Human-spacesuit interaction :

Abstract

Extravehicular activities (EVA), or space walks, are a critical and complex aspect of human spaceflight missions. To prepare for safe and successful execution of the required tasks, astronauts undergo extensive training in the Neutral Buoyancy Lab (NBL), which involves many hours of performing repetitive motions at various orientations, all while wearing a pressurized spacesuit. The current U.S. spacesuit-the Extravehicular Mobility Unit (EMU)-is pressurized to 29.6 kPa (4.3 psi) and requires astronauts to exert a substantial amount of energy in order to move the suit into a desired position. The pressurization of the suit therefore limits human mobility, causes discomfort, and leads to a variety of contact and strain injuries. Shoulder injuries are one of the most severe injuries that astronauts contend with, and are mainly attributed to the EMU's hard upper torso (HUT). While suit-related injuries have been observed for many years and some basic countermeasures have been implemented, there is still a lack of understanding of how humans move inside the spacesuit. The objective of this research is therefore to gain a greater understanding of this human-spacesuit interaction and potential for shoulder injury through two approaches: quantifying and analyzing the suit-induced pressures that arise in the shoulder region, and comparing the shoulder muscle forces that arise in the unsuited and suited conditions by modeling human-spacesuit interaction. The first approach provides an "inside look" of the pressure distributions and pressure profiles that arise at the interface between the human shoulder and the torso of the spacesuit, thereby suggesting which areas of the shoulder might be prone to contact injury. A commercially produced pressure sensing system is used to collect shoulder pressure data during a human subject experiment that involves three experienced subjects performing a series of upper body motions in both unsuited and suited conditions. Pressure distributions reveal that: 1) the least experienced subject generates the highest pressures, 2) for the majority of movements for all subjects, pressure is concentrated just above the clavicle over the soft musculature at the top of the shoulder, 3) the top of the shoulder is one of the regions in which maximum pressure is located most frequently, and 4) the shoulder blade is a secondary region of concern with regards to frequency of experiencing maximum pressure. Pressure profile analysis reveals that 1) for most subjects, general profile trends vary in shape across movement groups, 2) repetitions within each movement group are consistent in shape, and for most subjects also in magnitude, 3) the highest pressures are typically found near the top of the shoulder, and 4) the shoulder blade area is of concern for at least one subject. As these results are primarily observational in nature, a statistical analysis is performed to assess the effects of motion type and anthropometric region on peak pressure magnitudes. This analysis shows that results cannot be generalized across subjects as they are likely affected by individual anthropometry, suit fit, and the biomechanics of how each subject performs the motion. However, a number of interesting trends regarding which motions or regions yield higher pressures are found for each of the individual subjects. The results are specific to the subjects, suit sizes, and experimental conditions used in this particular experiment; however, the application of these quantitative and repeatable techniques during future experiments, suit fit sessions, or NBL runs would lead to a more complete understanding of human-spacesuit interaction at the shoulder interface. The second approach analyzes the effects of spacesuits on muscle forces in the shoulder region. Data regarding spacesuit joint torques and the joint angles of a suited subject are integrated into an upper-extremity musculoskeletal model in OpenSim to evaluate which muscles are most affected by the spacesuit. Looking specifically at a shoulder abduction/adduction motion, shoulder abductors, adductors, and stabilizer muscle groups are evaluated for significant changes in force from the unsuited to suited condition, and individual muscles within the shoulder region are also evaluated for significant changes from the unsuited to suited conditions. From a statistical analysis of the musculoskeletal simulation results, it is found that of the three investigated muscle groups-shoulder abductors, adductors, and stabilizers-only the abductors experience a statistically significant change in total muscle force between the unsuited and suited conditions. Looking specifically at the individual muscles that constitute the abductors and stabilizers, we find that only the middle deltoid experienced a statistically significant change in force from the unsuited to suited condition. A number of explanations are provided for the observed force profiles and the statistical results. The presented results are specific to the subject's motion data, suit torque data, and the musculoskeletal model that are used; however expanding this analysis to more subjects, other body joints, and a more complex musculoskeletal model would provide useful results for industry experts. Valuable information could be provided to EVA operations teams, flight doctors, and spacesuit designers regarding which movements or tasks should be avoided or performed minimally to prevent injury. The resulting muscle forces could also be used to set limits on the joint torques that are engineered in future spacesuits. Each of the approaches implemented in this thesis provides a different avenue for addressing the issue of shoulder injury in the spacesuit. While the pressure analysis contributes to the understanding of human-spacesuit interaction by informing on the anthropometric regions that might be most susceptible to contact injury, the musculoskeletal analysis provides insight as to which individual muscles are most susceptible to strain injury. Both of these quantitative, evidence-based approaches contribute to an increased understanding of the potential for shoulder injury in the spacesuit.