## 1 Introduction  
Lower extremity assistive devices(LEADs) have been developed in various forms such as a type in which an actuator is attached to each joint position [1; 2; 3; 4] or a treadmill-based exoskeleton [5; 6]. Such assistive devices, which have been developed for the purpose of muscle strength enhancement or power assistance, are intended for use under Earth gravity (1 g) conditions with a gravitational acceleration of approximately 9.81 m/s\({}^{2}\) (1 g) in the vertical direction. During the movement of a human body, this gravitational acceleration prevents muscle atrophy by continuously generating a resistance of the movement and makes it possible to maintain contact with the ground during walking, thereby producing a ground reaction force (GRF). However, the resistance force is barely generated when moving in a microgravity environment. The microgravity means \(10^{-6}\) times the gravitational acceleration on earth (1 g) and is used synonymously with zero gravity in general. A lack of resistance force in microgravity results in problems such as decrease in bone density and muscle atrophy during long-duration space flights [7]. When walking in microgravity conditions, the kinematic pattern changes [8; 9; 10], and even if kinematically the same gait is performed as 1 g walking, a difference in net joint torques is caused by the lack of a GRF. To address these issues, several devices have been developed to assist muscle-strengthening exercises by generating resistance to movement [11; 12]. Astronauts staying at the International Space Station (ISS) use these devices to perform more than two hours of exercise a day. The types of equipment for exercise include a cycle ergometer for pedaling, treadmill for walking or jogging, and a weight-bearing resistance device (RED) [11]. These devices are so designed that forces or torques are applied to the shoulders, feet, or joints of the human body to facilitate a user to perform actions such as squatting and walking under the resistance force. To develop an assistive device for resistance training in a microgravity environment, it is necessary to select the types and locations of appropriate actuators and determine the control inputs to implement the desired task. However, some issues such as the microgravity environment, time required to produce a prototype, and measuring muscle activation make it difficult to determine the feasibility of the designed actuation method and obtain corresponding control inputs by experimental methods. These problems can be dealt with more efficiently by using simulations. Some related studies have been performed on the prediction of the muscular force generated using given kinematics data and external force conditions [13], and on optimal motion prediction to achieve a specific motor task [14; 15; 16; 17]. In the design of a resistance training device under microgravity conditions, musculoskeletal simulations can be used to examine the actuator locations and acquire the corresponding control input, which enables the tracking of the desired motion data and muscle forces. The simulation of the human musculoskeletal system is categorized into data tracking and predictive simulation based on its purpose. The main purpose of a data tracking simulation is to predict the muscle force using noninvasive methods based on experimentally obtained motion data and the GRF data. The goal of the predictive simulation is to obtain optimal movement to perform a specific motor task without experimental data. Both methods exploit optimization methods for different purposes. In data tracking, optimization is used to determine the muscle forces in the redundant nature of a human body, where multiple muscles can be involved in the actuation of a joint. In the case of predictive simulation, optimization is used to predict the behavior of minimizing or maximizing the objective function composed of specific motor tasks. Optimization methods consist of dynamic optimization and static optimization.