What is microgravity?
“Micro-” means “very small,” so microgravity refers to the condition where gravity seems to be very small.(1)
Microgravity is the condition in which people or objects appear to be weightless. The effects of microgravity can be seen when astronauts and objects float in space.(1)
In microgravity, astronauts can float in their spacecraft – or outside, on a spacewalk. Heavy objects move around easily. For example, astronauts can move equipment weighing hundreds of pounds with their fingertips.(1)
Microgravity is sometimes called “zero gravity,” but this is misleading.(1)
Some people think that there is no gravity in space. In fact, a small amount of gravity can be found everywhere in space.(1)
Effects of microgravity on human body
Among the many technical, logistical and physiological challenges inherent to extended space exploration, the loss of gravitational force is a major prohibitive environmental factor that adversely affects the body of space travelers.(2)
In microgravity, without the continuous load of Earth’s gravity, the tissues that make up bones reshape themselves.(3)
The human body is intrinsically adapted to Earth’s gravity (~9.907 m/s2), thus exposure to conditions of reduced gravity, or microgravity (µG) can lead to a plethora of complications in normal bodily functions.(2)
The requirements of the human body, in particular the neuro-musculoskeletal system, are very different in space than on Earth. Interestingly, physiological spaceflight data suggest that it is more difficult to return to gravity than to adapt to microgravity conditions.(4)
In microgravity, musculoskeletal adaptations are appropriate to that environment but this has major effects on muscle function and posture.(4)
On Earth, the line of gravity normally passes through the ventral part of the L3 vertebral body.(4)
Astronauts move in a predominantly flexed position and the center of mass shifts posteriorly (Baroni et al., 2001), with increased recruitment of flexor muscles and a loss of extensors (Fitts et al., 2000 Aug, Fitts et al., 2001 Sep)(4).
A shift of muscle fibre types from tonic (type 1) to phasic (type 2) occurs (Fitts et al., 2001).(4)
Graviceptors, which are sensory receptors that contribute to providing a neural representation of the direction of gravity, with respect to the gravity vector (Binder, 2009), no longer function in microgravity.(4)
The astronaut therefore receives less information about his/her posture and has to rely on vision and feedback from dynamic receptors.(4)
Prolonged microgravity has negative effects on muscle strength and endurance, motor control, coordination and balance (Layne et al., 2001), which may place the astronaut at higher risk of injury. In the spine, primarily lumbar, intervertebral discs absorb more water (hyperhydration) than on Earth (Belavy et al., 2016), which can be associated with low back pain (LBP) inflight but is short-lived and has been reported in 70% of astronauts without a history of LBP and 100% of those with a history of LBP (Pool-Goudzwaard et al., 2015). (4)
The effects of microgravity on the intervertebral disc must be considered to allow safe re-loading of the spine postflight, as the astronaut must readapt abruptly to gravity on return to Earth.(4)
The incidence of herniated nucleus pulposus is 4.3 times higher in astronauts than in terrestrial populations, predominantly in the period immediately following return to Earth (Johnston et al., 2010).(4)
Microgravity-induced bone loss in humans:
Microgravity-induced bone loss has been suggested to be more severe than osteopenia on Earth, and prolonged exposure to unloading conditions can raise the risk of osteoporosis and bone fractures.(5)
The first observation of µG-induced bone loss was recorded in the mid-1970s, when Skylab crew members demonstrated the loss of 1–2% bone mass per month compared to pre-flight and ground controls(2)
Bone cells readjust their behaviors—the cells that build new bone slow down, while the cells that break down old or damaged bone tissue keep operating at their normal pace so that breakdown outpaces growth, producing weaker and more brittle bones.(3)
Bone density loss in microgravity (skeletal unloading) is a well-documented crew health concern since the Skylab mission, when it was observed that the flight crew had about 1-1.5% mineral loss per month. This was noted as being “significantly faster than normal osteoporotic individuals.”(6)
For every month in space, astronauts’ weight-bearing bones become roughly 1% less dense if they don’t take precautions to counter this loss. Muscles, usually activated by simply moving around on Earth, also weaken because they no longer need to work as hard. This loss of bone and muscle is called atrophy. (3)
The weightlessness experienced in microgravity reduces the loading on weight-bearing bones, resulting in adaptive changes that increase bone resorption and inhibit bone formation.(2)
As bone formation is reportedly unchanged or decreased, this results in an overall negative calcium balance that contributes to bone loss in space.(2)
Loss of calcium from the skeleton, increases the risk of kidney stones and bone fracture (both during the mission and potentially as a lifelong consequence).(6)
Space flight induced a loss of calcium which accelerated exponentially from about 50 mg/d at the end of 1 week to approx. 300 mg/d at the end of 12 weeks.(7)
The severity of bone loss also increases with spaceflight duration, and the time required for recovery to pre-flight BMD levels is reportedly longer than the actual mission.(2)
The data up to now suggest that bone loss is site-specific, and that there is no complete recovery after return to Earth.(8)
Bone loss was noted within the first month at zero gravity, whereas bone recovery was very slow at normal gravity on Earth.(8)
Reduced mechanical use, because of hypodynamia (decreased forces) and hypokinesia (fewer movements), is thought to be the main factor leading to bone loss in space. (8)
Microgravity has been thought to induce osteoporosis because of reduced weight-bearing.(8)
Muscle atrophy and Skeletal Deconditioning
µG decreases the effort required for movement, while causing mass fluid redistribution. As a result, muscles in the arms and legs experience atrophy, Exposure to µG also results in skeletal deconditioning, where significant reductions in bone mass increases the risk of fractures and osteoporosis, threatening the viability of long-duration missions and astronauts’ mobility upon return to Earth.(2)
Through data collected from astronauts, as well as animal and cellular experiments conducted in space, it is evident that microgravity induces skeletal deconditioning in weight-bearing bones.(2)
Bone segments mainly involved in locomotion and weight-bearing functions on Earth were particularly sensitive to bone loss.(8)
Bone mass loss is probably not linear. These losses cannot be attributed to any known factors at present. No relation was found with any of the potential confounding variables, including time previously spent in space.(8)
At the end of the first month, peripheral Quantitative Computed Tomography (QCT) measurements indicated a slight decrease of trabecular bone mass in the distal tibial metaphysis. However, after 6 months of spaceflight, a more marked loss of trabecular and cortical bones was observed in the tibia, and was still significant after 6 month recovery in the trabecular compartment, whereas a decrease was no longer observed in the cortical envelope. (9)
The lower weight-bearing bones appeared more sensitive than the upper ones in terms of spaceflight-induced bone loss.(9)
Role of Intensive physical training
Each astronaut aboard the space station engages the muscles, bones, and other connective tissues that comprise their musculoskeletal systems using Earth-like exercise regimens. Crews exercise for an average of two hours a day.(3)
Astronauts have biked on stationary bicycles and run on treadmills in space for decades.(3) Some of them are :
TVIS, a treadmill with a harness to keep the user tethered to the machine and add some gravity-like force.(3)
ARED ( Advanced Resistive Exercise Device )allows astronauts to mimic weightlifting in microgravity. ARED Kinematics analyzes how muscle strain, bone stress, and other internal factors affect the body while exercising in microgravity(3).– This device, while similar to the Interim Resistive Exercise Device (iRED), is capable of higher concentric resistance and eccentric-to-concentric ratio close to that recommended by expert panels and confirmed effective by exercise scientists. The aRED also collects data regarding the parameters associated with crew exercise and transmits it to the ground.(6)
Treadmill 2 (COLBERT) – An exercise treadmill that can also be used to collect data such as body loading, duration of session, and speed for each crewmember.(6)
Cycle-Ergometer with Vibration Isolation System (CEVIS) – A structurally isolated aerobic exercise cycle that serves as a countermeasure to cardiovascular deconditioning on orbit.(6)
Unfortunately, these machines are too large to bring aboard a spacecraft for long duration space flight.
Scientists are curious: Could exercises using minimal or no equipment provide adequate physical activity while taking up less room? How does NASA prepares their astronauts for it's mission?
Much research focuses on determining the right combination of diet, exercise, and medication to keep astronauts healthy during missions and when they return to Earth or set foot on the Moon or Mars.(3)
Understanding how to prevent and treat muscle atrophy and bone loss is particularly important as NASA plans missions to the Moon and Mars. Once they arrive, astronauts may need to perform strenuous activity in partial gravity after a long time in near weightlessness.(3) Understanding how the body experiences exercise in microgravity.
Full-body exercise affects the entire musculoskeletal system.(3)
Measuring the body during space workouts can help scientists understand how astronauts need to adapt exercises in microgravity to preserve and optimize their health during long duration spaceflight missions.(3)
Researchers found that pre-flight exercise training helps in the following:
-improves performance on station, just as pre-season training helps athletes in later competition.
-helps in determining optimal exercise programs to prepare astronauts before a mission
-limit the effects of microgravity during a mission, and
-enables safe and rapid recovery postflight.(3)
ROLE OF PHYSICAL THERAPY
At the European Space Agency (ESA), a physiotherapist plays a key role in the multidisciplinary ESA team responsible for astronaut health, with a focus on the neuro-musculoskeletal system.(4)
The physiotherapist prepares the astronaut for spaceflight, monitors their exercise performance whilst on the International Space Station (ISS), and reconditions the astronaut when they return to Earth.(4)
The physiotherapist and sports scientist are jointly responsible for the neuro-musculoskeletal health of astronauts.(4)
Three stages of conditioning and reconditioning during a mission cycle are:
Pre-conditioning
This is to provide the astronaut with exercise programs to prepare for a mission.
2. Counter-measures:
This limits the effects of microgravity during a mission and
3. Reconditioning:
This includes postflight retraining to enable safe and rapid recovery.
As the astronaut's proprioceptive feedback is decreased, the lumbar lordosis is flattened and the discs are hyperhydrated, precise positioning of the spine is challenging. In the absence of being able to correct the position manually, an important feature of the inflight programme is the physiotherapist providing real-time feedback to the astronaut by audio/video conference.(4)
The ability to control the position and motion of the trunk over the pelvis is important to allow optimum production, transfer and control of force and motion to the terminal segment Kibler et al., 2006). (4)
The correct performance of an exercise is as equally important in microgravity as it is on Earth, particularly to avoid injury when using exercise equipment.(4)
Assuming poor postures, such as flexed spinal positions in front of computers, induces decreased use of spinal extensor muscles, as seen in astronauts.(4)
ESA suggests the following (4):
Motor control | Balance and coordination trainings, their progressions and increasing the demands, complexity and intensity to these exercises | Successful and complete integration of postural optimization into functional movement, |
Postural control and alignment | Approximation of functional exercise movements. which includes endurance (with low body weight) | Bone loading |
Re-establishing good patterns of pain free normal movement. | Resistance training, | Achieving complex balance/movement stability |
Integrating motor control exercise (MCE) of multifidus and transversus | Extended 3D-spatial orientation (moving in larger radius) | Enable crew to return to complex and intense physical exercise |
Reorganizing all stabilizer muscles by trunk exercises6. | Construction of target movements | Crew physical functional empowerment |
Posture correction (closed chain exercises) | Training of muscular strength (incl. stability) | Undertake a “normal” pre-flight exercise programme |
Tonus normalization (axial training) | Resistance exercises: Squat, Back extensions/deadlift, construction of target movements- Hopping exercises (plyometric training prep/controlled) | Martial arts fitness |
Isolated training of stabilizers and integrating the stabilizer muscle system into complete muscle corset | Integration of optimized posture and movement strategies into more complex functional (athletic and daily life) motion | Endurance (Running, Cycling, Swimming) |
Muscle balance | Achieving target exercises/final target movement performance | Plyometric exercises (controlled) |
Active postural control | Body loading(Loaded squats/deadlifts/seated row), |
What contributes to bone loss in microgravity?
Data suggests a contribution of additional mediators to bone loss in microgravity. Over the 50 years of space travel, many factors, including altered calcium homeostasis, stress, altered metabolism and radiation have been suggested to contribute to bone loss in astronauts.(10)
Can we prevent bone loss in microgravity?
Countermeasures such as exercise, adequate nutrition, and medications have been recommended or required in order to prevent demineralization, especially during long-duration missions such as planetary and deep-space exploration.(6)
It is critical for crewmembers to have frequent access (potentially multiple daily sessions) to exercise equipment that can provide high levels of loading, and diversity in load application, on the skeletal system. These exercise countermeasures should be targeted primarily toward protecting the lower body and hip regions.” It has been observed that the areas of most concern for skeletal unloading are in the lower areas of the body (i.e., hips and trochanter).(6)
There is a consensus among exercise scientists that both endurance (aerobic) and resistance exercises are needed as countermeasures to maintain overall crew health and performance during and after spaceflight.(6)
Resistance exercise had a positive treatment effect and thus might be useful as a countermeasure to prevent the deleterious skeletal changes associated with long-duration spaceflight.(11)
An exercise countermeasure has the advantage of benefiting multiple body systems (musculoskeletal, cardiovascular, immunological) and can be targeted to those body regions needing protection.(6)
Countermeasures should be used to “mitigate undesirable physical, physiological, and psychological effects of space flight upon crewmembers.”(6)
The crew is required to exercise a minimum of time dependent on the program mission as dictated by the medical team, however previous requirements have been as little as 2.5 hours per workday with a strict exercise program.(6)
The food lab and nutritionists have developed appropriate nutritional foods to ensure that the crew have enough micro- and macronutrients to promote crew mental and physical health.(6)
Molecular therapies against microgravity-induced bone loss
The use of cellular models has shed light into the molecular mechanisms behind µG-induced bone loss, which in turn provides potential targets for therapeutic intervention.(2)
Scientists report that a drug called BP-NELL-PEG "successfully diminished" spaceflight-induced bone loss without causing any side effects in mice on the International Space Station (ISS). The treated mice showed increased bone density, while the bone density of untreated mice on the ISS declined significantly. (12)
"Our findings hold tremendous promise for the future of space exploration, particularly for missions involving extended stays in microgravity," co-author Dr. Chia Soo, a plastic surgeon at UCLA Health, said in a statement. (12)
BP-NELL-PEG is a tweaked version of the protein NELL-1. In animal studies, the protein has been shown to boost the activity of cells that form bone tissue, while inhibiting the cells that break bone down and jump-starting processes involved in bone repair. The drug is also set to be trialed in humans to treat adults undergoing surgery for degenerative disc disease, in which the discs of the spine wear out over time. (12)
Can one recover from microgravity induced bone loss?
Several studies reported that 2–5 years are required to recover microgravity-induced bone loss and that in some individuals the complete recovery was not achieved.
The active recovery phase, when bone resorption was suppressed and bone formation was active, appears to be limited to 6 months post-flight, much shorter than the time required for bone mass to return to pre-flight values.(10)
Researches:
From the data presented it is concluded that the process of demineralization observed in space flight is more severe than would be predicted on the basis of observations in immobilized, bed rested, or paralyzed subjects. It is, moreover, suggested that the process may not be totally reversible.(7)
A resistance exercise protocol which includes eccentric as well as concentric exercise, particularly when the eccentric exercise is emphasized, appears to result in greater strength gains than concentric exercise alone. Findings suggest eccentric exercise may be an important component of the in-flight resistance exercise protocol for long-duration spaceflight.(13)
One signaling pathway that plays an important role in maintaining muscle and bone homeostasis is that regulated by the secreted signaling proteins, myostatin (MSTN) and activin A. A signaling using a soluble form of the activin type IIB receptor (ACVR2B), which can bind each of these ligands, led to dramatic increases in both muscle and bone mass, with effects being comparable in ground and flight mice. Findings have implications for therapeutic strategies to combat the concomitant muscle and bone loss occurring in people afflicted with disuse atrophy on Earth as well as in astronauts in space, especially during prolonged missions.(14)
Drugs used to prevent bone loss on Earth, such as myostatin inhibitors, also may successfully prevent bone and muscle loss in astronauts in space.(3)
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Sources
Effects of microgravity on bone structure and function: https://www.nasa.gov/learning-resources/for-kids-and-students/what-is-microgravity-grades-5-8/#
The effects of microgravity on bone structure and function: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8983659/
Counteracting Bone and Muscle Loss in Microgravity, https://www.nasa.gov/missions/station/iss-research/counteracting-bone-and-muscle-loss-in-microgravity/
https://www.mskscienceandpractice.com/article/S1356-689X(16)30754-8/fulltext#secsectitle0065
https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-030-bone-loss.pdf
Prolonged weightlessness and calcium loss in man, science direct, Volume 6, Issue 9, September 1979, Pages 1113-1122.
Effects of long-term microgravity exposure on cancellous and cortical weight-bearing bones of cosmonauts, thelancet.comjournals VOLUME 355, ISSUE 9215, P1607-1611, MAY 06, 2000
Effects of 1- and 6-month spaceflight on bone mass and biochemistry in two humans, science direct, Volume 20, Issue 6, June 1997, Pages 547-551
A systematic review and meta-analysis of bone loss in space travellers https://www.nature.com/articles/s41526-020-0103-2
Resistance exercise as a countermeasure to disuse-induced bone loss https://pubmed.ncbi.nlm.nih.gov/15220316/
Strength gains following different combined concentric and eccentric exercise regimens, https://pubmed.ncbi.nlm.nih.gov/12688453/
Targeting myostatin/activin A protects against skeletal muscle and bone loss during spaceflight, https://pubmed.ncbi.nlm.nih.gov/32900939/
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