Pain and Vertebral Dysfunction in Dry Immersion: A Model of Microgravity Simulation Different from Bed Rest Studies

Lunar and mars gravity induce similar changes in spinal motor control as microgravity

Introduction: Once more, plans are underway to send humans to the Moon or possibly even to Mars. It is therefore, important to know potential physiological effects of a prolonged stay in space and to minimize possible health risks to astronauts. It has been shown that spinal motor control strategies change during microgravity induced by parabolic flight. The way in which spinal motor control strategies change during partial microgravity, such as that encountered on the Moon and on Mars, is not known. Methods: Spinal motor control measurements were performed during Earth, lunar, Mars, and micro-gravity conditions and two hypergravity conditions of a parabola. Three proxy measures of spinal motor control were recorded: spinal stiffness of lumbar L3 vertebra using the impulse response, muscle activity of lumbar flexors and extensors using surface electromyography, and lumbar curvature using two curvature distance sensors placed at the upper and lower lumbar spine. The participants were six females and six males, with a mean age of 33 years (standard deviation: 7 years). Results: Gravity condition had a statistically significant (Friedmann tests) effect spinal stiffness (p < 0.001); on EMG measures (multifidus (p = 0.047), transversus abdominis (p < 0.001), and psoas (p < 0.001) muscles) and on upper lumbar curvature sensor (p < 0.001). No effect was found on the erector spinae muscle (p = 0.063) or lower curvature sensor (p = 0.170). Post hoc tests revealed a significant increase in stiffness under micro-, lunar-, and Martian gravity conditions (all p's < 0.034). Spinal stiffness decreased under both hypergravity conditions (all p's ≤ 0.012) and decreased during the second hypergravity compared to the first hypergravity condition (p = 0.012). Discussion: Micro-, lunar-, and Martian gravity conditions resulted in similar increases in spinal stiffness, a decrease in transversus abdominis muscle activity, with no change in psoas muscle activity and thus modulation of spinal motor stabilization strategy compared to those observed under Earth's gravity. These findings suggest that the spine is highly sensitive to gravity transitions but that Lunar and Martian gravity are below that required for normal modulation of spinal motor stabilization strategy and thus may be associated with LBP and/or IVD risk without the definition of countermeasures.

Dry immersion induced acute low back pain and its relationship with trunk myofascial viscoelastic changes

Microgravity induces spinal elongation and Low Back Pain (LBP) but the pathophysiology is unknown. Changes in paraspinal muscle viscoelastic properties may play a role. Dry Immersion (DI) is a ground-based microgravity analogue that induces changes in m. erector spinae superficial myofascial tissue tone within 2 h. This study sought to determine whether bilateral m. erector spinae tone, creep, and stiffness persist beyond 2 h; and if such changes correlate with DI-induced spinal elongation and/or LBP. Ten healthy males lay in the DI bath at the Institute of Biomedical Problems (Moscow, Russia) for 6 h. Bilateral lumbar (L1, L4) and thoracic (T11, T9) trunk myofascial tone, stiffness and creep (MyotonPRO), and subjective LBP (0-10 NRS) were recorded before DI, after 1h, 6 h of DI, and 30min post. The non-standing spinal length was evaluated on the bath lifting platform using a bespoke stadiometer before and following DI. DI significantly modulated m. erector spinae viscoelastic properties at L4, L1, T11, and T9 with no effect of laterality. Bilateral tissue tone was significantly reduced after 1 and 6 h DI at L4, L1, T11, and T9 to a similar extent. Stiffness was also reduced by DI at 1 h but partially recovered at 6 h for L4, L1, and T11. Creep was increased by DI at 1 h, with partial recovery at 6 h, although only T11 was significant. All properties returned to baseline 30 min following DI. Significant spinal elongation (1.17 ± 0.20 cm) with mild (at 1 h) to moderate (at 6 h) LBP was induced, mainly in the upper lumbar and lower thoracic regions. Spinal length increases positively correlated (Rho = 0.847, p = 0.024) with middle thoracic (T9) tone reduction, but with no other stiffness or creep changes. Spinal length positively correlated (Rho = 0.557, p = 0.039) with Max LBP; LBP failed to correlate with any m. erector spinae measured parameters. The DI-induced bilateral m. erector spinae tone, creep, and stiffness changes persist beyond 2 h. Evidence of spinal elongation and LBP allows suggesting that the trunk myofascial tissue changes could play a role in LBP pathogenesis observed in real and simulated microgravity. Further study is warranted with longer duration DI, assessment of IVD geometry, and vertebral column stability.

Rückenschmerzen und erhöhtes Bandscheibenvorfallrisiko bei Astronauten während und nach Raumfahrtmissionen

Rückenschmerzen zu Beginn einer Raumfahrtmission sowie ein erhöhtes Risiko für Bandscheibenvorfälle (Diskusprolaps) nach der Rückkehr ist ein seit Langem bekanntes medizinisches Problem der bemannten Raumfahrt. Mit dem Bestreben, den Mond permanent zu besiedeln, wird der Erhalt der körperlichen Gesundheit in einer für den Menschen fremden Umgebung ein zentraler Faktor. Im Vergleich zu den Apollo-Flügen zum Mond in den 1970er-Jahren sollen die Aufenthalte auf dem Mond in Zukunft nicht nur ein paar Tage dauern, sondern Monate, was neue Gesundheitsrisiken mit sich bringt. Durch die Entfernung zur Erde und den dadurch eingeschränkten Zugang zu medizinischen Leistungen wird es ferner viel schwieriger oder gar unmöglich, bei Notfällen schnell einzugreifen. Deshalb sind neue Ideen zur Bewältigung der medizinischen Herausforderungen gefragt.

Spinal changes after 5-day dry immersion as shown by magnetic resonance imaging (DI-5-CUFFS)

Astronauts frequently report microgravity-induced back pain, which is generally more pronounced in the beginning of a spaceflight. The dry immersion (DI) model reproduces the early effects of microgravity in terms of global support unloading and fluidshift, both of which are involved in back pain pathogenesis. Here, we assessed spinal changes induced by exposure to 5 days of strict DI in 18 healthy men (25-43 years old) with (n = 9) or without (n = 9) thigh cuffs countermeasure. Intervertebral disc (IVD) height, spinal cord position, and apparent diffusion coefficient (ADC; reflecting global water motion) were measured using magnetic resonance imaging before and after DI. After DI, IVD height increased in thoracic (+3.3 ± 0.8 mm; C₇-T₁₂) and lumbar (+4.5 ± 0.4 mm; T₁₂-L₅) regions but not in the cervical region (C₂-C₇) of the spine. An increase in ADC after DI was observed at the L₁ (~6% increase, from 3.2 to 3.4 × 10^-3 mm²/s; p < 0.001) and L₂ (~3% increase, from 3.4 to 3.5 × 10^-3 mm²/s; p = 0.005) levels. There was no effect of thigh cuffs on spinal parameters. This change in IVD after DI follows the same "gradient" pattern of height increase from the cervical to the lumbar region as observed after bedrest and spaceflight. The increase in ADC at L₁ level positively correlated with reported back pain. These findings emphasize the utility of the DI model for studying early spinal changes observed in microgravity.

The First Female Dry Immersion (NAIAD-2020): Design and Specifics of a 3-Day Study

This article describes procedures and some results of the first study of females undergoing 3-day Dry Immersion. The experiment “NAIAD-2020” was carried out at the Institute of Biomedical Problems (Moscow, Russia) with the participation of six healthy women volunteers (age 30.17 ± 5.5 years, height 1.66 ± 0.1 m, weight 62.05 ± 8.4 kg, BMI 22.39 ± 2.2 kg/m²) with a natural menstrual cycle. During the study, a standard protocol was used, the same as for men, with a minimum period of time spent outside the immersion bath. Before, during and after Immersion, 22 experiments were carried out aimed at studying the neurophysiological, functional, metabolic and psychophysiological functions of the body, the results of which will be presented in future publications. The total time outside the bath for women did not exceed that for men. Systolic and diastolic pressure did not significantly change during the immersion. In the first 24 h after the end of the immersion, heart rate was significantly higher than the background values [F(4,20) = 14.67; P < 0.0001]. Changes in body temperature and water balance were consistent with the patterns found in men. No significant changes in height and weight were found during immersion. All women reported general discomfort and pain in the abdomen and back. The results of this study did not find significant risks to women’s health and showed the feasibility of using this model of the effects of space flight in women of reproductive age.

Ground-Based Analogs for Human Spaceflight

This mini-review provides an updated summary of various analogs for adaptations of humans to the microgravity of space. Microgravity analogs discussed in this paper include dry immersion, wet immersion, unilateral lower-extremity limb suspension, head down tilt (HDT), and supine bed rest. All Earth-based analogs are imperfect simulations of microgravity with their own advantages and disadvantages. This paper compares these five frequently used microgravity analogs to offer insights into their usefulness for various physiological systems. New developments for each human microgravity analog are explored and advantages of one analog are evaluated against other analogs. Furthermore, the newly observed risk of Spaceflight Associated Neuro-Ocular Syndrome (SANS) is included in this mini review with a discussion of the advantages and disadvantages of each method of simulation for the relatively new risk of SANS. Overall, the best and most integrated analog for Earth-based studies of the microgravity of space flight appears to be head-down tilt bed rest.

DI-5-Cuffs: Lumbar Intervertebral Disc Proteoglycan and Water Content Changes in Humans after Five Days of Dry Immersion to Simulate Microgravity

Most astronauts experience back pain after spaceflight, primarily located in the lumbar region. Intervertebral disc herniations have been observed after real and simulated microgravity. Spinal deconditioning after exposure to microgravity has been described, but the underlying mechanisms are not well understood. The dry immersion (DI) model of microgravity was used with eighteen male volunteers. Half of the participants wore thigh cuffs as a potential countermeasure. The spinal changes and intervertebral disc (IVD) content changes were investigated using magnetic resonance imaging (MRI) analyses with T1-T2 mapping sequences. IVD water content was estimated by the apparent diffusion coefficient (ADC), with proteoglycan content measured using MRI T1-mapping sequences centered in the nucleus pulposus. The use of thigh cuffs had no effect on any of the spinal variables measured. There was significant spinal lengthening for all of the subjects. The ADC and IVD proteoglycan content both increased significantly with DI (7.34 ± 2.23% and 10.09 ± 1.39%, respectively; mean ± standard deviation), p < 0.05). The ADC changes suggest dynamic and rapid water diffusion inside IVDs, linked to gravitational unloading. Further investigation is needed to determine whether similar changes occur in the cervical IVDs. A better understanding of the mechanisms involved in spinal deconditioning with spaceflight would assist in the development of alternative countermeasures to prevent IVD herniation.

Multi-System Adaptation to Confinement During the 180-Day Controlled Ecological Life Support System (CELSS) Experiment

Confinement experiments are essential to prepare long-term space exploration. The 180-day Chinese CELSS (Controlled Ecological Life Support System) study is unique in its design, including a closed-loop system and mid-mission simulation of Mars-like day-night cycle of 24 h 40 min for 36 days (days 72-108). Our aim was to study physiological and psychological consequences of this confinement in four healthy volunteers (one female). CELSS platform consisted of six interconnected modules including four greenhouses. Life support systems were controlled automatically. Body composition, fluid compartments, metabolic state, heart, large vessels, endothelial function, and muscle tone were studied using biological, functional, and/or morphological measurements. Behavioral activities were studied by ethological monitoring; psychological state was assessed by questionnaires. Body weight decreased by ∼2 kg mostly due to lean mass loss. Plasma volume and volume-regulating hormones were mostly stable. Carotid intima-media thickness (IMT) increased by 10-15%. Endothelium-dependent vasodilation decreased. Masseter tone increased by 6-14% suggesting stress, whereas paravertebral muscle tone diminished by 10 ± 6%. Behavioral flow reflecting global activity decreased 1.5- to 2-fold after the first month. Psychological questionnaires revealed decrease in hostility and negative emotions but increase in emotional adaptation suggesting boredom and monotony. One subject was clearly different with lower fitness, higher levels of stress and anxiety, and somatic signs as back pain, peak in masseter tone, increased blood cortisol and C-reactive protein. Comparison of CELSS experiment with Mars500 confinement program suggests the need for countermeasures to prevent increased IMT and endothelial deconditioning. Daily activity in greenhouse could act as countermeasure against psycho-physiological deconditioning.

Dry Immersion as a Ground-Based Model of Microgravity Physiological Effects

Dry immersion (DI) is one of the most widely used ground models of microgravity. DI accurately and rapidly reproduces most of physiological effects of short-term space flights. The model simulates such factors of space flight as lack of support, mechanical and axial unloading as well as physical inactivity. The current manuscript gathers the results of physiological studies performed from the time of the model's development. This review describes the changes induced by DI of different duration (from few hours to 56 days) in the neuromuscular, sensory-motor, cardiorespiratory, digestive and excretory, and immune systems, as well as in the metabolism and hemodynamics. DI reproduces practically the full spectrum of changes in the body systems during the exposure to microgravity. The numerous publications from Russian researchers, which until present were mostly inaccessible for scientists from other countries are summarized in this work. These data demonstrated and validated DI as a ground-based model for simulation of physiological effects of weightlessness. The magnitude and rate of physiological changes during DI makes this method advantageous as compared with other ground-based microgravity models. The actual and potential uses of the model are discussed in the context of fundamental studies and applications for Earth medicine.

Spinal Health during Unloading and Reloading Associated with Spaceflight

Spinal elongation and back pain are recognized effects of exposure to microgravity, however, spinal health has received relatively little attention. This changed with the report of an increased risk of post-flight intervertebral disc (IVD) herniation and subsequent identification of spinal pathophysiology in some astronauts post-flight. Ground-based analogs, particularly bed rest, suggest that a loss of spinal curvature and IVD swelling may be factors contributing to unloading-induced spinal elongation. In flight, trunk muscle atrophy, in particular multifidus, may precipitate lumbar curvature loss and reduced spinal stability, but in-flight (ultrasound) and pre- and post-flight (MRI) imaging have yet to detect significant IVD changes. Current International Space Station missions involve short periods of moderate-to-high spinal (axial) loading during running and resistance exercise, superimposed upon a background of prolonged unloading (microgravity). Axial loading acting on a dysfunctional spine, weakened by anatomical changes and local muscle atrophy, might increase the risk of damage/injury. Alternatively, regular loading may be beneficial. Spinal pathology has been identified in-flight, but there are few contemporary reports of in-flight back injury and no recent studies of post-flight back injury incidence. Accurate routine in-flight stature measurements, in- and post-flight imaging, and tracking of pain and injury (herniation) for at least 2 years post-flight is thus warranted. These should be complemented by ground-based studies, in particular hyper buoyancy floatation (HBF) a novel analog of spinal unloading, in order to elucidate the mechanisms and risk of spinal injury, and to evaluate countermeasures for exploration where injury could be mission critical.