Extravehicular activities (EVA), which are activities that require a crew member to leave the spacecraft, are a key component of many missions and require a substantial amount of preparation. Unfortunately, previous studies have shown that astronauts that are trained to carry out EVA have a significant incidence rate for shoulder injuries while wearing the protective spacesuit. A large percentage of these injuries occur during training prior to the mission, and a substantial portion of these require surgical intervention. A statistical analysis of the factors that may contribute to shoulder injuries identified that the current version of the Hard Upper Torso (HUT) in the spacesuit is strongly associated with astronauts developing shoulder issues. In order to make improve the design of the spacesuit, engineers need a thorough understanding of how the current HUT affects shoulder motion. This understanding can be obtained by applying musculoskeletal modeling techniques. Musculoskeletal modeling is a type of dynamic simulation that models how the human muscles and bones move to accomplish some task. I will use these types of models to understand how the spacesuit affects astronaut shoulder motion. My proposed research will consist of modeling the human-suit interactions in a one-g environment with an improved musculoskeletal model, simulating how these interactions change when the crew member performing underwater training, and comparing the results from the previous parts with simulations using a redesigned HUT. The musculoskeletal models I will use will be developed in OpenSim, an open-source software that already has a number of models for different parts of the human body. My musculoskeletal model will make improvements to an existing model by adding missing muscles and ligaments for a more complete description of the shoulder physiology. I will measure how the astronauts move with the current spacesuit using an optical motion capture system, and I will add various resistive loads that are associated with the suit's mass and joint stiffness. This model will be expanded on by including fluid forces that occur when astronauts performing EVA training in a submerged environment. These studies will provide information on how astronauts recruit their muscles to perform different tasks, as well as how shoulder posture and muscle forces relate to the likelihood of shoulder injury during EVA training. This information will be used to create hypothetical suit redesigns that can be used in the simulations to compare how shoulder motion differs between different HUT geometries. This research is important to several areas that are related to EVA mobility and astronaut health. First, quantifying the suit resistance, joint loads, and associated shoulder motion are vital to future redesigns of the EMU in order to reduce crew injury rates and increase the suit's mobility. This work will be important for astronaut medicine, as the information on muscle use and joint loads during EVA training can improve the prevention, diagnosis, and treatment potential shoulder problems. In addition, if future spacesuits incorporate technologies that allow for strength augmentation (such as those found in existing upper body exoskeletons), this information on joint forces and muscle use is necessary to ensure that these devices do not injure crew members.