## 4 Discussion
Accordingly, it can be used to check whether the desired target data can be tracked with the actuator configuration presented in the design of LEAD, and the corresponding control input can be obtained so that the capacity of the actuator can be selected or utilized for other controls. The Type 1 and Type 2 actuation method used in this study was chosen based on the actuation method of the strength assist system that has been developed so far. Systems such as Type 1 are called end-effector types used in the field of rehabilitation [41; 42], and the form of Type 2 is actually similar to the X1 [11] that was developed by NASA for long-term flight resistance training for astronauts. The form of Type 1 has the advantage of easy setup since there are relatively few fasteners, and Type 2 is typically smaller in volume than Type 1 and suitable for measuring the user motion. One of the most important factors in a robot that transmits force between the human body and the robot is user safety and comfort. In order to apply the simulation results obtained from this study to actual resistive training devices, appropriate actuators and control methods should be used, taking these factors into account. For this purpose, the Series Elastic Actuator (SEA) can be used. The SEA has a structure in which spring elements are attached to the transmission part of the actuator and calculates force or torque from spring displacement [43; 44]. The SEA, a type of compliant actuator, has been used for torque control of wearable exoskeleton robot [43; 45] because it improves safety due to its low impedance characteristics and also has excellent force control performance [46]. Hard-stops and motion limiting controllers are also required to prevent hyperextension or hyperflexion of the joints. In the control method, torque and force profiles should not be applied directly to the human joints as a periodic function over time. Because if the user does not produce torques of exactly the same magnitude with opposite direction, it can cause severe injury. Therefore, in practice, desired torque curves should be created based on the angle of the joint and then applied to the user by considering walking phase [46; 47]. It will require further experimentation and verification for actual implementation. Although both systems were able to achieve the simulation's objective of muscle activity and kinematics tracking (Figures 3, 4, 7 and 8), the advantages and disadvantages of each system are prominent from different perspectives. In addition to the muscle atrophy focused on this study, the Type 1 system is advantageous from the point of view of preventing bone loss. Because human bones are generally generated in the direction of compressive load, Type 1, which has a structure in which \(y\)-directional force acts on the sole, will cause more bone-loading in the longitudinal direction. However, Type 2 may have more advantages in performing partial joint movements or various motions. We have obtained a solution to the problem by performing the simulation for the design of a LEAD in a microgravity environment; however, there are also some limitations of this study. First, the model only has degrees of freedom in the sagittal plane, even though a three-dimensional model would more accurately reflect the human body. Although the motion in the sagittal plane is more dominant than in the other directions, the influence of the muscles that simultaneously affect two or more degrees of freedom may not be correctly evaluated. Second, the ideal actuator is used without regard to the effects of the mass or inertia, power transmission loss, and axis misalignment between the human body and assistive device. Therefore, problems arising in the actual world such as power loss caused by the misalignment of the rotation axis between the actuator and human joint could not be considered. Third, the interaction between the human and device is not considered.