The effects of exposure to microgravity and reconditioning of the lumbar multifidus and anterolateral abdominal muscles: implications for people with LBP

Detrimental Effects of Space Flight on the Lumbar Spine May Be Correlated to Baseline Degeneration: Insights From an Advanced MR Imaging Study

Introduction

Pain in lower back is a common condition reported by astronauts, both during and after space missions. Investigating the alterations in the spine and the mechanisms driving these changes is essential for a deeper understanding of how microgravity impacts the human spine. This knowledge could also open pathways for therapeutic or preventive interventions. Nevertheless, there is a limited evidence regarding changes in intervertebral discs (IVDs) due to space travel.

Materials and Methods

In this study, 2 astronauts were enrolled in a space travel. Before the space flight, a lower back magnetic resonance imaging (MRI) scan was performed. We repeated an MRI instantly after 17-days space travel, and again 3 months after landing. The water content and glycosaminoglycan (GAGs) levels in the lumbar IVDs were evaluated using DIXON water-only phase imaging and T1rho MRI sequences. Additionally, alterations in the size and quality of the paraspinal muscles (PSMs), including fatty infiltration, were examined.

Results

Varied alterations were observed in the IVDs and PSMs of both astronauts. One astronaut experienced a reduction in water and GAGs content, while the other showed an increase. These changes in the IVDs following spaceflight appeared to be linked to the degree of baseline degeneration. Regarding the PSMs, differences in size and fatty infiltration also varied between the two astronauts. Notably, these changes had not stabilized by the final follow-up at 3 months.

Conclusion

Our findings offer initial evidence indicating that even brief exposures to microgravity might be linked to biochemical alterations in IVDs and changes in the quality of PSMs, which could continue evolving for more than 3 months after returning from space.

Thoracolumbar spine muscle size and composition changes in long-duration space missions

Muscle atrophy occurs with extended exposure to microgravity. This study quantified the overall muscle size, lean muscle area and fat infiltration changes pre- to post-flight that occur in the thoracic and lumbar spine with long-duration spaceflight. Pre- and post-flight magnetic resonance imaging (MRI) scans were obtained from 9 crewmembers on long-duration (≥6 months) International Space Station (ISS) missions. Muscle size was measured by the cross-sectional area (CSA) and lean muscle tissue by the functional cross-sectional area (FCSA). Muscle-fat infiltration (MFI) was measured by the mean pixel intensities of the MRI in fat and water phases. A mixed model with random subject effect was used to analyze pre- to post-flight changes. Significant decreases were seen in the quadratus lumborum muscle size (-1.8 ± 0.6% per month, p = 0.002) and lean muscle tissue content in the paraspinal muscles (-0.7 ± 0.2% per month, p ≤ 0.001). Fat infiltration increased significantly in the transversospinalis (+4.1 ± 1.0% per month, p ≤ 0.01) muscle. Treadmill exercise had a tendency to reduce fat content in the paraspinal and quadratus lumborum muscles, while counteracting muscle build-up only in the paraspinal muscles. Cycle ergometer exercise suggested benefits for the psoas muscle. Resistance training appeared to benefit lean muscle mass of most thoracolumbar muscles. Our findings highlight the need for countermeasures to prevent muscle atrophy and detrimental effects in muscle composition during long-duration spaceflight.

Low Back Pain During and After Spaceflight: A Systematic Review with Meta-Analysis

Space flights can produce physiological changes in the spine, leading to the development of acute and chronic pain in passengers. However, there is a lack of comprehensive literature exploring physiological spine changes and acute and chronic pain in space passengers (astronauts and animals). The first aim of this study was to identify the physiological changes experienced by passengers (humans and animals) after space flight. The second aim was to identify the incidence of low back pain during and after space flight. This systematic review was conducted following PRISMA guidelines and was pre-registered in PROSPERO (ID 451144). We included Randomized Controlled Trials or longitudinal studies in humans and animals, and the variables must be assessed either in-flight or post-flight. We conducted a literature search in major databases combining the keywords: Pain; Space; Low Back Pain; Astronauts; Spine Changes; Microgravity; Physiological Changes; Humans; Animals. Risk of bias and quality of studies were analyzed, and the level of evidence was assessed using the GRADE system. After duplicates were removed, 115 abstracts were screened by two reviewers, and finally, 11 articles were included in this review. The evidence indicates that astronauts experience muscle atrophy in the lumbar multifidus with a moderate to large effect, especially in the L4-L5 and L5-S1 segments. Space flights also decrease the range of motion with a moderate effect, along with disc herniations and disc dehydration. 77% of astronauts experience pain during spaceflight, and 47% develop acute pain after spaceflight. Chronic pain was reported by 33% of the astronauts. After space flights, astronauts suffer from lumbar muscle atrophy, reduced range of motion, disc herniations, and disc dehydration, with a high incidence of both acute and chronic pain.

50% body weight loading reduces stature increases and lumbar disc expansion from 4 h hyper-buoyancy floatation versus 15 min sitting upright

Microgravity is associated with stature increases, back pain and post-flight intervertebral disc (IVD) herniation. This study aims to determine whether 30 s seated 50% body weight (BW) axial loading is comparable to 15 min sitting upright in 1 g upon changes in stature, anterior lumbar IVD height (via ultrasound), passive vertebral stiffness (VS), and back pain induced by 4 h hyper-buoyancy floatation (HBF) unloading. Sixteen (seven male) healthy volunteers had stature, lumbar IVD height (L2-S1), passive VS (C1-L5) and back pain assessed before and following 4 h HBF, and immediately after participants performed a 30 s seated squat with 50% of their BW or 15 min sitting upright. Four hours of HBF unloading induced significant increments in stature (+1.6 ± 0.5 cm; P < 0.001), IVD height (L2-L3: P = 0.002; L3-L4: P < 0.001; L4-L5: P = 0.013; L5-S1: P < 0.001) and back pain (2.90 ± 1.26; P < 0.001) with no differences between 1 and 1.5 BW. Stature, IVD height increments and back pain were similarly attenuated in both reloading groups. Passive VS was unchanged by 4 h HBF or reloading. HBF-induced back pain positively correlated with stature (P = 0.01) and lumbar IVD height changes (L2-L3: P = 0.03; L3-L4: P = 0.01; L5-S1: P = 0.02). Four hours of HBF increased stature, lumbar IVD height and induced moderate back pain that were similarly (albeit not entirely) ameliorated by both 15 min upright sitting and 30 s of 50% BW axial loading, with no changes in passive VS observed. IVD geometric changes appear key to space adaptation back pain and stature increments that can be rapidly modulated by brief periods of axial loading.

Mitigating skeletal muscle wasting in unloading and augmenting subsequent recovery

Skeletal muscle wasting is the hallmark pathophysiological adaptation to unloading or disuse that demonstrates the dependency on frequent mechanical stimulation (e.g. muscle activation and subsequent loading) for homeostasis of normally load-bearing muscles. In the absence of mitigation strategies, no mammalian organism is resistant to muscle atrophy driven by unloading. Given the profound impact of unloading-induced muscle wasting on physical capacity, metabolic health and immune function; mitigation strategies during unloading and/or augmentation approaches during recovery have broad healthcare implications in settings of bed-bound hospitalization, cast immobilization and spaceflight. This topical review aims to: (1) provide a succinct, state-of-the-field summary of seminal and recent findings regarding the mechanisms of unloading-induced skeletal muscle wasting; (2) discuss unsuccessful vs. promising mitigation and recovery augmentation strategies; and (3) identify knowledge gaps ripe for future research. We focus on the rapid muscle atrophy driven by relatively short-term mechanical unloading/disuse, which is in many ways mechanistically distinct from both hypermetabolic muscle wasting and denervation-induced muscle atrophy. By restricting this discussion to mechanical unloading during which all components of the nervous system remain intact (e.g. without denervation models), mechanical loading requiring motor and sensory neural circuits in muscle remain viable targets for both mitigation and recovery augmentation. We emphasize findings in humans with comparative discussions of studies in rodents which enable elaboration of key mechanisms. We also discuss what is currently known about the effects of age and sex as biological factors, and both are highlighted as knowledge gaps and novel future directions due to limited research.

Pain Experience and Sensory Changes in Astronauts During and After Short-Lasting Commercial Spaceflight: A Proof-of-Concept Study

Space travel has been associated with musculoskeletal pain, yet little is known about the nociceptive changes and pain experience during spaceflight. This preliminary study aims to investigate the pain experience and sensory alterations in astronauts following a 17-day mission to the International Space Station (ISS) on Axiom Space's AX-1 commercial space flight. Two participants were enrolled, and data were collected pre-flight, in-flight, post-flight, and three-month post-flight. Validated pain questionnaires assessed anxiety, catastrophizing, impact on physical and mental health, disability, and overall pain experience. Qualitative interviews were conducted post-landing and conditioned pain modulation (CPM) and quantitative sensory testing (QST) were performed. Both astronauts reported musculoskeletal pain during and after the flight, which was managed with anti-inflammatories and stretching techniques. Pain levels returned to baseline after three months. Pain questionnaires revealed heightened pain experiences in-flight and immediately post-flight, although their adequacy in assessing pain in space is uncertain. Qualitative interviews allowed astronauts to describe their pain experiences during the flight. Sensory changes included increased mechanical touch detection thresholds, temporal pain summation, heat pain thresholds, and differences in conditioned pain modulation post-flight. This preliminary study suggested that spaceflight may affect various aspects of sensory perception and regulation in astronauts, albeit in a variable manner. More data are needed to gain insight of on gain and loss of sensory functions during space missions. Further investigation into the multifactorial stressors affecting the somatosensory system during space travel could contribute to advancements in space and pain medicine.

Effect of microgravity on mechanical loadings in lumbar spine at various postures: a numerical study

The aim of this study was to quantitatively analyze the mechanical change of spinal segments (disc, muscle, and ligament) at various postures under microgravity using a full-body musculoskeletal modeling approach. Specifically, in the lumbar spine, the vertebra were modeled as rigid bodies, the intervertebral discs were modeled as 6-degree-of-freedom joints with linear force-deformation relationships, the disc swelling pressure was deformation dependent, the ligaments were modeled as piecewise linear elastic materials, the muscle strength was dependent on its functional cross-sectional area. The neutral posture and the "fetal tuck" posture in microgravity (short as "Neutral 0G" and "Fetal Tuck 0G", in our simulation, the G constant was set to 0 for simulating microgravity), and for comparison, the relaxed standing posture in 1G and 0G gravity (short as "Neutral 1G" and "Standing 0G") were simulated. Compared to values at Neutral 1G, the mechanical response in the lower spine changed significantly at Neutral 0G. For example, the compressive forces on lumbar discs decreased 62-70%, the muscle forces decreased 55.7-92.9%, while disc water content increased 7.0-10.2%, disc height increased 2.1-3.0%, disc volume increased 6.4-9.3%, and ligament forces increased 59.5-271.3% at Neutral 0G. The fetal tuck 0G reversed these changes at Neutral 0G back toward values at Neutral 1G, with magnitudes much larger than those at Neutral 1G. Our results suggest that microgravity has significant influences on spinal biomechanics, alteration of which may increase the risks of disc herniation and degeneration, muscle atrophy, and/or ligament failure.

Activation of Anterolateral Abdominal Muscles During Sling Bridge Exercises: Comparison of Different Pelvic Positions

CONTEXT

Inappropriate activation of the anterolateral abdominal muscles affects the stability of the lumbopelvic zone and increases the appearance of pain and lesion in the area. Therefore, ways to improve its effective contraction are crucial in rehabilitation. The aim of this study was to compare the activation of the transverse abdominis (TrA), internal oblique (IO), and external oblique (EO) muscles in 3 different pelvic positions (down pelvis [DP], horizontal pelvis [HP], and up pelvis [UP]) during sling bridge exercise (SBE) to determinate which position is more effective to promote a correct contraction of the anterolateral abdominal muscles.

DESIGN

Cross-sectional study.

METHODS

Fifteen participants performed 3 variations (DP, HP, and UP) of a one-legged exercise called "supine pelvic lift" on a sling device. The thicknesses of the TrA, IO, and EO were recorded at rest and at the 3 positions using ultrasound imaging. Thickness, change ratio, lateral slide of TrA, and preferential and contraction activation ratio of TrA, IO, and EO were analyzed.

RESULTS

TrA and IO showed greater activation (P = .01) in the UP position than the other pelvic positions. In addition, UP position decreased the activation of the EO (P = .01).

CONCLUSION

Based on the results of this study, SBE in the UP position has the potential to improve normal contraction patterns of the musculature and can be used in future intervention of the lumbopelvic zone.

Effect of trunk exercise upon lumbar IVD height and vertebral compliance when performed supine with 1 g at the CoM compared to upright in 1 g

Background

Spinal unloading in microgravity is associated with stature increments, back pain, intervertebral disc (IVD) swelling and impaired spinal kinematics. The aim of this study was to determine the effect of lateral stabilization, trunk rotation and isometric abdominal exercise upon lumbar IVD height, and both passive and active vertebral compliance when performed supine on a short-arm human centrifuge (SAHC)—a candidate microgravity countermeasure—with 1 g at the CoM, compared to that generated with equivalent upright exercise in 1 g.

Methods

12 (8 male) healthy subjects (33.8 ± 7 years, 178.4 ± 8.2 cm, 72.1 ± 9.6 kg) gave written informed consent. Subjects performed three sets of upper body trunk exercises either when standing upright (UPRIGHT), or when being spun on the SAHC. Lumbar IVD height and vertebral compliance (active and passive) were evaluated prior to SAHC (PRE SAHC) and following the first SAHC (POST SPIN 1) and second Spin (POST SPIN 2), in addition to before (PRE UPRIGHT), and after upright trunk exercises (POST UPRIGHT).

Results

No significant effect upon IVD height (L2–S1) when performed UPRIGHT or on the SAHC was observed. Trunk muscle exercise induced significant (p < 0.05) reduction of active thoracic vertebral compliance when performed on the SAHC, but not UPRIGHT. However, no effect was observed in the cervical, lumbar or across the entire vertebral column. On passive or active vertebral compliance.

Conclusion

This study, the first of its kind demonstrates that trunk exercise were feasible and tolerable. Whilst trunk muscle exercise appears to have minor effect upon IVD height, it may be a candidate approach to mitigate—particularly active—vertebral stability on Earth, and in μg via concurrent SAHC. However, significant variability suggests larger studies including optimization of trunk exercise and SAHC prescription with MRI are warranted.

Trial Registration

North Rhine ethical committee (Number: 6000223393) and registered on 29/09/2020 in the German Clinical Trials Register (DRKS00021750).

The Effects of Reconditioning Exercises Following Prolonged Bed Rest on Lumbopelvic Muscle Volume and Accumulation of Paraspinal Muscle Fat

Reduced muscle size and accumulation of paraspinal muscle fat content (PFC) have been reported in lumbopelvic muscles after spaceflights and head-down tilt (HDT) bed rest. While some information is available regarding reconditioning programs on muscle atrophy recovery, the effects on the accumulation of PFC are unknown. Recently, a device (the Functional Re-adaptive Exercise Device—FRED) has been developed which aims to specifically recruit lumbopelvic muscles. This study aimed to investigate the effects of a standard reconditioning (SR) program and SR program supplemented by FRED (SR + FRED) on the recovery of the lumbopelvic muscles following 60-day HDT bed rest. Twenty-four healthy participants arrived at the facility for baseline data collection (BDC) before the bed rest period. They remained in the facility for 13-day post-HDT bed rest and were randomly allocated to one of two reconditioning programs: SR or SR + FRED. Muscle volumes of the lumbar multifidus (LM), lumbar erector spinae (LES), quadratus lumborum (QL), and psoas major (PM) muscles were measured from axial T1-weighted magnetic resonance imaging (MRI) at all lumbar intervertebral disc levels. PFC was determined using a chemical shift-based lipid/water Dixon sequence. Each lumbopelvic muscle was segmented into four equal quartiles (from medial to lateral). MRI of the lumbopelvic region was conducted at BDC, Day-59 of bed rest (HDT59), and Day-13 after reconditioning (R13). Comparing R13 with BDC, the volumes of the LM muscle at L4/L5 and L5/S1, LES at L1/L2, and QL at L3/L4 had not recovered (all—p < 0.05), and the PM muscle remained larger at L1/L2 (p = 0.001). Accumulation of PFC in the LM muscle at the L4/L5 and L5/S1 levels remained higher in the centro-medial regions at R13 than BDC (all—p < 0.05). There was no difference between the two reconditioning programs. A 2-week reconditioning program was insufficient to fully restore all volumes of lumbopelvic muscles and reverse the accumulation of PFC in the muscles measured to BDC values, particularly in the LM muscle at the lower lumbar levels. These findings suggest that more extended reconditioning programs or alternative exercises may be necessary to fully restore the size and properties of the lumbopelvic muscles after prolonged bed rest.

The relationships between physical activity, lumbar multifidus muscle morphology, and low back pain from childhood to early adulthood: a 12-year longitudinal study

We investigated the longitudinal associations between physical activity (PA), lumbar multifidus morphology, and impactful low back pain (LBP) in young people. Nine-year-old children were recruited from 25 primary schools and followed up at age 13, 16, and 21 years. We measured PA with accelerometers at age 9, 13, and 16; quantified patterns of lumbar multifidus intramuscular adipose tissue (IMAT) change from 13 to 16 years using magnetic resonance imaging; and recorded LBP and its impact with standardised questionnaires and interviews. Associations were examined with crude and adjusted logistic or multinomial models and reported with odds ratios (OR) or relative risk ratios (RRR). We included data from 364 children (mean[SD] age = 9.7[.4] years). PA behaviour was not associated with LBP. Having persistently high IMAT levels at age 13 and 16 was associated with greater odds of LBP (OR[95% CI] = 2.98[1.17 to 7.58]). Increased time in moderate and vigorous intensity PA was associated with a lower risk of higher IMAT patterns (RRR[95% CI] = .67[.46 to .96] to .74[.55 to 1.00]). All associations became non-significant after adjusting for sex and body mass index (BMI). Future studies investigating the relationships between PA behaviour, lumbar multifidus IMAT, and impactful LBP should account for potential confounding by sex and BMI.

Alteration of lumbar muscle morphology and composition in relation to low back pain: a systematic review and meta-analysis

BACKGROUND CONTEXT

Previous studies have proposed that there is a relationship between low back pain (LBP) and morphology and composition of paraspinal muscles. However, results have been conflicting, especially regarding fatty infiltration of muscles.

PURPOSE

The primary goal of this study was to review and analyze results from imaging studies which investigated morphological and composition changes in the multifidus, erector spinae and psoas major muscles in people with LBP.

STUDY DESIGN/SETTING

Systematic review with meta-analysis.

PATIENT SAMPLE

A patient sample was not required OUTCOME MEASURES: This review did not have outcome measures.

METHODS

PubMed, Scopus, Web of Sciences, EMBASE and ProQuest were searched for eligible studies up to 31st July 2020 (all languages). A systematic search of electronic databases was conducted to identify studies investigating the association between the morphology and fat content of lumbar muscles in people with LBP compared with a (no LBP) control group. 13,795 articles were identified. Based on the screening for inclusion/ exclusion, 25 were included. The quality of the studies was evaluated using the Newcastle-Ottawa Scale. From the 25 articles, 20 were included in the meta-analysis.

RESULTS

Results showed that the total cross-sectional area of the multifidus was smaller in people with LBP (Standardized mean difference, SMD = -0.24, 95% CI = -0.5 to 0.03). Combined SMDs showed a medium effect of LBP on increasing multifidus muscle fat infiltration (SMD = 0.61, 95% CI = 0.30 to 0.91). There were no LBP related differences identified in the morphology or composition of the lumbar erector spine and psoas major muscles.

CONCLUSIONS

People with LBP were found to have somewhat smaller multifidus muscles with a significant amount of intramuscular fat infiltration. Varying sample size, age and BMI of participants, quality of studies and the procedures used to measure fat infiltration are possible reasons for inconsistencies in results of previous studies.

Identification of differentially expressed genes in mouse paraspinal muscle in response to microgravity

Lower back pain (LBP) is the primary reason leading to dyskinesia in patients, which can be experienced by people of all ages. Increasing evidence have revealed that paraspinal muscle (PSM) degeneration (PSMD) is a causative contributor to LBP. Current research revealed that fatty infiltration, tissue fibrosis, and muscle atrophy are the characteristic pathological alterations of PSMD, and muscle atrophy is associated with abnormally elevated oxidative stress, reactive oxygen species (ROS) and inflammation. Interestingly, microgravity can induce PSMD and LBP. However, studies on the molecular mechanism of microgravity in the induction of PSMD are strongly limited. This study identified 23 differentially expressed genes (DEGs) in the PSM (longissimus dorsi) of mice which were flown aboard the Bion M1 biosatellite in microgravity by bioinformatics analysis. Then, we performed protein-protein interaction, Gene Ontology function, and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis for the DEGs. We found that Il6ra, Tnfaip2, Myo5a, Sesn1, Lcn2, Lrg1, and Pik3r1 were inflammatory genes; Fbox32, Cdkn1a, Sesn1, and Mafb were associated with muscle atrophy; Cdkn1a, Sesn1, Lcn2, and Net1 were associated with ROS; and Sesn1 and Net1 were linked to oxidative stress. Furthermore, Lcn2, Fbxo32, Cdkn1a, Pik3r1, Sesn1, Net1, Il6ra, Myo5a, Lrg1, and Pfkfb3 were remarkably upregulated, whereas Tnfaip2 and Mafb were remarkably downregulated in PSMD, suggesting that they might play a significant role in regulating the occurrence and development of PSMD. These findings provide theoretical basis and therapeutic targets for the treatment of PSMD.

Gluteal Muscle Atrophy and Increased Intramuscular Lipid Concentration Are Not Mitigated by Daily Artificial Gravity Following 60-Day Head-Down Tilt Bed Rest

Exposure to spaceflight and head-down tilt (HDT) bed rest leads to decreases in the mass of the gluteal muscle. Preliminary results have suggested that interventions, such as artificial gravity (AG), can partially mitigate some of the physiological adaptations induced by HDT bed rest. However, its effect on the gluteal muscles is currently unknown. This study investigated the effects of daily AG on the gluteal muscles during 60-day HDT bed rest. Twenty-four healthy individuals participated in the study: eight received 30 min of continuous AG; eight received 6 × 5 min of AG, interspersed with rest periods; eight belonged to a control group. T1-weighted Dixon magnetic resonance imaging of the hip region was conducted at baseline and day 59 of HDT bed rest to establish changes in volumes and intramuscular lipid concentration (ILC). Results showed that, across groups, muscle volumes decreased by 9.2% for gluteus maximus (GMAX), 8.0% for gluteus medius (GMED), and 10.5% for gluteus minimus after 59-day HDT bed rest (all p < 0.005). The ILC increased by 1.3% for GMAX and 0.5% for GMED (both p < 0.05). Neither of the AG protocols mitigated deconditioning of the gluteal muscles. Whereas all gluteal muscles atrophied, the ratio of lipids to intramuscular water increased only in GMAX and GMED muscles. These changes could impair the function of the hip joint and increased the risk of falls. The deconditioning of the gluteal muscles in space may negatively impact the hip joint stability of astronauts when reexpose to terrestrial gravity.

Intermittent short-arm centrifugation is a partially effective countermeasure against upright balance deterioration following 60-day head-down tilt bed rest

This study investigated whether artificial gravity (AG), induced by short-radius centrifugation, mitigated deterioration in standing balance and anticipatory postural adjustments (APAs) of trunk muscles following 60-day head-down tilt bed rest. Twenty-four participants were allocated to one of three groups: control group (n = 8); 30-min continuous AG daily (n = 8); and intermittent 6 × 5 min AG daily (n = 8). Before and immediately after bed rest, standing balance was assessed in four conditions: eyes open and closed on both stable and foam surfaces. Measures including sway path, root mean square, and peak sway velocity, sway area, sway frequency power, and sway density curve were extracted from the center of pressure displacement. APAs were assessed during rapid arm movements using intramuscular or surface electromyography electrodes of the rectus abdominis; obliquus externus and internus abdominis; transversus abdominis; erector spinae at L1, L2, L3, and L4 vertebral levels; and deep lumbar multifidus muscles. The relative latency between the EMG onset of the deltoid and each of the trunk muscles was calculated. All three groups had poorer balance performance in most of the parameters (all P < 0.05) and delayed APAs of the trunk muscles following bed rest (all P < 0.05). Sway path and sway velocity were deteriorated, and sway frequency power was less in those who received intermittent AG than in the control group (all P < 0.05), particularly in conditions with reduced proprioceptive feedback. These data highlight the potential of intermittent AG to mitigate deterioration of some aspects of postural control induced by gravitational unloading, but no protective effects on trunk muscle responses were observed.NEW & NOTEWORTHY This study presents novel insights into the effect of artificial gravity (AG) on the deterioration of standing balance and anticipatory postural adjustments (APAs) of trunk muscles induced by 60-day strict head-down bed rest. The results indicated severe balance dysfunction and delayed APAs during rapid arm movement. AG partially mitigated the deterioration in standing balance and may thus be considered as a potential countermeasure for future planetary surface explorations. Optimization of AG protocols might enhance effects.

Neurosurgery and spinal adaptations in spaceflight: A literature review

BACKGROUND

Spaceflight places astronauts in multiple environments capable of inducing pathological changes. Alterations in the spine have a significant impact on astronauts' health during and after spaceflight. Low back pain is an established and common intra-flight complaint. Intervertebral disc herniation occurs at higher rates in this population and poses significant morbidity. Morphological changes within intervertebral discs, vertebral bodies, and spinal postural muscles affect overall spine function and astronaut performance. There remains a paucity of research related to spaceflight-induced pathologies, and currently available reviews concern the central nervous system broadly while lacking emphasis on spinal function.

OBJECTIVE

Our aim was to review and summarize available data regarding changes in spinal health with exposure to spaceflight, especially focusing on effects of microgravity. The authors also present promising diagnostic and treatment approaches wherein the neurosurgeon could positively impact astronauts' health and post-flight outcomes.

MATERIALS AND METHODS

Articles included in this review were identified via search engine using MEDLINE, PubMed, Cochrane Review, Google Scholar, and references within other relevant articles. Search criteria included "spine and spaceflight", "vertebral column and spaceflight", "vertebral disc and spaceflight", and "muscle atrophy and spaceflight", with results limited to articles written in English from 1961 to 2020. References of selected articles were included as appropriate.

RESULTS

Fifty-six articles were included in this review. Compositional changes at the intervertebral discs, vertebral bone, and paraspinal muscles contribute to undesirable effects on astronaut spinal function in space and contribute to post-flight pathologies. Risk of intervertebral disc herniation increases, especially during post-flight recovery. Vertebral bone degeneration in microgravity may increase risk for herniation and fracture. Paraspinal muscle atrophy contributes to low back pain, poorer spine health, and reduced stability.

CONCLUSION

Anatomical changes in microgravity contribute to the development of spinal pathologies. Microgravity impacts sensory neurovestibular function, neuromuscular output, genetic expression, among other systems. Future developments in imaging and therapeutic interventions may better analyze these changes and offer targeted therapeutic interventions to decrease the burden of pain and other diseases of the spine in this population.

Lumbar muscle atrophy and increased relative intramuscular lipid concentration are not mitigated by daily artificial gravity after 60-day head-down tilt bed rest

Exposure to axial unloading induces adaptations in paraspinal muscles, as shown after spaceflights. This study investigated whether daily exposure to artificial gravity (AG) mitigated lumbar spine flattening and muscle atrophy associated with 60-day head-down tilt (HDT) bed rest (Earth-based space analog). Twenty-four healthy individuals participated in the study: 8 received 30-min continuous AG; 8 received 6 × 5-min AG interspersed with rest periods; and 8 received no AG exposure (control group). Magnetic resonance imaging (MRI) of the lumbopelvic region was conducted at baseline (BDC) and at day 59 of HDT (HDT59). Longitudinal relaxation time (T1)-weighted images were used to assess morphology of the lumbar spine (spinal length, intervertebral disk angles, disk area) and volumes of the lumbar multifidus (LM), lumbar erector spinae (LES), quadratus lumborum (QL), and psoas major (PM) muscles from L₁/L₂ to L₅/S₁ vertebral levels. A chemical shift-based two-point lipid/water Dixon sequence was used to evaluate muscle composition. Results showed that spinal length and disk area increased (P < 0.05); intervertebral disk angles (P < 0.05) and muscle volumes of LM, LES, and QL reduced (P < 0.01); and lipid-to-water ratio for the LM and LES muscles increased (P < 0.01) after HDT59 in all groups. Neither of the AG protocols mitigated the lumbar spinae deconditioning induced by HDT bed rest. The increase in lipid-to-water ratio in LM and LES muscles indicates an increased relative intramuscular lipid concentration. Altered muscle composition in atrophied muscles may impair lumbar spine function after body unloading, which could increase injury risk to vulnerable soft tissues. This relationship needs further investigation.NEW & NOTEWORTHY This study presents novel insights into the morphological adaptations occurring in the lumbar spine after 60-day head-down bed rest and the potential role of artificial gravity (AG) to mitigate them. Results demonstrated no protective effect of AG protocols used in this study. In atrophied paraspinal muscles, the ratio of lipids versus intramuscular water increased in the postural lumbar muscles, which could impair muscle function during upright standing. These findings have relevance for future space explorations.

Back Pain in Outer Space

Space travel has grown during the past 2 decades, and is expected to surge in the future with the establishment of an American Space Force, businesses specializing in commercial space travel, and National Aeronautics and Space Administration's planned sustained presence on the moon. Accompanying this rise, treating physicians are bracing for a concomitant increase in space-related medical problems, including back pain. Back pain is highly prevalent in astronauts and space travelers, with most cases being transient and self-limiting (space adaptation back pain). Pathophysiologic changes that affect the spine occur during space travel and may be attributed to microgravity, rapid acceleration and deceleration, and increased radiation. These include a loss of spinal curvature, spinal muscle atrophy, a higher rate of disc herniation, decreased proteoglycan and collagen content in intervertebral discs, and a reduction in bone density that may predispose people to vertebral endplate fractures. In this article, the authors discuss epidemiology, pathophysiology, prevention, treatment, and future research.

Trunk Skeletal Muscle Changes on CT with Long-Duration Spaceflight

Astronauts exposed to microgravity for extended time are susceptible to trunk muscle atrophy, which may compromise strength and function on mission and after return. This study investigates changes in trunk skeletal muscle size and composition using computed tomography (CT) and dual-energy X-ray absorptiometry (DXA) among 16 crewmembers (1 female, 15 male) on 4-6 month missions. Muscle cross-sectional area and muscle attenuation were measured using abdominal CT scans at pre-flight, post-flight return, 1 year post-flight, and 2-4 years post-flight. Longitudinal muscle changes were analyzed using mixed models. In six crewmembers, CT and DXA data were used to calculate subject height-normalized skeletal muscle indices. Changes in these indices were analyzed using paired t-tests and compared by imaging modality using Pearson correlations. Trunk muscle area decreased at post-flight return (- 4.7 ± 1.1%, p < 0.001) and recovered to pre-flight values at 1-4 years post-flight. Muscle attenuation changes were not significant. Skeletal muscle index from CT decreased (- 5.2 ± 1.0%, p = 0.004) while appendicular skeletal muscle index from DXA did not change significantly. In summary, trunk muscle atrophies with long-duration microgravity exposure but recovers to pre-flight values within 1-4 years. The CT measures highlight size decreases not detected with DXA, emphasizing the importance of advanced imaging modalities in assessing muscle health with spaceflight.