Short-arm centrifugation as a partially effective musculoskeletal countermeasure during 5-day head-down tilt bed rest—results from the BRAG1 study

Systematic review of the effectiveness of standalone passive countermeasures on microgravity-induced physiologic deconditioning

A systematic review of literature was conducted to evaluate the effectiveness of passive countermeasures in ameliorating the cardiopulmonary and musculoskeletal effects of gravitational unloading on humans during spaceflight. This systematic review is the third of a series being conducted by the European Space Agency to evaluate the effectiveness of countermeasures to physiologic deconditioning during spaceflight. With future long-duration space missions on the horizon, it is critical to understand the effectiveness of existing countermeasures to promote astronaut health and improve the probability of future mission success. An updated search for studies examining passive countermeasures was conducted in 2021 to supplement results from a broader search conducted in 2017 for all countermeasures. Ground-based analogue and spaceflight studies were included in the search. A total of 647 articles were screened following removal of duplicates, of which 16 were included in this review. Data extraction and analysis, quality assessment of studies, and transferability of reviewed studies to actual spaceflight based on their bed-rest protocol were conducted using dedicated tools created by the Aerospace Medicine Systematic Review Group. Of the 180 examined outcomes across the reviewed studies, only 20 were shown to have a significant positive effect in favour of the intervention group. Lower body negative pressure was seen to significantly maintain orthostatic tolerance (OT) closer to baseline as comparted to control groups. It also was seen to have mixed efficacy with regards to maintaining resting heart rate close to pre-bed rest values. Whole body vibration significantly maintained many balance-related outcome measures close to pre-bed rest values as compared to control. Skin surface cooling and centrifugation both showed efficacy in maintaining OT. Centrifugation also was seen to have mixed efficacy with regards to maintaining VO2max close to pre-bed rest values. Overall, standalone passive countermeasures showed no significant effect in maintaining 159 unique outcome measures close to their pre-bed rest values as compared to control groups. Risk of bias was rated high or unclear in all studies due to poorly detailed methodologies, poor control of confounding variables, and other sources of bias (i.e. inequitable recruitment of participants leading to a higher male:female ratios). The bed-rest transferability (BR) score varied from 2-7, with a median score of 5. Generally, most studies had good BR transferability but underreported on factors such as control of sunlight or radiation exposure, diet, level of exercise and sleep-cycles. We conclude that: (1) Lack of standardisation of outcome measurement and methodologies has led to large heterogeneity amongst studies; (2) Scarcity of literature and high risk of bias amongst existing studies limits the statistical power of results; and (3) Passive countermeasures have little or no efficacy as standalone measures against cardiopulmonary and musculoskeletal deconditioning induced by spaceflight related to physiologic deterioration due to gravity un-loading.

Paving the way to better understand the effects of prolonged spaceflight on operational performance and its neural bases

Space exploration objectives will soon move from low Earth orbit to distant destinations like Moon and Mars. The present work provides an up-to-date roadmap that identifies critical research gaps related to human behavior and performance in altered gravity and space. The roadmap summarizes (1) key neurobehavioral challenges associated with spaceflight, (2) the need to consider sex as a biological variable, (3) the use of integrative omics technologies to elucidate mechanisms underlying changes in the brain and behavior, and (4) the importance of understanding the neural representation of gravity throughout the brain and its multisensory processing. We then highlight the need for a variety of target-specific countermeasures, and a personalized administration schedule as two critical strategies for mitigating potentially adverse effects of spaceflight on the central nervous system and performance. We conclude with a summary of key priorities for the roadmaps of current and future space programs and stress the importance of new collaborative strategies across agencies and researchers for fostering an integrative cross- and transdisciplinary approach from cells, molecules to neural circuits and cognitive performance. Finally, we highlight that space research in neurocognitive science goes beyond monitoring and mitigating risks in astronauts but could also have significant benefits for the population on Earth.

MRI-based quantification of posterior ocular globe flattening during 60 days of strict 6° head-down tilt bed rest with and without daily centrifugation

Spaceflight associated neuro-ocular syndrome (SANS) is associated with acquired optic disc edema, hyperopia, and posterior globe flattening in some astronauts during long-duration spaceflight possibly due to the headward fluid redistribution in microgravity. The goal of this study was to assess whether strict head-down tilt (HDT) bed rest as a spaceflight analog would produce globe flattening and whether centrifugation could prevent these changes. Twenty-four healthy subjects separated into three groups underwent 60 days of strict 6° HDT bed rest: one control group with no countermeasure (n = 8) and two countermeasure groups exposed to 30 min daily of short-arm centrifugation as a means of artificial gravity (AG), either intermittent (iAG, n = 8) or continuous (cAG, n = 8). Magnetic resonance images (MRI) were collected at baseline, HDT-day 14, HDT-day 52, and 3 days after bed rest. An automated method was applied to quantify posterior globe volume displacement compared with baseline scans. On average, subjects showed an increasing degree of globe volume displacement with bed rest duration (means ± SE: 1.41 ± 1.01 mm³ on HDT14 and 4.04 ± 1.19 mm³ on HDT52) that persisted post-bed rest (5.51 ± 1.26 mm³). Application of 30 min daily AG did not have a significant impact on globe volume displacement (P = 0.42 for cAG and P = 0.93 for iAG compared with control). These results indicate that strict 6° HDT bed rest produced displacement of the posterior globe with a trend of increasing displacement with longer duration that was not prevented by daily 30 min exposure to AG.NEW & NOTEWORTHY Head-down tilt (HDT) bed rest is commonly used as a spaceflight analog for investigating spaceflight associated neuro-ocular syndrome (SANS). Posterior ocular globe flattening has been identified in astronauts with SANS but until now has not been investigated during HDT bed rest. In this study, posterior ocular globe volume displacement was quantified before, during, and after HDT bed rest and countermeasures were tested for their potential to reduce the degree of globe flattening.

The application of artificial gravity in medicine and space

Gravity plays a crucial role in physiology. The lack of gravity, like in long duration spaceflight missions, cause pathologies in e.g., the musculoskeletal system, cardiovascular deconditioning, immune system deprivation or brain abnormalities, to just mention a few. The application of artificial gravity through short-arm human centrifugation (SAHC) has been studied as a possible countermeasure to treat spaceflight deconditioning. However, hypergravity protocols applied by using SAHC have also been used to treat different, ground-based pathologies. Such gravitational therapies have been applied in Uruguay for more than four decades now. The aim of this overview is to summarize the most important findings about the effects of gravitational therapy in different, mainly vascular based pathologies according to the experience in the Gravitational Therapy Center and to discuss the current research in the field of hypergravity applications in medicine but also as multisystem countermeasure for near weightlessness pathologies. New insight is needed on the use of hypergravity in medicine and space research and application.

Comparison of trunk muscle exercises in supine position during short arm centrifugation with 1 g at centre of mass and upright in 1 g

Spaceflight is associated with reduced antigravitational muscle activity, which results in trunk muscle atrophy and may contribute to post-flight postural and spinal instability. Exercise in artificial gravity (AG) performed via short-arm human centrifugation (SAHC) is a promising multi-organ countermeasure, especially to mitigate microgravity-induced postural muscle atrophy. Here, we compared trunk muscular activity (mm. rectus abdominis, ext. obliques and multifidi), cardiovascular response and tolerability of trunk muscle exercises performed during centrifugation with 1 g at individual center of mass on a SAHC against standard upright exercising. We recorded heart rate, blood pressure, surface trunk muscle activity, motion sickness and rating of perceived exertion (BORG) of 12 participants (8 male/4 female, 34 ± 7 years, 178.4 ± 8.2 cm, 72.1 ± 9.6 kg). Heart rate was significantly increased (p < 0.001) during exercises without differences in conditions. Systolic blood pressure was higher (p < 0.001) during centrifugation with a delayed rise during exercises in upright condition. Diastolic blood pressure was lower in upright (p = 0.018) compared to counter-clockwise but not to clockwise centrifugation. Target muscle activation were comparable between conditions, although activity of multifidi was lower (clockwise: p = 0.003, counter-clockwise: p < 0.001) and rectus abdominis were higher (clockwise: p = 0.0023, counter-clockwise: < 0.001) during centrifugation in one exercise type. No sessions were terminated, BORG scoring reflected a relevant training intensity and no significant increase in motion sickness was reported during centrifugation. Thus, exercising trunk muscles during centrifugation generates comparable targeted muscular and heart rate response and appears to be well tolerated. Differences in blood pressure were relatively minor and not indicative of haemodynamic challenge. SAHC-based muscle training is a candidate to reduce microgravity-induced inter-vertebral disc pathology and trunk muscle atrophy. However, further optimization is required prior to performance of a training study for individuals with trunk muscle atrophy/dysfunction.

Introducing the Concept of Exercise Holidays for Human Spaceflight - What Can We Learn From the Recovery of Bed Rest Passive Control Groups

In an attempt to counteract microgravity-induced deconditioning during spaceflight, exercise has been performed in various forms on the International Space Station (ISS). Despite significant consumption of time and resources by daily exercise, including around one third of astronauts' energy expenditure, deconditioning-to variable extents-are observed. However, in future Artemis/Lunar Gateway missions, greater constraints will mean that the current high volume and diversity of ISS in-flight exercise will be impractical. Thus, investigating both more effective and efficient multi-systems countermeasure approaches taking into account the novel mission profiles and the associated health and safety risks will be required, while also reducing resource requirements. One potential approach is to reduce mission exercise volume by the introduction of exercise-free periods, or "exercise holidays". Thus, we hypothesise that by evaluating the 'recovery' of the no-intervention control group of head-down-tilt bed rest (HDTBR) campaigns of differing durations, we may be able to define the relationship between unloading duration and the dynamics of functional recovery-of interest to future spaceflight operations within and beyond Low Earth Orbit (LEO)-including preliminary evaluation of the concept of exercise holidays. Hence, the aim of this literature study is to collect and investigate the post-HDTBR recovery dynamics of current operationally relevant anthropometric outcomes and physiological systems (skeletal, muscular, and cardiovascular) of the passive control groups of HDTBR campaigns, mimicking a period of 'exercise holidays', thereby providing a preliminary evaluation of the concept of 'exercise holidays' for spaceflight, within and beyond LEO. The main findings were that, although a high degree of paucity and inconsistency of reported recovery data is present within the 18 included studies, data suggests that recovery of current operationally relevant outcomes following HDTBR without exercise-and even without targeted rehabilitation during the recovery period-could be timely and does not lead to persistent decrements differing from those experienced following spaceflight. Thus, evaluation of potential exercise holidays concepts within future HDTBR campaigns is warranted, filling current knowledge gaps prior to its potential implementation in human spaceflight exploration missions.

Intramuscular lipid concentration increased in localized regions of the lumbar muscles following 60 day bedrest

BACKGROUND CONTEXT

Prolonged bedrest induces accumulation of intramuscular lipid concentration (ILC) in the lumbar musculature; however, spatial distribution of ILC has not been determined. Artificial gravity (AG) mitigates some adaptations induced by 60 day bedrest by creating a head-to-feet force while participants are in a supine position.

PURPOSE

To quantify the spatial distribution of accumulation of ILC in the lumbar musculature after 60 day bedrest, and whether this can be mitigated by AG exposure.

STUDY DESIGN

Prospective longitudinal study.

PATIENT SAMPLE

Twenty-four healthy individuals (8 females) participated in the study: Eight received 30 min continuous AG (cAG); Eight received 6 × 5 min AG (iAG), interspersed with rests; Eight were not exposed to AG (CRTL).

OUTCOME MEASURES

From 3T magnetic resonance imaging (MRI), axial images were selected to assess lumbar multifidus (LM), lumbar erector spinae (LES), quadratus lumborum (QL), and psoas major (PM) muscles from L1/L2 to L5/S1 intervertebral disc levels. Chemical shift-based 2-echo lipid and/or water Dixon sequence was used to measure tissue composition. Each lumbar muscle was segmented into four equal quartiles (from medial to lateral).

METHODS

Participants arrived at the facility for the baseline data collection before undergoing a 60 day strict 6° head-down tilt (HDT) bedrest period. MRI of the lumbopelvic region was conducted at baseline and Day-59 of bedrest. Participants performed all activities, including hygiene, in 6° HDT and were discouraged from moving excessively or unnecessarily.

RESULTS

At the L4/L5 and L5/S1 intervertebral disc levels, 60-day bedrest induced a greater increase in ILC in medial and lateral regions (∼+4%) of the LM than central regions (∼+2%; p<.05). A smaller increase in ILC was induced in the lateral region of LES (∼+1%) at L1/L2 and L2/L3 than at the centro-medial region (∼+2%; p<.05). There was no difference between CRTL and intervention groups.

CONCLUSIONS

Inhomogeneous spatial distribution of accumulation of ILC was found in the lumbar musculature after 60 day bedrest. These findings might reflect pathophysiological mechanisms related to muscle disuse and contribute to localized lumbar spine dysfunction. Altered spatial distribution of ILC may impair lumbar spine function after prolonged body unloading, which could increase injury risk to vulnerable soft tissues, such as the lumbar intervertebral discs. These novel results may represent a new biomarker of lumbar deconditioning for astronauts, bedridden, sedentary individuals, or those with chronic back pain. Changes are potentially modifiable but not by the AG protocols tested here.

Simulation study on the effect of resistance exercise on the hydrodynamic microenvironment of osteocytes in microgravity

Osteoporosis occurs in astronauts after long-term space flight owing to the lack of gravity. The mechanical microenvironment of osteocytes in load-bearing bone are changed during resistance exercise, which prevents massive bone loss in the human body. A cylindrical fluid-structure coupling finite element model for osteons with a two-stage pore structure (i.e., Haversian canal, lacunar-canalicular system) was established with the software COMSOL. In the Earth's gravity field and in microgravity, considering the effects of pulsating pressure of arterioles, a comparative study was performed on the changes in hydrodynamic microenvironment of osteocytes during human body high-intensity exercise at different frequencies (defined as causing bone to produce 3000 με) and the body is at rest. Positive and negative liquid pressure (with respect to one atmosphere pressure) alternately acted on osteocytes during human exercising, but only positive pressure acted on osteocytes during human resting. The variation range of liquid pressure acted on osteocytes during human exercising was significantly higher than that during resting. The liquid flow velocity around osteocytes during body exercise was about four orders of magnitude higher than that during resting. In microgravity, moderate physical exercise can obviously improve the hydrodynamic microenvironment of osteocytes in load-bearing bone, which could compensate for the lack of mechanical stimulation to osteocytes caused by the lack of gravity, thereby promoting the normal physiological function of osteocytes. To a certain extent, these results revealed the biomechanical mechanism by which exercise has an effect in fighting osteoporosis in astronauts.

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.

Daily 30-min exposure to artificial gravity during 60 days of bed rest does not maintain aerobic exercise capacity but mitigates some deteriorations of muscle function: results from the AGBRESA RCT

PURPOSE

Spaceflight impairs physical capacity. Here we assessed the protective effect of artificial gravity (AG) on aerobic exercise capacity and muscle function during bed rest, a spaceflight analogue.

METHODS

24 participants (33 ± 9 years, 175 ± 9 cm, 74 ± 10 kg, 8 women) were randomly allocated to one of three groups: continuous AG (cAG), intermittent AG (iAG) or control (CTRL). All participants were subjected to 60 days of six-degree head-down tilt bed rest, and subjects of the intervention groups completed 30 min of centrifugation per day: cAG continuously and iAG for 6 × 5 min, with an acceleration of 1g at the center of mass. Physical capacity was assessed before and after bed rest via maximal voluntary contractions, cycling spiroergometry, and countermovement jumps.

RESULTS

AG had no significant effect on aerobic exercise capacity, flexor muscle function and isometric knee extension strength or rate of force development (RFD). However, AG mitigated the effects of bed rest on jumping power (group * time interaction of the rmANOVA p < 0.001; iAG - 25%, cAG - 26%, CTRL - 33%), plantar flexion strength (group * time p = 0.003; iAG - 35%, cAG - 31%, CTRL - 48%) and plantar flexion RFD (group * time p = 0.020; iAG - 28%, cAG - 12%, CTRL - 40%). Women showed more pronounced losses than men in jumping power (p < 0.001) and knee extension strength (p = 0.010).

CONCLUSION

The AG protocols were not suitable to maintain aerobic exercise capacity, probably due to the very low cardiorespiratory demand of this intervention. However, they mitigated some losses in muscle function, potentially due to the low-intensity muscle contractions during centrifugation used to avoid presyncope.

Nonuniform loss of muscle strength and atrophy during bed rest: a systematic review

Muscle atrophy and decline in muscle strength appear very rapidly with prolonged disuse or mechanical unloading after acute hospitalization or experimental bed rest. The current study analyzed data from short-, medium-, and long-term bed rest (5-120 days) in a pooled sample of 318 healthy adults and modeled the mathematical relationship between muscle strength decline and atrophy. The results show a logarithmic disuse-induced loss of strength and muscle atrophy of the weight-bearing knee extensor muscles. The greatest rate of muscle strength decline and atrophy occurred in the earliest stages of bed rest, plateauing later, and likely contributed to the rapid neuromuscular loss of function in the early period. In addition, during the first 2 wk of bed rest, muscle strength decline is much faster than muscle atrophy: on day 5, the ratio of muscle atrophy to strength decline as a function of bed rest duration is 4.2, falls to 2.4 on day 14, and stabilizes to a value of 1.9 after ∼35 days of bed rest. Positive regression revealed that ∼79% of the muscle strength loss may be explained by muscle atrophy, while the remaining is most likely due to alterations in single fiber mechanical properties, excitation-contraction coupling, fiber architecture, tendon stiffness, muscle denervation, neuromuscular junction damage, and supraspinal changes. Future studies should focus on neural factors as well as muscular factors independent of atrophy (single fiber excitability and mechanical properties, architectural factors) and on the role of extracellular matrix changes. Bed rest results in nonuniform loss of isometric muscle strength and atrophy over time, where the magnitude of change was greater for muscle strength than for atrophy. Future research should focus on the loss of muscle function and the underlying mechanisms, which will aid in the development of countermeasures to mitigate or prevent the decline in neuromuscular efficiency.NEW & NOTEWORTHY Our study contributes to the characterization of muscle loss and weakness processes reflected by a logarithmic decline in muscle strength induced by chronic bed rest. Acute short-term hospitalization (≤5 days) associated with periods of disuse/immobilization/prolonged time in the supine position in the hospital bed is sufficient to significantly decrease muscle mass and size and induce functional changes related to weakness in maximal muscle strength. By bringing together integrated evaluation of muscle structure and function, this work identifies that 79% of the loss in muscle strength can be explained by muscle atrophy, leaving 21% of the functional loss unexplained. The outcomes of this study should be considered in the development of daily countermeasures for preserving neuromuscular integrity as well as preconditioning interventions to be implemented before clinical bed rest or chronic gravitational unloading (e.g., spaceflights).

Sixty days of head-down tilt bed rest with or without artificial gravity do not affect the neuromuscular secretome

Artificial gravity is a potential countermeasure to attenuate effects of weightlessness during long-term spaceflight, including losses of muscle mass and function, possibly to some extent attributable to disturbed neuromuscular interaction. The 60-day AGBRESA bed-rest study was conducted with 24 participants (16 men, 8 women; 33 ± 9 years; 175 ± 9 cm; 74 ± 10 kg; 8 control group, 8 continuous (cAG) and 8 intermittent (iAG) centrifugation) to assess the impact of bed rest with or without daily 30-min continuous/intermittent centrifugation with 1G at the centre of mass. Fasting blood samples were collected before and on day 6, 20, 40 and 57 during 6° head-down tilt bed rest. Concentrations of circulating markers of muscle wasting (GDF-8/myostatin; slow skeletal muscle troponin T; prostaglandin E2), neurotrophic factors (BDNF; GDNF) and C-terminal Agrin Fragment (CAF) were determined by ELISAs. Creatine kinase activity was assessed by colorimetric enzyme assay. Repeated-measures ANOVAs were conducted with TIME as within-subject, and INTERVENTION and SEX as between-subject factors. The analyses revealed no significant effect of bed rest or sex on any of the parameters. Continuous or intermittent artificial gravity is a safe intervention that does not have a negative impact of the neuromuscular secretome.

Cardiorespiratory and Neuromuscular Demand of Daily Centrifugation: Results From the 60-Day AGBRESA Bed Rest Study

Purpose

Long stays in space require countermeasures for the degrading effects of weightlessness on the human body, and artificial gravity (AG) has been proposed as an integrated countermeasure. The aim of this study was to assess the cardiorespiratory and neuromuscular demand of AG elicited via daily centrifugation during 60 days of bed rest.

Methods

Twenty four participants (33 ± 9 y, 175 ± 9 cm, 74 ± 10 kg, 8 female) were subjected to 60 days of strict six-degree head-down tilt (HDT) bed rest and were randomly allocated to one of three experimental groups: 30 min of daily centrifugation with an acceleration of 1 g at the center of mass and 2 g at the feet applied continuously (cAG) or intermittently in 6 epochs of 5 min each, separated by 3 min breaks (iAG), or non-centrifuged control (CTRL). Cardiorespiratory demand during centrifugation was assessed at the beginning (HDT3) and end (HDT60) of the bed rest phase via spirometry and heart rate monitoring, leg muscle activation was monitored via electromyography.

Results

On average, analyses of variance revealed that heart rate during centrifugation increased by 40% (iAG) and 60% (cAG) compared to resting values (p < 0.001), while oxygen uptake did not change significantly (p = 0.96). There was a preference for calf over knee extensor muscle activation (active time soleus 57 ± 27%, gastrocnemius medialis 45 ± 27% and vastus lateralis 27 ± 27%, p < 0.001), with large inter-individual differences in leg muscle active time. AG could not prevent the increase in resting heart rate after bed rest. For most of the recorded parameters, there were little differences between cAG and iAG, with the increase in heart rate during centrifugation being a notable exception (greater increase for cAG, p = 0.01).

Conclusion

Daily 30 min bouts of artificial gravity elicited by centrifugation put a substantial demand on the heart as a pump without increasing oxygen consumption. If centrifugation is to be used as a countermeasure for the deteriorating effects of microgravity on physical performance, we recommend combining it with strenuous exercise.

Head-down tilt bed rest with or without artificial gravity is not associated with motor unit remodeling

PURPOSE

The objective of this study was to assess whether artificial gravity attenuates any long-duration head-down 6⁰ bed rest (HDBR)-induced alterations in motor unit (MU) properties.

METHODS

Twenty-four healthy participants (16 men; 8 women; 26-54 years) underwent 60-day HDBR with (n = 16) or without (n = 8) 30 min artificial gravity daily induced by whole-body centrifugation. Compound muscle action potential (CMAP), MU number (MUNIX) and MU size (MUSIX) were estimated using the method of Motor Unit Number Index in the Abductor digiti minimi and tibialis anterior muscles 5 days before (BDC-5), and during day 4 (HDT4) and 59 (HDT59) of HDBR.

RESULTS

The CMAP, MUNIX, and MUSIX at baseline did not change significantly in either muscle, irrespective of the intervention (p > 0.05). Across groups, there were no significant differences in any variable during HDBR, compared to BDC-5.

CONCLUSION

Sixty days of HDBR with or without artificial gravity does not induce alterations in motor unit number and size in the ADM or TA muscles in healthy individuals.

Effects of 21 days of bed rest and whey protein supplementation on plantar flexor muscle fatigue resistance during repeated shortening contractions

PURPOSE

Space flight and bed rest (BR) lead to a rapid decline in exercise capacity. Whey protein plus potassium bicarbonate diet-supplementation (NUTR) could attenuate this effect by improving oxidative metabolism. We evaluated the impact of 21-day BR and NUTR on fatigue resistance of plantar flexor muscles (PF) during repeated shortening contractions, and whether any change was related to altered energy metabolism and muscle oxygenation.

METHODS

Ten healthy men received a standardized isocaloric diet with (n = 5) or without (n = 5) NUTR. Eight bouts of 24 concentric plantar flexions (30 s each bout) with 20 s rest between bouts were employed. PF muscle size was assessed by means of peripheral quantitative computed tomography. PF muscle volume was assessed with magnetic resonance imaging. PF muscle force, contraction velocity, power and surface electromyogram signals were recorded during each contraction, as well as energy metabolism (31P nuclear magnetic resonance spectroscopy) and oxygenation (near-infrared spectroscopy). Cardiopulmonary parameters were measured during an incremental cycle exercise test.

RESULTS

BR caused 10-15% loss of PF volume that was partly recovered 3 days after re-ambulation, as a consequence of fluid redistribution. Unexpectedly, PF fatigue resistance was not affected by BR or NUTR. BR induced a shift in muscle metabolism toward glycolysis and some signs of impaired muscle oxygen extraction. NUTR did not attenuate the BR-induced-shift in energy metabolism.

CONCLUSIONS

Twenty-one days' BR did not impair PF fatigue resistance, but the shift to glycolytic metabolism and indications of impaired oxygen extraction may be early signs of developing reduced muscle fatigue resistance.

A Comparison of Squatting Exercise on a Centrifuge and With Earth Gravity

Purpose: Long-duration space missions require countermeasures against the muscular wasting and cardiovascular deconditioning associated with microgravity. Replacing gravitational acceleration by means of centrifugation is a promising alternative as it challenges all physiological systems at once. The aim of this study is to examine the metabolic energy costs of squatting on a centrifuge in comparison with squatting in an upright standing posture under natural gravity.

Methods: 24 subjects (11 male, 13 female) performed continuous squatting exercise for 9 min with increasing cadence (10, 12, and 15 squats min^-1). This was done under three conditions: Upright under natural gravity and lying supine on a centrifuge at two radii (2.5 and 3.5 m) at 1 g of centrifugal acceleration at the subject’s average center of mass during the exercise.

Results: Generally, subjects did not suffer from motion sickness. Exercise under natural gravity led to a higher Δ V’O₂/body mass (7.1 ± 2.0, ml min^-1 kg^-1, mean ± SD) compared with exercise on the centrifuge (6.1 ± 1.6, ml min^-1 kg^-1, mean ± SD). Exercise efficiency was also reduced under natural 1 g at 28.2 ± 1.0% compared to 40.4 ± 1.5% on the centrifuge. As expected, oxygen consumption increased with increasing cadences. The Coriolis effect had a negligible impact as there was no significant difference in V’O₂ between the two radii. However, during centrifugation and upward movement the right leg was more loaded than the leg left and vice versa during downward movement (centrifuge running clockwise looking down, so to the subjects’ right).

Conclusion: The lower V’O₂ on the centrifuge may be attributed to the unloading of trunk muscles while subjects were lying on the sled, which in the upright condition leaning against the sled were still working to stabilize the torso. Subjects tolerated high rotational rates combined with exercise very well.

Pulsed electromagnetic fields prevented the decrease of bone formation in hindlimb-suspended rats by activating sAC/cAMP/PKA/CREB signaling pathway

Microgravity is one of the main threats to the health of astronauts. Pulsed electromagnetic fields (PEMFs) have been considered as one of the potential countermeasures for bone loss induced by space flight. However, the optimal therapeutic parameters of PEMFs have not been obtained and the action mechanism is still largely unknown. In this study, a set of optimal therapeutic parameters for PEMFs (50 Hz, 0.6 mT 50% duty cycle and 90 min/day) selected based on high-throughput screening with cultured osteoblasts was used to prevent bone loss in rats induced by hindlimb suspension, a commonly accepted animal model to simulate the space environment. It was found that hindlimb suspension for 4 weeks led to significant decreases in femoral and vertebral bone mineral density (BMD) and their maximal loads, severe deterioration in bone micro-structure, and decreases in levels of bone formation markers and increases in bone resorption markers. PEMF treatment prevented about 50% of the decreased BMD and maximal loads, preserved the microstructure of cancellous bone and thickness of cortical bone, and inhibited decreases in bone formation markers. Histological analyses revealed that PEMFs significantly alleviated the reduction in osteoblast number and inhibited the increase in adipocyte number in the bone marrow. PEMFs also blocked decreases in serum levels of parathyroid hormone and its downstream signal molecule cAMP, and maintained the phosphorylation levels of protein kinase A (PKA) and cAMP response element-binding protein (CREB). The expression level of soluble adenylyl cyclases (sAC) was also maintained. It therefore can be concluded that PEMFs partially prevented the bone loss induced by weightless environment by maintaining bone formation through signaling of the sAC/cAMP/PKA/CREB pathway. Bioelectromagnetics. 39:569-584, 2018. © 2018 Wiley Periodicals, Inc.

Intraocular pressure and cardiovascular alterations investigated in artificial gravity as a countermeasure to spaceflight associated neuro-ocular syndrome

Artificial gravity (AG) has been proposed as a countermeasure to spaceflight-associated neuro-ocular syndrome (SANS). The etiology of SANS is unknown but mimicking gravitational loading through AG may mitigate these negative adaptations. Seventeen subjects (nine men, eight women; 18-32 yr) were analyzed in four experimental conditions: 1) standing, 2) supine, 3) AG with the center of rotation at the eye (AGEC), and 4) AG with 2 Gs at the feet (AG2G). In both AG conditions, subjects were spun to produce 1 G at their center of mass. Data included self-administered intraocular pressure (IOP, Tono-pen AVIA, Depew, NY), heart rate (HR), and mean arterial blood pressure (MAP, Omron Series 10, Omron Healthcare, Kyoto, Japan). Data were analyzed with repeated measures ANOVAs with Tukey-Kramer corrections for multiple pairwise comparisons. IOP was 15.7 ± 1.4 mmHg (mean ± 95% confidence interval) standing, 18.8 ± 1.3 mmHg supine, 18.5 ± 1.7 mmHg in AGEC, and 17.5 ± 1.5 mmHg in AG2G. Postures showed a main effect [F(3,48) = 11.0, P < 0.0005], with standing significantly lower than supine ( P = 0.0009), AGEC ( P = 0.002), and AG2G (0.036). Supine, AGEC, and AG2G were not statistically different. HR and MAP were lower in supine compared with all other postures ( P = 0.002 to P < 0.0005), but there were no differences between standing, AGEC, and AG2G. IOP in supine and standing was consistent with previous studies, but contrary to our hypothesis, remained elevated in both AG conditions. Cardiovascular parameters and hydrostatic gradients determine IOP, which remain unchanged compared with standing. These results suggest additional influence on IOP from previously unconsidered factors. NEW & NOTEWORTHY This is the first study, to the authors' knowledge, to measure intraocular pressure in short-radius centrifuge artificial gravity (AG), which has been proposed as a countermeasure to the spaceflight-associated neuro-ocular syndrome (SANS). If the etiology of SANS is related to intraocular pressure, these results have implications for whether or not short-radius AG can be used to prevent ocular changes relevant to it. Our results indicate this proposed countermeasure merits further investigation.

Moderate Exercise Based on Artificial Gravity Preserves Orthostatic Tolerance and Exercise Capacity During Short-Term Head-Down Bed Rest

Numerous countermeasures have been proposed to minimize microgravity-induced physical deconditioning, but their benefits are limited. The present study aimed to investigate whether personalized aerobic exercise based on artificial gravity (AG) mitigates multisystem physical deconditioning. Fourteen men were assigned to the control group (n=6) and the countermeasure group (CM, n=8). Subjects in the CM group were exposed to AG (2 Gz at foot level) for 30 min twice daily, during which time cycling exercise of 80-95 % anaerobic threshold (AT) intensity was undertaken. Orthostatic tolerance (OT), exercise tests, and blood assays were determined before and after 4 days head-down bed rest (HDBR). Cardiac systolic function was measured every day. After HDBR, OT decreased to 50.9 % and 77.5 % of pre-HDBR values in control and CM groups, respectively. Exercise endurance, maximal oxygen consumption, and AT decreased to 96.5 %, 91.5 % and 91.8 % of pre-HDBR values, respectively, in the control group. Nevertheless, there were slight changes in the CM group. HDBR increased heart rate, sympathetic activity, and the pre-ejection period, but decreased plasma volume, parasympathetic activity and left-ventricular ejection time in the control group, whereas these effects were eliminated in the CM group. Aldosterone had no change in the control group but increased significantly in the CM group. Our study shows that 80-95 % AT aerobic exercise based on 2 Gz of AG preserves OT and exercise endurance, and affects body fluid regulation during short-term HDBR. The underlying mechanisms might involve maintained cardiac systolic function, preserved plasma volume, and improved sympathetic responses to orthostatic stress.

Human Pathophysiological Adaptations to the Space Environment

Space is an extreme environment for the human body, where during long-term missions microgravity and high radiation levels represent major threats to crew health. Intriguingly, space flight (SF) imposes on the body of highly selected, well-trained, and healthy individuals (astronauts and cosmonauts) pathophysiological adaptive changes akin to an accelerated aging process and to some diseases. Such effects, becoming manifest over a time span of weeks (i.e., cardiovascular deconditioning) to months (i.e., loss of bone density and muscle atrophy) of exposure to weightlessness, can be reduced through proper countermeasures during SF and in due time are mostly reversible after landing. Based on these considerations, it is increasingly accepted that SF might provide a mechanistic insight into certain pathophysiological processes, a concept of interest to pre-nosological medicine. In this article, we will review the main stress factors encountered in space and their impact on the human body and will also discuss the possible lessons learned with space exploration in reference to human health on Earth. In fact, this is a productive, cross-fertilized, endeavor in which studies performed on Earth yield countermeasures for protection of space crew health, and space research is translated into health measures for Earth-bound population.

Excretion of Zinc and Copper Increases in Men during 3 Weeks of Bed Rest, with or without Artificial Gravity

Background: Zinc and copper have many physiologic functions and little or no functional storage capability, so persistent losses of either element present health concerns, especially during extended-duration space missions.Objectives: We evaluated the effects of short-term bed rest (BR), a spaceflight analog, on copper and zinc metabolism to better understand the role of these nutrients in human adaptation to (simulated) spaceflight. We also investigated the effect of artificial gravity on copper and zinc homeostasis.Methods: Zinc and copper balances were studied in 15 men [mean ± SD age: 29 ± 3 y; body mass index (in kg/m2): 26.4 ± 2.2] before, during, and after 21 d of head-down tilt BR, during which 8 of the participants were subjected to artificial gravity (AG) by centrifugation for 1 h/d. Control subjects were transferred onto the centrifuge but were not exposed to centrifugation. The study was conducted in a metabolic ward; all urine and feces were collected. Data were analyzed by 2-factor repeated-measures ANOVA.Results: Urinary zinc excretion values for control and AG groups were 33% and 14%, respectively, higher during BR than before BR, and fecal zinc excretion values for control and AG groups were 36% and 19%, respectively, higher during BR, resulting in 67% and 82% lower net zinc balances for controls and AG, respectively (both P < 0.01), despite lower nutrient intake during BR. Fecal copper values for control and AG groups were 40% and 33%, respectively, higher during BR than before BR (P < 0.01 for both). Urinary copper did not change during BR, but a 19% increase was observed after BR compared with before BR in the AG group (P < 0.05).Conclusions: The increased fecal excretion of copper and zinc by men during BR suggests that their absorption of these minerals from the diet was reduced, secondary to the release of minerals from bone and muscle. These findings highlight the importance of determining dietary requirements for astronauts on space missions and ensuring provision and intake of all nutrients.

The impact of microgravity on bone in humans

Experiencing real weightlessness in space is a dream for many of us who are interested in space research. Although space traveling fascinates us, it can cause both short-term and long-term health problems. Microgravity is the most important influence on the human organism in space. The human body undergoes dramatic changes during a long-term spaceflight. In this review, we will mainly focus on changes in calcium, sodium and bone metabolism of space travelers. Moreover, we report on the current knowledge on the mechanisms of bone loss in space, available models to simulate the effects of microgravity on bone on Earth as well as the combined effects of microgravity and cosmic radiation on bone. The available countermeasures applied in space will also be evaluated.