Understanding human-space suit interaction to prevent injury during extravehicular activity

Abstract

Extravehicular Activity (EVA) is a critical component of human spaceflight. Working in gas-pressurized space suits, however, causes fatigue, unnecessary energy expenditure, and injury. The problem of injury is particularly acute and is exacerbated with the additional hours astronauts spend training inside the suit, especially underwater in NASA's Neutral Buoyancy Laboratory (NBL). Although space suit performance and improved system designs have been investigated, relatively little is known about how the astronaut moves and interacts with the space suit, what factors lead to injury, and how to prevent injury. At the outset of this research effort there were no technologies suitable to evaluate human movement and contact within the space suit during dynamic movements. The objective of this thesis is to help understand human-space suit interaction and design hardware to assess and ultimately mitigate injury. This is accomplished through two specific aims. The first specific aim is to use data mining techniques to uncover trends in space suit configuration, training environment, and anthropometry, which may lead to injury. Two groups of subjects were analyzed: those whose reported shoulder injury incidence is specifically attributable to the NBL or working in the space suit, and those whose shoulder problems began in active duty, meaning working in the suit could have been a contributing factor. The first statistical model correctly identifies 39% of injured subjects, while the second model correctly identifies 68% of injured subjects. For both models, percent of training incidence in the space suit planar hard upper torso (HUT) was the most important predictor variable. Frequency of training and recovery between training were also identified as significant metrics. These variables can be monitored and modified operationally to reduce the impacts on the astronaut's health. Several anthropometric dimensions were also found to have explanatory power for injury. Expanded chest depth was included in both models, while bi-deltoid breadth was relevant for identifying injured NBL subjects and shoulder circumference was relevant for identifying injured Active subjects. These dimensions may be targeted as particularly important to accommodate in future designs of the HUT or any advanced concept space suits. Finally, for the NBL subjects, previous record of injury was found to be an important factor. Further descriptive analysis implies that analyzing the HUT style and size together may be critical for future detailed studies on fit and accommodation. These results quantitatively elucidate the underlying mechanisms of shoulder injuries for astronauts working inside the space suit. The second specific aim is to develop a wearable pressure sensing capability to quantitatively measure areas on the body's surface that the space suit impacts during normal EVA movement. A low-pressure sensing system was designed and constructed for the upper body during dynamic movements inside the space suit environment. Sensors were designed to measure between 5-60 kPa with approximately 1 kPa resolution. The sensors are constructed from hyper-elastic silicone imbedded with a microfluidic channel. The channel is filled with liquid conductive metal, galinstan, such that an applied pressure corresponds to a change in resistance of the liquid metal. The system of 12 pressure sensors accommodates anthropometry from a 50th percentile female to a 95th percentile male upper body dimensions with near shirt-sleeve mobility. The wiring was intentionally designed to achieve the best trade between flexibility, resistance, and stretch ability, but ultimately was the greatest limitation in system durability. The electronics architecture utilizes onboard data storage with more than 4 hours of use. The entire system was designed with extreme environments in mind, where considerations of shock, battery hazards, and material properties in mixed gas, pressurized atmosphere were minimized to ensure user safety. The pressure sensing system was used in a human subject experiment to characterize human-suit interaction. Three experienced subjects were asked to perform a series of 3 isolated joint movements and 2 functional tasks, all focused on upper body movement. Movements were repeated 12 times each and pressure responses were evaluated both by quantifying peak pressure and full profile responses. Comparing subjective feedback to the quantitative pressure data allows a sense of the variability of movement and minor changes in loading on the body while performing suited motions. Users generally felt they were consistent for all movements. However, using a nonparametric H-test, 53% of movements were found to be biomechanically inconsistent (p < 0.05). This experiment provided the first "window" inside the suit to evaluate contact pressures and sequential indexing of the person inside the suit for realistic EVA movement. It cannot be extrapolated how changes in contact pressure would affect a subject's propensity for injury as injuries accumulate over long time scales. However, changes in pressure may be due to alterations in biomechanical strategies or fatigue, both of which could be precursors for injury and discomfort. This work focuses on the upper body, but the methods may be extended to the full body as future work. It provides solutions that could be applied beyond the field of aerospace to assess human-garment interactions and recommending armor protection for defense applications to alleviate fall impacts for medical applications. The contributions to the field include the development of a protection system that assesses and prevents injury inside gas-pressurized space suits.