that pesticide spraying requires a larger muscle activity of the upper limb compared to the muscle tension required for fruit thinning. However, the RPE scale of groups A and B for pesticide spraying was lower than that for fruit thinning, indicating that the participants felt that pesticide spraying was easier than fruit thinning. This phenomenon is likely due to the short-term lowering of the upper limb during pesticide spraying. Specifically, the participants moved their hand from 0\({}^{\circ}\) to 100\({}^{\circ}\), swinging back and forth, which provided the AD with a brief rest, and consequently, the participants felt less tired.  
The PULE reduced the %MVC of the AD by 22.06 for the jujube thinning activity. These results are superior to those of the upper limb assist device developed by the Institute of Agricultural Machinery of Japan, which reduced the %MVC of grape thinning by 8 [32]. The proposed PULE also decreased the LD muscle activity for the jujube thinning activity by 46.42%. These results are worse than the 62% decrease in the static overhead work with a 2-kg load [17]. This phenomenon is likely due to the long-term raising of the upper limb during the jujube thinning activity, as static overhead work with a 2-kg load requires raising the upper limb for only 30 s.  
In groups A and B, the PULE reduced the work required by the AD muscle by 17.64% to 19.86%, but there is no significant difference. The reason for this is likely because the torque of the PULE is constant and cannot be adjusted. If the torque can be modified, the upper limb lifting action may be able to benefit from a greater auxiliary gain for the participants.  
Moreover, although a 20-yr. age difference existed between groups A and B, the PULE effect was similar for both fruit thinning and pesticide spraying activities. In other words, the PULE effect did not exhibit any significant difference between groups A and B in both the activities. This result indicates that the age of the participants (25-64 yr.) was not a factor in the PULE effect.