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The human body is adapted to Earth's gravity, and the lack of gravity in space causes multiple health problems, including muscle atrophy. This is because the anti-gravity muscles that allow us to stand and walk are not used as much in a weightless environment. As a result, astronauts can lose up to 20% of their muscle mass within 5-11 days of being in space. This loss of muscle mass corresponds to a loss of strength, which can be dangerous if an astronaut needs to perform a strenuous activity during an emergency. To prevent muscle atrophy, astronauts must engage in intensive exercise, particularly strength training, for about two hours a day.
| Characteristics | Values |
|---|---|
| Cause | Lack of continuous load of Earth's gravity |
| Effect | Loss of bone and muscle mass, cardiovascular dysfunction, impaired fracture healing, impaired immune response, vestibular disorders in the ears |
| Muscles Affected | Gastrocnemius (calf muscles), quadriceps, back and neck muscles |
| Loss of Muscle Mass | Up to 20% on spaceflights lasting 5-11 days |
| Loss of Bone Mass | 1.5% per month in zero gravity |
| Prevention | Intensive exercise, particularly strength training, combined with an adequate diet |
| Treatment | Drugs used to prevent bone loss on Earth, such as myostatin inhibitors |
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What You'll Learn
Anti-gravitational muscles are not used as much in zero gravity
The human body has evolved to function under the constant influence of Earth's gravity. Gravity provides the loading forces that stimulate cells and tissues, and these mechanical stimuli are essential for the functioning of many physiological systems.
In a zero-gravity environment, the body experiences a lack of mechanical stimulation, which can lead to several health issues. One of the most notable effects is muscle atrophy, particularly in the anti-gravitational muscles. These anti-gravitational muscles, including the gastrocnemius (calf muscles), quadriceps, and muscles of the back and neck, are responsible for maintaining posture and allowing movement against the force of gravity.
In the absence of gravity, these muscles are not used as much, and their disuse can lead to atrophy. Without the weight load on the back and leg muscles, they begin to weaken and shrink. In some cases, degeneration can be rapid, with astronauts potentially losing up to 20% of their muscle mass within 5 to 11 days. This atrophy is caused by a decrease in neural activity and mechanical stress, resulting in a reduction in the number of sarcomeres (the structural units of muscles) and a decrease in force development.
To counteract the effects of reduced gravity, astronauts engage in intensive exercise regimens, including strength training, treadmill workouts, and resistance training. Additionally, studies are being conducted on the use of electrical muscle stimulation to maintain muscle strength and mass during long-duration space missions.
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The heart degenerates, causing decreased blood pressure and oxygen flow
The human body has evolved to function under the influence of gravity. As a result, the lack of gravity in space can cause several health issues, including muscle atrophy and bone loss. The heart is also affected by the lack of gravity, and this can have serious consequences.
The heart is a muscle, and like other muscles in the body, it is affected by the lack of gravity in space. With no weight load, the heart muscle weakens and shrinks, leading to degeneration. This degeneration can cause a decrease in blood pressure and may hamper the flow of oxygen to the brain. This is a serious issue, as oxygen is crucial for the proper functioning of the brain and other organs.
The heart's degeneration is a result of it having to pump less blood in a zero-gravity environment. Fluids tend to accumulate at the top of the body, reducing the amount of blood that needs to be pumped back up to the heart and brain. This accumulation of fluids can also blur an astronaut's vision until the brain learns to compensate and correct the image.
To counteract the effects of zero gravity on the heart and other muscles, regular resistance training is essential. Astronauts aboard the International Space Station (ISS) are required to perform at least two hours of physical activity per day, including jogging on a treadmill, riding a stationary bicycle, and lifting weights. These exercises help to maintain bone and muscle mass and reduce the risk of health complications due to muscle atrophy.
The negative effects of zero gravity on the heart are a concern for long-term space missions, such as those to Mars or asteroids. Researchers are working to understand how the human body reacts and adapts to the harsh space environment to develop measures to prevent or counteract the loss of muscle mass in the heart and other muscles.
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Bone and muscle mass loss
Exposure to zero gravity or microgravity causes bone and muscle mass loss. This is due to the lack of continuous load from Earth's gravity, which causes bone and muscle tissues to atrophy. Bone cells readjust their behaviours: the cells that build new bone slow down, while the cells that break down old or damaged bone tissue continue to operate at their normal pace, resulting in weaker and more brittle bones. Bones may lose up to 1.5% of their mass in a month in zero gravity, compared to about 3% a decade on Earth. This bone loss mainly affects the lower vertebrae of the spine, the hip joint, and the femur.
Muscles, which are usually activated by simply moving around on Earth, weaken in zero gravity because they no longer need to work as hard. Some muscles degenerate rapidly, and without regular exercise, astronauts may lose up to 20% of their muscle mass within 5 to 11 days. This loss of muscle mass is caused by a decrease in neural activity, as well as contraction- and/or stretch-dependent mechanical stress. This results in impaired coordination of antagonist muscles and altered mechanics, leading to walking difficulties.
To counteract bone and muscle loss in zero gravity, astronauts engage in resistance training and other exercises that stimulate the muscles, bones, and other connective tissues that comprise their musculoskeletal systems. Astronauts aboard the ISS are required to use treadmills, bicycle ergometers, and resistance training equipment. One study suggests that astronauts should walk or slowly run with rear foot-strike landing to stimulate the soleus muscle adequately and reduce chances of its atrophy. Another study, called Zero T2, aims to find out if exercises using minimal or no equipment could provide adequate physical activity while taking up less room.
Drugs may also be used to prevent bone and muscle loss in astronauts. For example, myostatin inhibitors, which are used to prevent bone loss on Earth, may also successfully prevent bone and muscle loss in astronauts and animal models in space.
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Changes in muscle structure and properties
In a zero-gravity environment, the human body experiences a loss of mechanical stimulation of cells and tissues. This results in a decrease in neural activity, as well as mechanical stress caused by contraction and/or stretch. The anti-gravitational muscles, which are responsible for maintaining posture and allowing movement against the force of gravity, are particularly affected.
Inhibiting anti-gravitational muscle activities results in the remodelling of sarcomeres, which are the structural units of muscles. This leads to a decrease in their number, causing a reduction in force development and eventual muscular atrophy. The soleus and adductor longus muscles are specifically impacted, with a reduction in the amplitude of electromyograms.
The effects of zero gravity on muscle structure and properties are not limited to the number of sarcomeres. Gravitational unloading causes deterioration in motor control, resulting in impaired coordination of antagonist muscles and altered mechanics. Walking difficulties have been observed in crews after spaceflight, despite regular exercise on the International Space Station (ISS). This indicates that exercise-based countermeasures may not be sufficient to prevent all neuromuscular changes.
Research has shown that rapid volumetric skeletal muscle loss occurs in the quadriceps, gastrocnemius, and posterior back muscles following 6-9 days of spaceflight. This loss is predominantly observed in the lower limb and trunk muscles due to their role in ambulation and postural support under standard gravity.
To summarise, zero gravity causes changes in muscle structure and properties, including the remodelling of sarcomeres, decreased neural activity, impaired motor control, and volumetric muscle loss. These changes can lead to muscular atrophy and walking difficulties, impacting the physical health and performance of astronauts.
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Muscle atrophy prevention methods
Zero gravity or microgravity causes muscle atrophy by inhibiting anti-gravitational muscle activities. In the absence of gravity, there is no weight load on the back and leg muscles, so they begin to weaken and shrink. This results in a loss of muscle mass and strength.
- Exercise and physical activity: Astronauts aboard the ISS are required to use treadmills, bicycle ergometers, and resistance training equipment to counter the effects of reduced gravity on the neuromuscular system. Periodic passive stretching of the soleus is also effective. On Earth, seniors are encouraged to exercise safely with simple exercises like practicing getting off the floor by themselves to improve flexibility, balance, coordination, and muscle power.
- Stimulation of the soleus muscle: Adequately stimulating the soleus muscle can reduce the chances of its atrophy. While exercising, astronauts should walk or slowly run with rear foot-strike landing (using a bungee cord can help).
- Nutrition: A long-term ketogenic diet has been found to alleviate ageing-induced sarcopenia in mice by improving mitochondrial function and antioxidant capacity. Dietary supplementation with L-carnitine reduces muscle damage, soreness, and cellular damage. Omega-3 polyunsaturated fatty acids (omega-3 PUFA) have been shown to reduce the development of sarcopenia in the elderly by positively regulating intracellular metabolic signalling.
- Drugs and supplements: Magnesium lithospermate B has been found to prevent obesity-related skeletal muscle atrophy in mouse models. Puerarin, a flavonoid isoflavone extracted from pueraria, has been shown to enhance muscle strength and body mass in diabetic rats. Triptolide has been found to prevent skeletal muscle atrophy by preventing inflammation. Salidroside acts as an anti-inflammatory and antioxidant agent, inhibiting the production of pro-inflammatory cytokines and alleviating muscle atrophy. Baicalin, a flavonoid glycoside, has anti-inflammatory and anti-apoptotic effects.
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Frequently asked questions
Yes, zero gravity can cause muscle atrophy. In a zero-gravity environment, there is no weight load on the back and leg muscles, so they begin to weaken and shrink.
In a zero-gravity environment, the muscles, especially the anti-gravitational ones, are not used as much, resulting in their atrophy and changes to their structure and properties.
Muscle atrophy caused by zero gravity can be prevented through intensive exercise, particularly strength training exercises, combined with an adequate diet.
