Professor California Polytechnic State University, San Luis Obispo, United States
Introduction:: Physical pain and injury are significant risks for astronauts traveling beyond the Earth’s atmosphere. From launch to outer space, the human body is subjected to a variety of conditions unlike those on Earth. The spine is of particular concern and many astronauts have reported back issues during and after their flights aboard the International Space Station. Due to the lack of data and small sample size, several Earth-based simulations have been developed to assess spine changes associated with microgravity. These include horizontal bed rest, head-down tilt bed rest, hyper-buoyancy flotation, liquid immersion, dry immersion, animal limb unloading, and a rotary cell culture system. This study explores how accurately head-down tilt bed rest and dry immersion—two of the most well-regarded simulation methods—represent actual symptoms experienced in a microgravity environment during spaceflight.
Materials and Methods:: Launch vehicle ascent was based on the STS-121 liftoff to main engine cutoff [1] and compared to injury assessment reference values [2]. Microgravity-associated back changes were assessed in a literature review with respect to bodily location and mission phase. Relevant sources include a NASA study on 50 astronauts across 58 to 341-day missions [3]. Another study included 3T magnetic resonance imaging and dynamic fluoroscopy of 6 astronauts assessed 30 days pre-flight, 1 day post-flight, and 1 year post-flight [4]. Finally, ultrasound scans of 7 astronauts during flight were compared to caliper measurements, MRI, and ultrasound scans before and after flight [5]. Bed rest methods were analyzed from various sources in the context of a 6-degree head-down tilt [6, 7, 8] up to 1370 days [9]. Days to weeks of dry immersion in a semi-recumbent posture was also assessed [6, 10]. All data were taken from previous experiments, studies, and spaceflight missions, with analysis completed in MATLAB (The MathWorks, Inc., Natick, MA).
Results, Conclusions, and Discussions:: Both microgravity and head-down tilt bed rest were associated with disc expansion as well as muscle and bone atrophy [3, 9]. However, microgravity correlated with 5 cm body lengthening [10], 11% decreased spine curvature, and no increase in disc height [4], while bed rest led to 2.2 cm body lengthening [7], 3% decreased spine curvature [4, 11], and increased disc height [7] (Figure 1). Further variation presented in documented pain, with a lower proportion of reports during microgravity [12] than bed rest [8]. Dry immersion corresponded to muscle atrophy, 1.5 cm spine lengthening, and reported pain values between those in microgravity and bed rest [10] (Figure 1). A decreased spinal range of motion was also seen in both microgravity [4] and dry immersion [10]. In general, head-down tilt bed rest and dry immersion were associated with similar symptomatic trends as microgravity, though often at dissimilar magnitudes. This may be due to differences in gravitational acceleration forces, weight distribution, tissue pressure, and the fact that the simulation methods do not account for launch and descent. Although the maximum launch vehicle acceleration of 29.3 m/s2 [1] was found to be less than the injury threshold of 105 m/s2 [2] (Figure 2), the additional compressive forces before and after microgravity exposure cannot be ignored. Additional research into each method is recommended, especially with respect to range of motion in bed rest as well as spine curvature, disc swelling, disc height, and bone atrophy in dry immersion.
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References (Optional): : [1] NASA, 1-4, https://www.nasa.gov/pdf/466711main_AP_ST_ShuttleAscent.pdf. [2] Lalwala M et al., Ann Biomed Eng, 51:430–442, 2023. [3] Nusbaum D M et al., NASA, JSC-E-DAA-TN61165, 2018. [4] Bailey J F et al., Spine J, 18:7-14, 2018. [5] Garcia K M et al., J Ultrasound Med, 37:987–99, 2018. [6] Watenpaugh D E et al., J Appl Physiol, 120:904–914, 2016. [7] Styf J R et al., Aviat Space Environ Med, 68:24-9, 1997. [8] Hutchinson K J et al., Aviat Space Environ Med, 66:256-9, 1995. [9] Hargens A R et al., J Appl Physiol, 120:891–903, 2016. [10] Treffel L et al., Pain Res Manag, 2017:10, 2017. [11] Cleve Clin, 2022, https://my.clevelandclinic.org/health/diseases/23908-lordosis. [12] Green D A et al., Front Physiol, 8:1126, 2018.
