## 2 Materials and Methods  
### Biomechanics Design of Exoskeleton  
The exoskeleton is designed according to the biomechanical conditions representing the working conditions of workers. Upper-extremity exoskeletons mainly support the upper arm against gravity; therefore, they help to decrease the moment around the shoulder joint. First, the shoulder moments without wearing the exoskeleton are determined for the pre-defined tasks, named Task 1 and Task 2, as shown in Figure 1. The moment for Task 1 can be calculated with the formula,  
\[\mathrm{M_{s,T1}}=\mathrm{g}\big{[}(\mathrm{m_{tool}})\times\big{(}\mathrm{L_{ upperarm}}\big{)}+(\mathrm{m_{hand}})\times\big{(}\mathrm{L_{upperarm}} \big{)}+(\mathrm{m_{forearm}})\times\big{(}\mathrm{L_{upperarm}}\big{)} \tag{1}\] \[+\big{(}\mathrm{m_{upperarm}}\big{)}\times\big{(}\mathrm{L_{upperarm }}\times 0.436\big{)}\big{]}\]  
and, for Task 2,  
\[\mathrm{M_{s,T2}}=\mathrm{g}\big{[}(\mathrm{m_{tool}})\times\big{(}\mathrm{L_{ upperarm}}+\mathrm{L_{forearm}}+\mathrm{L_{hand}}\times 0.506\big{)}+(\mathrm{m_{hand}}) \times\big{(}\mathrm{L_{upperarm}}+\mathrm{L_{forearm}}+\mathrm{L_{hand}} \times 0.506\big{)}\]  
where \(\mathrm{g}\) is the gravitational acceleration. The coefficients 0.436, 0.43, and 0.506 that are seen in these equations are the ratios of the centroids measured from the proximal joint for the upper arm, forearm, and hand, respectively. These ratios are taken from the anthropometric data presented in Neumann [20]. The deltoid muscles are the main muscles activated to perform both tasks, while the biceps brachii and trapezius muscles are the supporting muscles for creating balance. The biceps brachii muscles' main function is to balance the forearm, hand, and tool in given positions (flexion forearm). Therefore, the biceps brachii do not affect the required support force exerted by the exoskeleton. The trapezius muscle works to elevate and stabilize the scapula. The deltoid muscles' activation force is required for the design of the exoskeleton. For this purpose, a method of section is applied for the arm and free body diagrams for the segments of Task 1 and Task 2, which can be seen in Figure 2.  
Since the tasks are performed in static equilibrium, the inertial effects can be neglected and the following equalities can be written. \[\mathrm{\sum\stackrel{{\rightarrow}}{{\mathrm{M_{s,t1}}}}}= \stackrel{{\rightarrow}}{{0}}\text{ and }\mathrm{\sum\stackrel{{ \rightarrow}}{{\mathrm{M_{s,t2}}}}}=\stackrel{{\rightarrow}}{{0}} \tag{3}\]  
\[\mathrm{g}\big{  
and 0.85 of the upper arm length is taken as the arm rest attachment distance from the shoulder joint. Then,  
\[\frac{\mathrm{M_{s,t1}}}{2}=\mathrm{F_{exo,t1}}\times(\mathrm{H}\ \times 0.186 \times 0.85) \tag{6}\]  
\[\frac{\mathrm{M_{s,t2}}}{2}=\mathrm{F_{exo,t2}}\times(\mathrm{H}\ \times 0.186 \times 0.85) \tag{7}\]  
\(\mathrm{F_{exo,t1}}\) and \(\mathrm{F_{exo,t2}}\) represent the force supported by the arm support. The exoskeleton is designed such that it can be worn by the human like a backpack and a 3D solid sketch of the exoskeleton is shown in Figure 3. The mechanism that generates torque is located around the shoulder area and connected to the waist belt and arm rest via connecting rods. The waist belt connector is attached to the waist belt via a ball-and-socket joint so that the exoskeleton can move freely when the user moves their arm. The arm rest connecting rod is attached to a unit that is called a torque generator. The torque generator is designed such that it is initially deactivated at the anatomical position. It is activated when the user lifts their upper arm (flexion) 120 degrees from its anatomical position. A varying torque is generated upon this activation while supporting the upper arm until the mechanism releases itself. The torque increases when lowering the upper arm and the torque generator is deactivated at 89 degrees from the anatomical position.