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Changes in seated height in microgravity
Applied Ergonomics 83 (2020) 102995
Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: http://www.elsevier.com/locate/apergo
Changes in seated height in microgravity Karen S. Young a, *, Sudhakar Rajulu b a b
Leidos, Inc, United States National Aeronautics and Space Administration, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Anthropometry Astronaut Spinal elongation NASA
Many physiological factors, such as spinal elongation, bone atrophy, and muscle loss, occur when humans are exposed to a microgravity environment. These physiological changes can result in slight to drastic changes in body dimensions. Any drastic change in body dimensions is critical information for current and future space hardware designers. These changes can affect accommodation, safety, and performance of a crewmember while in space. This study measured the overall change in seated height and stature for crewmembers exposed to a microgravity environment. Seated height data were obtained from 29 crewmembers that included 8 Interna tional Space Station increment crew (2 females and 6 males) and 21 Shuttle crew (1 female and 20 males). The results indicate that all participating crewmembers experienced statistically significant change in seated height. The corresponding change, 6% from preflight, should be considered for vehicle designs as the necessary seated microgravity adjustment.
1. Introduction Spinal elongation is an important factor to consider when designing for microgravity conditions and can affect safety, performance, and accommodation of a crewmember while in space. Spinal elongation occurs due to the lack of gravity/compression on the spinal column. This can potentially cause a straightening of the natural spinal curve or cause fluid to shift in the inter-vertebral disks that most likely result in changes in height. These changes are known to be the primary mechanism for changes in height in microgravity. The adapted morphology of the spine in microgravity has been also associated with the health issues such as persistent back pain, which can ultimately interfere with crew safety and performance, i.e. don/doff of the spacesuit and performance during Extravehicular Activity (EVA). However, due to the difficulty of mea surements during microgravity exposure, only a limited number of studies have been performed in the past to systematically quantify height and other anthropometric changes in microgravity. Previous experiments have taken place during the Apollo era and during bed-rest studies which have acted as an analogue to space flight. Spinal elongation and the associated changes, were previously investigated during Apollo-Soyuz Test Project (ASTP), Skylab, and bed rest studies (Webb Associates, 1978; Brown, 1975, 1977). In the ASTP studies (1975), which consisted of one 9-day flight, stature was measured periodically throughout the mission with the crewmembers’
feet against the docking module hatch and bodies supine on the control and display panel, with the observer using a checklist as a square edge to read from a measurement decal taped to the control and display panel. It was observed that in as little as 2 days a typical crewmember can exhibit increases in stature of up to 3%. After the first few days, stature remained at a steady state (Web Associates, 1978; Brown, 1975, 1977; Hutchinson et al., 1995; NASA, 1985; Pepper et al., 1994; Styf et al., 1997; Thornton et al., 1977, Thornton and Moore, 1987). The initial changes in stature were attributed to the immediate loss of spinal cur vature, with additional changes after Day 6 attributed to expansion of intervertebral discs. This data, however, was collected for only a limited number of crewmembers in standing postures, seated heights were not measured. Another study performed during Skylab 4 (1973) examined changes in stature for three crewmembers (Thornton et al., 1977; Thornton and Moore, 1987). Measurements were taken with crewmembers barefoot, standing fully erect with their back against a wall and applying a downward pressure with their hands to maintain solid contact with the floor. A mark was made on the wall at the top of the subject’s head (Fig. 1). The study indicated a quick increase in stature during the first day of weightlessness, after which stature remained steady at 4–6 cm greater than the original measurement (Thornton et al., 1977). Based on analysis of the stature changes during the Skylab and ASTP studies, a ‘growth’ of approximately 3% of stature was identified, attained over
* Corresponding author. 2101 NASA Parkway, Houston, TX, 77058. E-mail address: [email protected] (K.S. Young). https://doi.org/10.1016/j.apergo.2019.102995 Received 19 June 2019; Received in revised form 1 November 2019; Accepted 3 November 2019 Available online 15 November 2019 0003-6870/© 2019 Elsevier Ltd. All rights reserved.
K.S. Young and S. Rajulu
Applied Ergonomics 83 (2020) 102995
crewmembers. Furthermore, if spinal elongation is not included in the design of the spaceflight hardware, potential adverse impacts to per formance, comfort, and safety can occur; for example, crewmembers may not be able to safely ingress/egress the seat or don/doff the spacesuit potentially causing injury or the inability to perform their task (s). The amount of clearance adjustments needed between the seats could also affect crew complements and crew accommodation; for instance, the Orion module had a height restriction in place such that when a crewmember of 99th percentile male seated height occupied the top seat, the seated height of the occupant of the bottom seat had to be less than 99th percentile male seated height when the effect of spinal elongation was incorporated (Margerum and Rajulu, 2008). In contrast, if spinal elongation effect were nonexistent, a crewmember of any seated height could occupy the bottom seat underneath a 99th percentile male crewmember in the top seat due to the additional 6.6 cm (2.6 inch) of clearance gained. Another problem encountered by astronauts in space is the ensuing fit issue in space suits as a result of the microgravity induced spinal elongation. Some crewmembers have experienced trouble donning suits after extended periods in microgravity. Based on this, an arbitrary adjustment of 2.54 cm (1 inch) is currently added to suit torso length as a sizing adjustment to account for microgravity effects (Thornton et al., 1977). Suit accommodation needs to take spinal elongation into account to optimize individual performance, fit, and comfort of the crewmember in the suit. Otherwise, crewmembers may not fit well in the suit in microgravity and this may lead to decreased performance and potential safety or injury concerns. Given the necessity for accurately quantifying the effects of spinal elongation, the present study was primarily aimed at measuring seated height and stature from subjects exposed to a microgravity environment and providing the new information to designers via updated re quirements. Due to the payload imperatives or geometric constraints, the margin for clearance or adjustability is often restricted in the spaceflight vehicle or spacesuit. Thus, the design optimization is a major issue and the accurate assessments of the body shape changes in crew members are crucial. Data gained from this study would provide valu able information to future vehicle designs including MPCV regarding seat layout and crew accommodation. The implications on suit design, lunar vehicle design, and crew selection are important as well.
Fig. 1. Sketch of in-flight height determination (Skylab 4).
the first 2 days of weightlessness (Thornton and Moore, 1987). The 3% microgravity-induced factor has been adopted in NASA Man System Integration Standards (NASA, 1985) and Human Systems Integration Requirements (NASA, 2016). Bed rest studies were often performed to simulate the effects of microgravity on the body. This is accomplished by offloading the body vertically which removes the vertical gravity component. The vertical offloading of the spine allows the spine to lengthen, similar to the known effect that a person is taller in the morning than in the evening. The observed “circadian rhythm” in height changes corresponded to 1.93 cm (1.1%) increase of the subjects’ stature (Tyrrell et al., 1985). Similarly, loading a seated subject’ shoulders with a 10 kg weight for 5 min induced a stature decrease by 0.63 cm on average (Magnusson and Pope, 1996). A ground-based bed rest study (1995) observed 8 subjects (182.6 � 5.5 cm [mean � SD]) exposed to a 6-degree head-down tilt for 16 days to simulate the effects of spaceflight. The study found stature changes of 2.1 � 0.5 cm by day 3 and remained consistent throughout the rest of the duration (Hutchinson et al., 1995). An additional bed rest study used a 6-degree head-down tilt as well as horizontal bed config urations for 6 subjects who experienced 1.2 � 0.1 cm of stature increases after 1 day of bed rest and 2.2 � 0.8 cm after 3 days (Styf et al., 1997). Spine length was recorded as well, measured from L5 to C7, with an increase of 2.5 � 0.8 cm after 3 days. It was suggested that spinal elon gation is primarily responsible for changes in stature, further indicating that 30% to 60% of the change is due to increases in intervertebral disc height while the remainder of the change is due to decreases in the curvature of the spine. However, while the previous studies have focused on height mea surements in a standing posture, seated posture, which is hypothesized to be directly influenced by spinal elongation, has not been addressed. Seated postures may exhibit different characteristics than standing up right because of the changes in body posture, fluid shifts, and muscle loss. Seated height was deemed as a critical measurement for the seat layout in the design of the Orion Multi-Purpose Crew Vehicle (MPCV) due to the layout of the seats in a capsule-like vehicle. Specifically, four crewmembers were to be stacked two abreast with the commander and pilot seats on the top (fore) and the two remaining seats underneath (aft), limiting the amount of seated height clearance for the crew members seated in the aft seats. The interior wall dimensions of these vehicles were fixed because of other design constraints. Therefore an accurate quantification of the effects of microgravity on seated height was a necessity to provide an optimal level of clearance for all
2. Materials and methods 2.1. Subjects A total of 32 crewmembers consented to participate in the experi ment, including backup crewmembers that did not fly during the experiment. Data were collected from 29 of the 32 consented crew members, 3 females and 26 males with a mean age of 48 (�4 years), during the final 6 Shuttle flights (2009–2011). The crewmembers were measured between flight day 5 and flight day 152. The NASA Johnson Space Center’s (JSC) Institutional Review Board approved the method ology of the study and all participating crewmembers provided written consent. All participating crewmembers were measured for seated height once during the Shuttle mission, except for the International Space Station (ISS) crewmembers. Three ISS crewmembers were measured once during multiple Shuttle missions. In-flight measurements took place towards the end of the Shuttle mission, about a 10–14 day missions for each crewmember; during which seated height measurements were collected from 8 ISS (2 female, 6 male) and 21 Shuttle (1 female, 20 male) crewmembers. A subset of crewmembers also collected stature (i. e., standing height) as an optional task. This portion of the experiment was performed by 23 crewmembers including 7 ISS (2 female, 5 males) and 16 Shuttle (1 female, 15 males). Due to flight schedules and time constraints, the remaining 6 crewmembers did not perform the optional 2
K.S. Young and S. Rajulu
Applied Ergonomics 83 (2020) 102995
task of measuring their stature.
after informed consent had been given, between 180 days prior to launch, and postflight measurements were collected at approximately within 20 days after landing. Time of day for ground measurements varied due to training schedules; morning was requested for consistency and to remove confounding effect of gravity loading throughout the day, but it was not always obtainable. In-flight measurements were taken at a predefined time during flight for each crewmember, predominantly early in the morning and not within 1 h of exercising to avoid any spinal compression from the devices. A minimum of one in-flight session per crewmember was required. For each set of measurements, the crewmember was restrained in the Shuttle commander seat while another crewmember assisted and measured the distance from the top of the seatback to the top of the subject’s head using the modified anthropometer. The subject was instructed to sit erect, hip and knees at 90� , and feet on deck of the Shuttle looking directly ahead (head in Frankfort plane). Following the measurement, the operator was instructed to take a photograph using the camera mounted in the predefined position aiming sideways, orthogonal to the sagittal plane (Fig. 4). The data collection occurred as late as possible during the Shuttle mission to ensure that maximal spinal elongation had occurred. Each subject performed three measurement sessions: preflight, inflight, and postflight. In each measurement session, two anthrop ometer measurements and two photographed measurements were taken while restrained in the seat. After the first anthropometer measurement was recorded a digital picture immediately followed it, and then the subject exited the seat and was reseated. The procedure was repeated to collect a second anthropometer measurement and digital photograph. Stature was measured while the subject stood against a flat surface (Shuttle mid-deck lockers) and their height was marked on a piece of paper, similar to a child’s growth chart on a wall. Stature was measured preflight, postflight, and in-flight when time permitted.
2.2. Equipment The seated height was measured with a subject flexed 90� at the hips and flexed 90� at the knees. In microgravity, it is difficult to achieve this posture due to instability and lack of tactile feedback to ensure contact with the seat pan. As the ISS lacks any seat-like structures, the com mander seat in the Shuttle was used, as this allows for the proper posture for the standard seated height measurement. However, because of the retiring of the Shuttle fleet, the measurements were limited to ShuttleISS docked operations. The optimal restraint method was determined by performing simu lated microgravity flights prior to the deployment in the Shuttle and ISS. (Young et al., 2010). Three flights were performed to evaluate several different restraint methods. Seated height and pressure on the seat pan were measured while the subject was seated during the microgravity portions of the flight. The final restraint method was determined, which ensured the subjects were in contact with the seat pan and not ‘floating’ off the seat. The selected method involved the Shuttle seat restraints rerouted around the joint connecting the seat pan to the seat back. This method altered the direction and location of the anchor point, to pull down and back along the pelvis instead of along the upper thighs. This method was incorporated into the experiment procedures during in-flight and ground-based measurements of the crewmembers for consistency (Fig. 2). Seated height was measured with a custom modified instrument based on the GPM SERITEX anthropometer (Zurich, Switzerland; Fig. 3) with a measurement precision of 0.1 cm. A custom base was designed to mount the anthropometer beam to the seatback where the headrest at taches. The modified anthropometer was tested in both the laboratory and Shuttle cockpit trainer for accuracy and repeatability for use in the Shuttle seat configuration. The in-flight procedure involved the collec tion of manual measurements as well as pictures taken with a 12.4 megapixel digital camera (Nikon D2XS) mounted in a prescribed stan dard position (Fig. 4). The photographs were used to verify that the setup and posture of the crewmember during the data collection of the measurement was correct, as well as provide a backup reference to verify the actual measurement, if required. Specifically, measurements were approximated by scaling of objects of known sizes in photographs of seated crewmembers.
3. Results 3.1. Seated height elongation Preflight seated height ranged from 85.2 cm to 99.4 cm (mean: 91.9 cm, SD: 3.6 cm). Change in seated height was observed from inflight compared to preflight measurements for all participating crew members. When compared to preflight measurements, the seated height in-flight was observed to be larger by a range from 0.7 cm to 6.7 cm (mean: 3.8 cm SD: 1.3 cm) (Fig. 5). The differences were statistically significant based on a paired t-test (t ¼ 17.08664; P < 0.05). The inflight change in seated height corresponds to 1%–7% of the preflight measurements (mean: 4%, SD: 1%), and the 95th percentile change corresponds to 6% (i.e., 95% of the observations showed seated height
2.3. Procedures Seated height was measured during three stages of a mission, pre, in-, and postflight. Pre and postflight measurements were collected at the Shuttle trainers at JSC. Preflight measurements were collected any time
Fig. 2. A) Restraint method selected for in-flight data collection B) Nominal routing. 3
K.S. Young and S. Rajulu
Applied Ergonomics 83 (2020) 102995
Fig. 3. Graphical representation and actual modified anthropometer.
Fig. 4. Graphical depiction of in-flight data collection.
change of 6% or less; Fig. 6). The postflight measurements of seated height ranged from 84.9 cm to 100.5 cm (mean: 92.0 cm, SD: 3.8 cm). Compared to the preflight measurements, the difference in seated height corresponds to a range between 2.1 and 2.4 cm (mean: 0.1 cm, SD: 1.0 cm) A paired t-test indicated that seated heights measured in the postflight session were not significantly different than preflight seated height measurements (t ¼ 0.6515; P > 0.05, Fig. 5).
The precision error of measurement was estimated using the tech nical error of measurement (TEM). The TEM is defined as the standard deviation of differences between repeated measures. The TEM was calculated to be 0.31 cm (preflight), 0.02 cm (inflight) and 0.03 cm (postflight), respectively. The values correspond to 0.34%, 0.05% and 0.06% of the mean seated height. The calculated TEMs are overall smaller or equivalent to the past studies on body length measurements (Ulijaszek and Kerr, 1999; Conkle et al., 2017; Carsley et al., 2019). Also, 4
K.S. Young and S. Rajulu
Applied Ergonomics 83 (2020) 102995
Fig. 5. Seated height measurement per session.
Fig. 7. Stature measurements per session.
Fig. 6. Seated height distribution.
Fig. 8. Stature distribution.
the coefficient of reliability (the proportion of inter-subject variance not associated with the measurement error) was estimated as 0.99 for all measurement sessions, which is above the acceptable level 0.95 (Uli jaszek and Kerr, 1999; Carsley et al., 2019).
between 3.5 cm and 3.5 cm (mean: 0.3 cm, SD: 1.6 cm), and the dif ferences are not statistically significant (t ¼ 1.8318; P > 0.05). The precision error of measurement was not estimated, as stature was measured only once per subject without replications.
3.2. Stature elongation
3.3. Seated height vs stature elongation
While not all subjects collected stature data in-flight, those subjects without in-flight stature were removed from the stature analysis. The preflight standing height measurements ranged from 162.7 cm to 189.6 cm (mean: 178.1 cm, SD: 7.4 cm). Compared to preflight, the inflight stature measurements are larger by a range from 1.5 cm to 6.1 cm (mean: 3.8 cm, SD: 1.2 cm, Fig. 7), and the differences are sta tistically significant (t ¼ 14.9934; P < 0.05). The change in standing height corresponds to 1%–3% increase (mean: 2.1% SD: 0.7%), with the 95th percentile change corresponding to 3% (Fig. 8). The postflight measurements of standing height ranged from 162.8 cm to 189.7 cm (mean: 178.4 cm, SD: 7.3 cm). Compared to the preflight measurements, the difference in stature corresponds to a range
The in-flight change (delta) in standing compared to seated height change (delta) is illustrated in Fig. 9. Overall, the correlation between the two delta measurements did not show a statistical significance, (Pearson’s correlation coefficient r ¼ 0.2815, P > 0.05). 4. Discussion This on-orbit study was the first formal experiment since the Skylab mission to collect anthropometric measurements in-orbit. Changes in seated height were successfully collected from 29 crewmembers. Even though only one measurement session occurred for subjects during each Shuttle mission, the data collected proved that significant height 5
K.S. Young and S. Rajulu
Applied Ergonomics 83 (2020) 102995
seats upon return to Earth orbit after a long-duration mission, such as going to Moon or Mars or beyond. This experiment also has provided potential information regarding suit sizing adjustments in space. An improper suit fit had been (anecdotally) shown to affect the crew member’s performance and comfort during Extra-Vehicular Activities (EVAs). Based on historical data, suit engineers currently provide a 2.54 cm (1.0 inch) increase allowance in a crewmember’s in-orbit suit versus 1g suit configurations. However, the results from this study have shown that crewmembers’ stature can increase anywhere between 1.5 cm and 6.1 cm (corresponding to 1%–3% of stature). The process used and the experience gained from this experiment will help future research onboard the ISS in which anthropometric measurements are collected, potentially based on the photographic method, anthropometer, or non-hardware methods. Future projects will also allow for evaluation of alternative measurement locations on ISS to improve the measurement technique. Future work should also consider more subjects and additional measurements, such as overall torso change in a standing posture, which will provide salient information for suit fit adjustments in space. Further, it would be valuable to determine the trend over time, with several data collection days throughout the Expedition, for several individuals rather than collecting only one measurement session per crewmember. Additional future work would be to gather information regarding spinal disk characteristics versus elongation. The results from this study confirmed height increase due to expo sure to microgravity, with a design recommendation of 6% of seated height for the seated microgravity factor. The data obtained from the experiment will be used to provide designers information regarding how microgravity affects anthropometric measurements and to update all related documents that have a reference to spinal elongation. Engineers will be able to use this data to design future vehicles, suits, tools, equipment, etc. that properly account for microgravity induced adjust ments to ensure that the crewmembers will have a sufficient and safe amount of seat clearance to ingress their seats after an extended expo sure to microgravity.
Fig. 9. Seated height delta versus stature delta.
changes do occur for both stature and seated height and occur across all crewmembers during their duration in microgravity. In addition, the results indicate that observed lengthening are reversed with exposure to normal gravity. This study found seated height changes ranged from 1% to 7%, with the 95th percentile corresponding to 6%. Based on the results, individual changes in seated height demonstrated large variations, due to a large number of possible factors that could potentially contribute to the change, such as age, microgravity experience, level of muscularity, bone structure, etc. Even the types and magnitudes of the body motions/ex ercise, such as low back hyperextension, can potentially change the body height, as reported by a ground-based study (Magnusson and Pope, 1996). The data however demonstrated multiple crewmembers who consistently experienced around a 6% increase in seated height (5.1 cm–5.7 cm), while only one case (6.7 cm corresponding to 7%) showed an increase exceeding 6%. Given the measurement distribution, an allowance of 6% of seated height is regarded as a worst-case design consideration that would ensure that most crew would be able to ingress the seat after an extended exposure to microgravity. However, it should be noted that small sized subjects were not included in this study because of a lack of availability, which could potentially limit the applicability. The seated height measurements for subjects in this study (85.2–99.4 cm) do not completely encompass the allowed crew popu lation range for the Orion vehicle of 77.7 cm–101.3 cm (NASA, 2016). This study also found that the change observed in seated height in comparison to stature was only weakly correlated (r ¼ 0.247). There fore, it is hypothesized that the seated height increase must be treated as a separate entity from standing height spinal elongation, which justifies the necessity to maintain two separate design criteria for each mea surement. The 95th percentile stature change corresponds to 3% of stature, which was the maximum change experienced by multiple crewmembers. Thus 3% stature increase was identified as the stature microgravity factor for design consideration. The historical findings that have consistently reported the 3% change in stature also support this hypothesis. A limitation is that standing height was not measured from all participating crewmembers due to the crew time constraints. Based on the results, individual changes would be difficult to predict due to the large number of possible factors that could contribute to the change, such as age, microgravity experience, launch vehicle, mission length, day of data collection, bone/muscle structure, etc. Therefore, the worse-case or value that would encompass the vast majority of the subject and potential future subjects was determined from the data. The results have shown the importance of conducting human factors experiments in space to expand our current knowledge about micro gravity effects on human physical parameters. Without adequate data on microgravity-induced body size changes, the next crew exploration vehicle, such as Orion with one crewmember stacked above the other, could have created potential problems for crewmembers ingressing their
Funding This work was supported by the National Aeronautics and Space Administration (NASA) Johnson Space Center and partially funded by Human Health and Performance Contract, NNJ15HK11B. Declaration of competing interest No potential conflict of interest was reported by the authors. Acknowledgement We thank K. Han Kim at Leidos for the technical assistance to the manuscript. References Brown, J., 1975. ASTP002: Skylab 4 and ASTP crew height. Retrieved December 28, 2007, from NASA Life Sciences Data Archive Web site. http://lsda.jsc.nasa.gov. Brown, J., 1977. Crew height measurement. In: Nicogossian, A. (Ed.), The Apollo-Soyuz Test Project Medical Report. NASA, Washington, DC, pp. 119–121. Carsley, S., Parkin, P.C., Tu, K., Pullenayegum, E., Persaud, N., Maguire, J.L., 2019. Reliability of routinely collected anthropometric measurements in primary care. BMC Med. Res. Methodol. 19 (1), 84. Conkle, J., Ramakrishnan, U., Flores-Ayala, R., Suchdev, P.S., Martorell, R., 2017. Improving the quality of child anthropometry: manual anthropometry in the body imaging for nutritional assessment study (BINA). PLoS One 12 (12), e0189332. Hutchinson, K., Watenpaugh, D., Murthy, G., Convertino, V., Hargens, A., 1995. Back pain during 6 degree head-down tilt approximates that during actual microgravity. Aviat. Space Environ. Med. 66, 256–259. Magnusson, M., Pope, M.H., 1996. Body height changes with hyperextension. Clin. Biomech. 11 (4), 236–238.
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Applied Ergonomics 83 (2020) 102995
Margerum, S., Rajulu, S., 2008. Human Factors Analysis of Crew Height and Crew Weight Limitations in Space Vehicle Design. Human Factors and Ergonomics Society (HFES), New York City, New York. NASA, 1985. Man-Systems Integration Standards, Johnson Space Center, NASA-STD3000 (Rev B). Houston, Texas. NASA, 2016. Orion Multi-Purpose Crew Vehicle (MPCV) Program: Human-Systems Integration Requirements (HSIR). MPCV 70024 Rev C., Houston, TX. Pepper, L., Wing, P., Tsang, I., Gagnon, F., Susak, L., 1994. DSO 483: Back Pain Pattern in Microgravity. DSO Status Report, September 1994. Styf, J., Ballard, R., Fechner, K., Watenpaugh, D., Kahan, N., Hargens, A., 1997. Height increase, neuromuscular function, and back pain during 6 degree head-down tilt with traction. Aviat. Space Environ. Med. 68 (1), 24–29. Thornton, W., Hoffler, G., Rummel, J., 1977. Anthropometric changes and fluid shifts. In: Johnston, R., Dietlin, L. (Eds.), Biomedical Results from Skylab. NASA, Washington, DC, pp. 330–338.
Thornton, W., Moore, T., 1987. Height changes in microgravity. In: Results of the Life Sciences DSOs Conducted Aboard the Space Shuttle 1981-1986. Biomedical Research Institute, Houston, TX, pp. 55–57. NASA. Tyrrell, A.R., et al., 1985. Circadian variation in stature and the effects of spinal loading. Spine 10 (2), 161–164. Ulijaszek, S.J., Kerr, D.A., 1999. Anthropometric measurement error and the assessment of nutritional status. Br. J. Nutr. 82 (3), 165–177. Webb Associates, 1978. Anthropometric Source Book, vol. I. Anthropometry for Designers, Yellow Springs, Ohio. Young, K., Mesloh, M., Rajulu, S., 2010. Development of methodology to gather seated anthropometry data in a microgravity environment (simulated microgravity flight report). In: Conference Proceedings at the Applied Human Factors and Ergonomics (AHFE 2010), Miami, Florida.
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Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: http://www.elsevier.com/locate/apergo
Changes in seated height in microgravity Karen S. Young a, *, Sudhakar Rajulu b a b
Leidos, Inc, United States National Aeronautics and Space Administration, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Anthropometry Astronaut Spinal elongation NASA
Many physiological factors, such as spinal elongation, bone atrophy, and muscle loss, occur when humans are exposed to a microgravity environment. These physiological changes can result in slight to drastic changes in body dimensions. Any drastic change in body dimensions is critical information for current and future space hardware designers. These changes can affect accommodation, safety, and performance of a crewmember while in space. This study measured the overall change in seated height and stature for crewmembers exposed to a microgravity environment. Seated height data were obtained from 29 crewmembers that included 8 Interna tional Space Station increment crew (2 females and 6 males) and 21 Shuttle crew (1 female and 20 males). The results indicate that all participating crewmembers experienced statistically significant change in seated height. The corresponding change, 6% from preflight, should be considered for vehicle designs as the necessary seated microgravity adjustment.
1. Introduction Spinal elongation is an important factor to consider when designing for microgravity conditions and can affect safety, performance, and accommodation of a crewmember while in space. Spinal elongation occurs due to the lack of gravity/compression on the spinal column. This can potentially cause a straightening of the natural spinal curve or cause fluid to shift in the inter-vertebral disks that most likely result in changes in height. These changes are known to be the primary mechanism for changes in height in microgravity. The adapted morphology of the spine in microgravity has been also associated with the health issues such as persistent back pain, which can ultimately interfere with crew safety and performance, i.e. don/doff of the spacesuit and performance during Extravehicular Activity (EVA). However, due to the difficulty of mea surements during microgravity exposure, only a limited number of studies have been performed in the past to systematically quantify height and other anthropometric changes in microgravity. Previous experiments have taken place during the Apollo era and during bed-rest studies which have acted as an analogue to space flight. Spinal elongation and the associated changes, were previously investigated during Apollo-Soyuz Test Project (ASTP), Skylab, and bed rest studies (Webb Associates, 1978; Brown, 1975, 1977). In the ASTP studies (1975), which consisted of one 9-day flight, stature was measured periodically throughout the mission with the crewmembers’
feet against the docking module hatch and bodies supine on the control and display panel, with the observer using a checklist as a square edge to read from a measurement decal taped to the control and display panel. It was observed that in as little as 2 days a typical crewmember can exhibit increases in stature of up to 3%. After the first few days, stature remained at a steady state (Web Associates, 1978; Brown, 1975, 1977; Hutchinson et al., 1995; NASA, 1985; Pepper et al., 1994; Styf et al., 1997; Thornton et al., 1977, Thornton and Moore, 1987). The initial changes in stature were attributed to the immediate loss of spinal cur vature, with additional changes after Day 6 attributed to expansion of intervertebral discs. This data, however, was collected for only a limited number of crewmembers in standing postures, seated heights were not measured. Another study performed during Skylab 4 (1973) examined changes in stature for three crewmembers (Thornton et al., 1977; Thornton and Moore, 1987). Measurements were taken with crewmembers barefoot, standing fully erect with their back against a wall and applying a downward pressure with their hands to maintain solid contact with the floor. A mark was made on the wall at the top of the subject’s head (Fig. 1). The study indicated a quick increase in stature during the first day of weightlessness, after which stature remained steady at 4–6 cm greater than the original measurement (Thornton et al., 1977). Based on analysis of the stature changes during the Skylab and ASTP studies, a ‘growth’ of approximately 3% of stature was identified, attained over
* Corresponding author. 2101 NASA Parkway, Houston, TX, 77058. E-mail address: [email protected] (K.S. Young). https://doi.org/10.1016/j.apergo.2019.102995 Received 19 June 2019; Received in revised form 1 November 2019; Accepted 3 November 2019 Available online 15 November 2019 0003-6870/© 2019 Elsevier Ltd. All rights reserved.
K.S. Young and S. Rajulu
Applied Ergonomics 83 (2020) 102995
crewmembers. Furthermore, if spinal elongation is not included in the design of the spaceflight hardware, potential adverse impacts to per formance, comfort, and safety can occur; for example, crewmembers may not be able to safely ingress/egress the seat or don/doff the spacesuit potentially causing injury or the inability to perform their task (s). The amount of clearance adjustments needed between the seats could also affect crew complements and crew accommodation; for instance, the Orion module had a height restriction in place such that when a crewmember of 99th percentile male seated height occupied the top seat, the seated height of the occupant of the bottom seat had to be less than 99th percentile male seated height when the effect of spinal elongation was incorporated (Margerum and Rajulu, 2008). In contrast, if spinal elongation effect were nonexistent, a crewmember of any seated height could occupy the bottom seat underneath a 99th percentile male crewmember in the top seat due to the additional 6.6 cm (2.6 inch) of clearance gained. Another problem encountered by astronauts in space is the ensuing fit issue in space suits as a result of the microgravity induced spinal elongation. Some crewmembers have experienced trouble donning suits after extended periods in microgravity. Based on this, an arbitrary adjustment of 2.54 cm (1 inch) is currently added to suit torso length as a sizing adjustment to account for microgravity effects (Thornton et al., 1977). Suit accommodation needs to take spinal elongation into account to optimize individual performance, fit, and comfort of the crewmember in the suit. Otherwise, crewmembers may not fit well in the suit in microgravity and this may lead to decreased performance and potential safety or injury concerns. Given the necessity for accurately quantifying the effects of spinal elongation, the present study was primarily aimed at measuring seated height and stature from subjects exposed to a microgravity environment and providing the new information to designers via updated re quirements. Due to the payload imperatives or geometric constraints, the margin for clearance or adjustability is often restricted in the spaceflight vehicle or spacesuit. Thus, the design optimization is a major issue and the accurate assessments of the body shape changes in crew members are crucial. Data gained from this study would provide valu able information to future vehicle designs including MPCV regarding seat layout and crew accommodation. The implications on suit design, lunar vehicle design, and crew selection are important as well.
Fig. 1. Sketch of in-flight height determination (Skylab 4).
the first 2 days of weightlessness (Thornton and Moore, 1987). The 3% microgravity-induced factor has been adopted in NASA Man System Integration Standards (NASA, 1985) and Human Systems Integration Requirements (NASA, 2016). Bed rest studies were often performed to simulate the effects of microgravity on the body. This is accomplished by offloading the body vertically which removes the vertical gravity component. The vertical offloading of the spine allows the spine to lengthen, similar to the known effect that a person is taller in the morning than in the evening. The observed “circadian rhythm” in height changes corresponded to 1.93 cm (1.1%) increase of the subjects’ stature (Tyrrell et al., 1985). Similarly, loading a seated subject’ shoulders with a 10 kg weight for 5 min induced a stature decrease by 0.63 cm on average (Magnusson and Pope, 1996). A ground-based bed rest study (1995) observed 8 subjects (182.6 � 5.5 cm [mean � SD]) exposed to a 6-degree head-down tilt for 16 days to simulate the effects of spaceflight. The study found stature changes of 2.1 � 0.5 cm by day 3 and remained consistent throughout the rest of the duration (Hutchinson et al., 1995). An additional bed rest study used a 6-degree head-down tilt as well as horizontal bed config urations for 6 subjects who experienced 1.2 � 0.1 cm of stature increases after 1 day of bed rest and 2.2 � 0.8 cm after 3 days (Styf et al., 1997). Spine length was recorded as well, measured from L5 to C7, with an increase of 2.5 � 0.8 cm after 3 days. It was suggested that spinal elon gation is primarily responsible for changes in stature, further indicating that 30% to 60% of the change is due to increases in intervertebral disc height while the remainder of the change is due to decreases in the curvature of the spine. However, while the previous studies have focused on height mea surements in a standing posture, seated posture, which is hypothesized to be directly influenced by spinal elongation, has not been addressed. Seated postures may exhibit different characteristics than standing up right because of the changes in body posture, fluid shifts, and muscle loss. Seated height was deemed as a critical measurement for the seat layout in the design of the Orion Multi-Purpose Crew Vehicle (MPCV) due to the layout of the seats in a capsule-like vehicle. Specifically, four crewmembers were to be stacked two abreast with the commander and pilot seats on the top (fore) and the two remaining seats underneath (aft), limiting the amount of seated height clearance for the crew members seated in the aft seats. The interior wall dimensions of these vehicles were fixed because of other design constraints. Therefore an accurate quantification of the effects of microgravity on seated height was a necessity to provide an optimal level of clearance for all
2. Materials and methods 2.1. Subjects A total of 32 crewmembers consented to participate in the experi ment, including backup crewmembers that did not fly during the experiment. Data were collected from 29 of the 32 consented crew members, 3 females and 26 males with a mean age of 48 (�4 years), during the final 6 Shuttle flights (2009–2011). The crewmembers were measured between flight day 5 and flight day 152. The NASA Johnson Space Center’s (JSC) Institutional Review Board approved the method ology of the study and all participating crewmembers provided written consent. All participating crewmembers were measured for seated height once during the Shuttle mission, except for the International Space Station (ISS) crewmembers. Three ISS crewmembers were measured once during multiple Shuttle missions. In-flight measurements took place towards the end of the Shuttle mission, about a 10–14 day missions for each crewmember; during which seated height measurements were collected from 8 ISS (2 female, 6 male) and 21 Shuttle (1 female, 20 male) crewmembers. A subset of crewmembers also collected stature (i. e., standing height) as an optional task. This portion of the experiment was performed by 23 crewmembers including 7 ISS (2 female, 5 males) and 16 Shuttle (1 female, 15 males). Due to flight schedules and time constraints, the remaining 6 crewmembers did not perform the optional 2
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task of measuring their stature.
after informed consent had been given, between 180 days prior to launch, and postflight measurements were collected at approximately within 20 days after landing. Time of day for ground measurements varied due to training schedules; morning was requested for consistency and to remove confounding effect of gravity loading throughout the day, but it was not always obtainable. In-flight measurements were taken at a predefined time during flight for each crewmember, predominantly early in the morning and not within 1 h of exercising to avoid any spinal compression from the devices. A minimum of one in-flight session per crewmember was required. For each set of measurements, the crewmember was restrained in the Shuttle commander seat while another crewmember assisted and measured the distance from the top of the seatback to the top of the subject’s head using the modified anthropometer. The subject was instructed to sit erect, hip and knees at 90� , and feet on deck of the Shuttle looking directly ahead (head in Frankfort plane). Following the measurement, the operator was instructed to take a photograph using the camera mounted in the predefined position aiming sideways, orthogonal to the sagittal plane (Fig. 4). The data collection occurred as late as possible during the Shuttle mission to ensure that maximal spinal elongation had occurred. Each subject performed three measurement sessions: preflight, inflight, and postflight. In each measurement session, two anthrop ometer measurements and two photographed measurements were taken while restrained in the seat. After the first anthropometer measurement was recorded a digital picture immediately followed it, and then the subject exited the seat and was reseated. The procedure was repeated to collect a second anthropometer measurement and digital photograph. Stature was measured while the subject stood against a flat surface (Shuttle mid-deck lockers) and their height was marked on a piece of paper, similar to a child’s growth chart on a wall. Stature was measured preflight, postflight, and in-flight when time permitted.
2.2. Equipment The seated height was measured with a subject flexed 90� at the hips and flexed 90� at the knees. In microgravity, it is difficult to achieve this posture due to instability and lack of tactile feedback to ensure contact with the seat pan. As the ISS lacks any seat-like structures, the com mander seat in the Shuttle was used, as this allows for the proper posture for the standard seated height measurement. However, because of the retiring of the Shuttle fleet, the measurements were limited to ShuttleISS docked operations. The optimal restraint method was determined by performing simu lated microgravity flights prior to the deployment in the Shuttle and ISS. (Young et al., 2010). Three flights were performed to evaluate several different restraint methods. Seated height and pressure on the seat pan were measured while the subject was seated during the microgravity portions of the flight. The final restraint method was determined, which ensured the subjects were in contact with the seat pan and not ‘floating’ off the seat. The selected method involved the Shuttle seat restraints rerouted around the joint connecting the seat pan to the seat back. This method altered the direction and location of the anchor point, to pull down and back along the pelvis instead of along the upper thighs. This method was incorporated into the experiment procedures during in-flight and ground-based measurements of the crewmembers for consistency (Fig. 2). Seated height was measured with a custom modified instrument based on the GPM SERITEX anthropometer (Zurich, Switzerland; Fig. 3) with a measurement precision of 0.1 cm. A custom base was designed to mount the anthropometer beam to the seatback where the headrest at taches. The modified anthropometer was tested in both the laboratory and Shuttle cockpit trainer for accuracy and repeatability for use in the Shuttle seat configuration. The in-flight procedure involved the collec tion of manual measurements as well as pictures taken with a 12.4 megapixel digital camera (Nikon D2XS) mounted in a prescribed stan dard position (Fig. 4). The photographs were used to verify that the setup and posture of the crewmember during the data collection of the measurement was correct, as well as provide a backup reference to verify the actual measurement, if required. Specifically, measurements were approximated by scaling of objects of known sizes in photographs of seated crewmembers.
3. Results 3.1. Seated height elongation Preflight seated height ranged from 85.2 cm to 99.4 cm (mean: 91.9 cm, SD: 3.6 cm). Change in seated height was observed from inflight compared to preflight measurements for all participating crew members. When compared to preflight measurements, the seated height in-flight was observed to be larger by a range from 0.7 cm to 6.7 cm (mean: 3.8 cm SD: 1.3 cm) (Fig. 5). The differences were statistically significant based on a paired t-test (t ¼ 17.08664; P < 0.05). The inflight change in seated height corresponds to 1%–7% of the preflight measurements (mean: 4%, SD: 1%), and the 95th percentile change corresponds to 6% (i.e., 95% of the observations showed seated height
2.3. Procedures Seated height was measured during three stages of a mission, pre, in-, and postflight. Pre and postflight measurements were collected at the Shuttle trainers at JSC. Preflight measurements were collected any time
Fig. 2. A) Restraint method selected for in-flight data collection B) Nominal routing. 3
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Fig. 3. Graphical representation and actual modified anthropometer.
Fig. 4. Graphical depiction of in-flight data collection.
change of 6% or less; Fig. 6). The postflight measurements of seated height ranged from 84.9 cm to 100.5 cm (mean: 92.0 cm, SD: 3.8 cm). Compared to the preflight measurements, the difference in seated height corresponds to a range between 2.1 and 2.4 cm (mean: 0.1 cm, SD: 1.0 cm) A paired t-test indicated that seated heights measured in the postflight session were not significantly different than preflight seated height measurements (t ¼ 0.6515; P > 0.05, Fig. 5).
The precision error of measurement was estimated using the tech nical error of measurement (TEM). The TEM is defined as the standard deviation of differences between repeated measures. The TEM was calculated to be 0.31 cm (preflight), 0.02 cm (inflight) and 0.03 cm (postflight), respectively. The values correspond to 0.34%, 0.05% and 0.06% of the mean seated height. The calculated TEMs are overall smaller or equivalent to the past studies on body length measurements (Ulijaszek and Kerr, 1999; Conkle et al., 2017; Carsley et al., 2019). Also, 4
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Fig. 5. Seated height measurement per session.
Fig. 7. Stature measurements per session.
Fig. 6. Seated height distribution.
Fig. 8. Stature distribution.
the coefficient of reliability (the proportion of inter-subject variance not associated with the measurement error) was estimated as 0.99 for all measurement sessions, which is above the acceptable level 0.95 (Uli jaszek and Kerr, 1999; Carsley et al., 2019).
between 3.5 cm and 3.5 cm (mean: 0.3 cm, SD: 1.6 cm), and the dif ferences are not statistically significant (t ¼ 1.8318; P > 0.05). The precision error of measurement was not estimated, as stature was measured only once per subject without replications.
3.2. Stature elongation
3.3. Seated height vs stature elongation
While not all subjects collected stature data in-flight, those subjects without in-flight stature were removed from the stature analysis. The preflight standing height measurements ranged from 162.7 cm to 189.6 cm (mean: 178.1 cm, SD: 7.4 cm). Compared to preflight, the inflight stature measurements are larger by a range from 1.5 cm to 6.1 cm (mean: 3.8 cm, SD: 1.2 cm, Fig. 7), and the differences are sta tistically significant (t ¼ 14.9934; P < 0.05). The change in standing height corresponds to 1%–3% increase (mean: 2.1% SD: 0.7%), with the 95th percentile change corresponding to 3% (Fig. 8). The postflight measurements of standing height ranged from 162.8 cm to 189.7 cm (mean: 178.4 cm, SD: 7.3 cm). Compared to the preflight measurements, the difference in stature corresponds to a range
The in-flight change (delta) in standing compared to seated height change (delta) is illustrated in Fig. 9. Overall, the correlation between the two delta measurements did not show a statistical significance, (Pearson’s correlation coefficient r ¼ 0.2815, P > 0.05). 4. Discussion This on-orbit study was the first formal experiment since the Skylab mission to collect anthropometric measurements in-orbit. Changes in seated height were successfully collected from 29 crewmembers. Even though only one measurement session occurred for subjects during each Shuttle mission, the data collected proved that significant height 5
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seats upon return to Earth orbit after a long-duration mission, such as going to Moon or Mars or beyond. This experiment also has provided potential information regarding suit sizing adjustments in space. An improper suit fit had been (anecdotally) shown to affect the crew member’s performance and comfort during Extra-Vehicular Activities (EVAs). Based on historical data, suit engineers currently provide a 2.54 cm (1.0 inch) increase allowance in a crewmember’s in-orbit suit versus 1g suit configurations. However, the results from this study have shown that crewmembers’ stature can increase anywhere between 1.5 cm and 6.1 cm (corresponding to 1%–3% of stature). The process used and the experience gained from this experiment will help future research onboard the ISS in which anthropometric measurements are collected, potentially based on the photographic method, anthropometer, or non-hardware methods. Future projects will also allow for evaluation of alternative measurement locations on ISS to improve the measurement technique. Future work should also consider more subjects and additional measurements, such as overall torso change in a standing posture, which will provide salient information for suit fit adjustments in space. Further, it would be valuable to determine the trend over time, with several data collection days throughout the Expedition, for several individuals rather than collecting only one measurement session per crewmember. Additional future work would be to gather information regarding spinal disk characteristics versus elongation. The results from this study confirmed height increase due to expo sure to microgravity, with a design recommendation of 6% of seated height for the seated microgravity factor. The data obtained from the experiment will be used to provide designers information regarding how microgravity affects anthropometric measurements and to update all related documents that have a reference to spinal elongation. Engineers will be able to use this data to design future vehicles, suits, tools, equipment, etc. that properly account for microgravity induced adjust ments to ensure that the crewmembers will have a sufficient and safe amount of seat clearance to ingress their seats after an extended expo sure to microgravity.
Fig. 9. Seated height delta versus stature delta.
changes do occur for both stature and seated height and occur across all crewmembers during their duration in microgravity. In addition, the results indicate that observed lengthening are reversed with exposure to normal gravity. This study found seated height changes ranged from 1% to 7%, with the 95th percentile corresponding to 6%. Based on the results, individual changes in seated height demonstrated large variations, due to a large number of possible factors that could potentially contribute to the change, such as age, microgravity experience, level of muscularity, bone structure, etc. Even the types and magnitudes of the body motions/ex ercise, such as low back hyperextension, can potentially change the body height, as reported by a ground-based study (Magnusson and Pope, 1996). The data however demonstrated multiple crewmembers who consistently experienced around a 6% increase in seated height (5.1 cm–5.7 cm), while only one case (6.7 cm corresponding to 7%) showed an increase exceeding 6%. Given the measurement distribution, an allowance of 6% of seated height is regarded as a worst-case design consideration that would ensure that most crew would be able to ingress the seat after an extended exposure to microgravity. However, it should be noted that small sized subjects were not included in this study because of a lack of availability, which could potentially limit the applicability. The seated height measurements for subjects in this study (85.2–99.4 cm) do not completely encompass the allowed crew popu lation range for the Orion vehicle of 77.7 cm–101.3 cm (NASA, 2016). This study also found that the change observed in seated height in comparison to stature was only weakly correlated (r ¼ 0.247). There fore, it is hypothesized that the seated height increase must be treated as a separate entity from standing height spinal elongation, which justifies the necessity to maintain two separate design criteria for each mea surement. The 95th percentile stature change corresponds to 3% of stature, which was the maximum change experienced by multiple crewmembers. Thus 3% stature increase was identified as the stature microgravity factor for design consideration. The historical findings that have consistently reported the 3% change in stature also support this hypothesis. A limitation is that standing height was not measured from all participating crewmembers due to the crew time constraints. Based on the results, individual changes would be difficult to predict due to the large number of possible factors that could contribute to the change, such as age, microgravity experience, launch vehicle, mission length, day of data collection, bone/muscle structure, etc. Therefore, the worse-case or value that would encompass the vast majority of the subject and potential future subjects was determined from the data. The results have shown the importance of conducting human factors experiments in space to expand our current knowledge about micro gravity effects on human physical parameters. Without adequate data on microgravity-induced body size changes, the next crew exploration vehicle, such as Orion with one crewmember stacked above the other, could have created potential problems for crewmembers ingressing their
Funding This work was supported by the National Aeronautics and Space Administration (NASA) Johnson Space Center and partially funded by Human Health and Performance Contract, NNJ15HK11B. Declaration of competing interest No potential conflict of interest was reported by the authors. Acknowledgement We thank K. Han Kim at Leidos for the technical assistance to the manuscript. References Brown, J., 1975. ASTP002: Skylab 4 and ASTP crew height. Retrieved December 28, 2007, from NASA Life Sciences Data Archive Web site. http://lsda.jsc.nasa.gov. Brown, J., 1977. Crew height measurement. In: Nicogossian, A. (Ed.), The Apollo-Soyuz Test Project Medical Report. NASA, Washington, DC, pp. 119–121. Carsley, S., Parkin, P.C., Tu, K., Pullenayegum, E., Persaud, N., Maguire, J.L., 2019. Reliability of routinely collected anthropometric measurements in primary care. BMC Med. Res. Methodol. 19 (1), 84. Conkle, J., Ramakrishnan, U., Flores-Ayala, R., Suchdev, P.S., Martorell, R., 2017. Improving the quality of child anthropometry: manual anthropometry in the body imaging for nutritional assessment study (BINA). PLoS One 12 (12), e0189332. Hutchinson, K., Watenpaugh, D., Murthy, G., Convertino, V., Hargens, A., 1995. Back pain during 6 degree head-down tilt approximates that during actual microgravity. Aviat. Space Environ. Med. 66, 256–259. Magnusson, M., Pope, M.H., 1996. Body height changes with hyperextension. Clin. Biomech. 11 (4), 236–238.
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Thornton, W., Moore, T., 1987. Height changes in microgravity. In: Results of the Life Sciences DSOs Conducted Aboard the Space Shuttle 1981-1986. Biomedical Research Institute, Houston, TX, pp. 55–57. NASA. Tyrrell, A.R., et al., 1985. Circadian variation in stature and the effects of spinal loading. Spine 10 (2), 161–164. Ulijaszek, S.J., Kerr, D.A., 1999. Anthropometric measurement error and the assessment of nutritional status. Br. J. Nutr. 82 (3), 165–177. Webb Associates, 1978. Anthropometric Source Book, vol. I. Anthropometry for Designers, Yellow Springs, Ohio. Young, K., Mesloh, M., Rajulu, S., 2010. Development of methodology to gather seated anthropometry data in a microgravity environment (simulated microgravity flight report). In: Conference Proceedings at the Applied Human Factors and Ergonomics (AHFE 2010), Miami, Florida.
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