Often, when we think of astronauts in space, we picture them floating around gracefully (much like ballet dancers but in slow motion) across the never-ending emptiness of space, marveling at the unworldly views of the orbiting planets and the glamorous galaxies. We imagine weightlessness, bodies gliding with ease and Freedom. However, behind that elegance is a hidden cost, and a quite hefty one; the very absence of gravity or microgravity imposes an extremely harsh toll on the human body.
How do Bones and Muscles Function under Normal Conditions?
Quite like many specialized cells in the body, bones and muscles are in a state of dynamic turnover: bones involve 3 cells mainly, osteoblasts that build bone, osteocytes that are mature bone cells responsible for regulating the balance between bone formation and bone resorption, and osteoclasts that break down damaged bone cells.
In contrast, muscle fibers don’t have different cells but repair and grow in a process called Muscle Protein Synthesis (MPS) while old or damaged muscle fibers are degraded and expelled from the body by a process known as Muscle Protein Breakdown (MPB), keeping the body in homeostasis. On Earth, our muscles stay in shape because they are under constant gravitational effect.
Every time we stand, walk, sit upright, hold out an arm, or lift a leg, our muscles have to contract to support our body weight. These repeated contractions that occur every time we make even a slight movement act as a natural workout against gravity. Similarly, gravity also pulls on our bones and joints, and that pressure is what keeps our musculoskeletal system strong and efficient.
Thus, under normal conditions, the consistent resistance provided by our own body against gravity is what keeps the muscles and bones intact. This exertion of pressure or weight is called mechanical loading; bones and muscles add mass when we put some stress on them, and atrophy or deteriorate when we remove that mechanical stress.
What is Microgravity?
The term microgravity, sometimes also referred to as zero gravity, might be a bit misleading, as it hints at there being “less or no gravity in space”. Contrary to popular belief, space is not completely devoid of gravity. In fact, gravity, a force between any two masses, is present everywhere: between the moon and the Earth, the planets and the Sun, and between the stars in a galaxy.
The weightlessness that astronauts seem to experience is not because there is no gravity in space, but because they are in a state of free fall – the astronauts and their surroundings, i.e., the spacecraft, are all falling together under gravity’s pull. This is microgravity.
The Shrinking Strength of Bones and Muscles in Space
During spaceflight, due to being in constant free fall, the musculoskeletal system does not experience any sort of compression or tension; instead, it retracts to a ‘mechanical rest state’. Muscles don’t need to work as hard to support the body’s weight; thus, as a result of this reduced mechanical loading, they start to ‘rust’ and wither away.
In 1994, an experiment using bioartificial muscle cells was cultured to determine whether muscle deterioration was intrinsic to muscle fibers themselves or related to other factors such as growth hormone levels. It was then flown into space, on the Space Shuttle Atlantis, mission STS-66, for a span of 9 to 10 days. Two years later, in May 1996, a follow-up was performed on Space Shuttle Endeavour, mission STS-77.
In technical terms, the findings of this experiment were that the anabolic process of Muscle Protein Synthesis (MPS) is relatively less active in space than the catabolic process of Muscle Protein Breakdown (MPS), hence resulting in a direct net loss of muscle fibers. This shows that microgravity directly affects muscle physiology and induces muscle atrophy, rather than being a secondary effect.
Similarly, bone homeostasis is also severely affected in long durations of spaceflight. According to NASA, there is a 1-2% reduction in bone density per month spent in space. For some context, the most common crewed space expeditions are the ISS missions, which typically last around six months.
Under microgravity, osteocytes, the cells that modulate the processes of bone formation and bone resorption, experience reduced connectivity, which in turn affects bone remodeling, while also going into a state of programmed cell death, also known as apoptosis.
To put it simply, the lack of weight on bones results in a higher rate of bones being broken down by osteoclasts, while the rate of bone formation by osteoblasts is greatly reduced. This results in an overall net loss of bone mass and density, particularly in weight-bearing bones such as the femur and the spine, making them more porous and hence, more susceptible to injuries and fractures.
Clinically, this is termed as ‘osteoporosis’ – a chronic condition in which the balance of bone making and degrading is adversely affected. Space-induced osteoporosis is far more rapid and severe than on Earth: an astronaut who has spent just six months in space will have symptoms similar to those of an osteoporosis-affected elderly woman on Earth.
Other Effects of Microgravity on the Human Body
As bone cells are broken down, calcium previously stored in bones is released into the bloodstream; an excess of calcium can cause kidney stones. Potassium citrate is often prescribed as a suitable preventive measure for this complication.
Moreover, fluid inside the body shifts upward due to the lack of force pulling it down, which may lead to nasal congestion, sinus pressure, and vision problems. In order to counter this, astronauts wear compression cuffs on their thighs, which control fluid shifts in the body.
Current Protective Measures and Potential Future Solutions
Astronauts on ISS missions exercise for about 2 hours daily in order to maintain the strength of their muscles and bones and mitigate the effects of microgravity. Exercising machines like the TVIS treadmill, which has a harness that keeps the person attached to the machine during exercise and mimics gravity by doing so, and the Advanced Resistive Exercise Device (ARED), another machine that enables astronauts to do weightlifting in space, are indeed useful inventions in this matter. However, these machines are too heavy to take on a long-term space mission.
A lot of experimentation is currently being conducted to study countermeasures for the complications of bone and muscle atrophy, such as Vertebral Strength, an experiment that studies scans of astronauts’ muscles and bones pre-flight and post-flight, providing researchers with a comparison for further study.
References:
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3743123/
- https://www.nasa.gov/humans-in-space/the-human-body-in-space/
- https://www.nasa.gov/learning-resources/for-kids-and-students/what-is-microgravity-grades-5-8/
- https://pubmed.ncbi.nlm.nih.gov/10336885/
- https://frpmc.edu.pk/academicResearcher/assets/pdf/volume2%20issue1%202025/8-%20Kashif%20et%20al.pdf
- https://www.nasa.gov/missions/station/iss-research/counteracting-bone-and-muscle-loss-in-microgravity/
- https://www.esa.int/Enabling_Support/Preparing_for_the_Future/Space_for_Earth/Space_for_health/Musculo-skeletal_system_Bone_and_Muscle_loss
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Syeda Ayesha Kakakhel is an undergraduate mathematics student at NUST with a strong passion for science communication. She is interested in learning about the intersections of different fields of science, particularly the connections between medicine, astrophysics, and data science. She reads historical and contemporary fiction and writes both scientific and non-scientific pieces.
