Exercise during space flight requires specially-designed equipment.
Weightlessness experienced by space travelers produces short-term as well as long term effects on body systems. Organ systems that are particularly affected include the musculoskeletal, cardiovascular, urinary, and nervous systems. For these reasons, astronauts must exercise during space flight. Exercises performed in space include aerobic activities such as running, and anaerobic activities such as resistance training.
Physiological Changes during Space Flight
Bones lose calcium during periods of weightlessness.
Weightlessness causes skeletal unloading, which is to say elimination of most of the forces on bones and muscles. This results in muscle atrophy and loss of bone calcium. Shifting of body fluids toward the head puts unusual stresses on the heart, and results in loss of blood volume. As fluids are excreted, calcium released from bones can cause kidney stones. Since blood volume is lowered and the body's ability to move blood from the legs to the heart is reduced, astronauts returning to earth may experience orthostatic intolerance. This means that the blood pressure in the upper body, including in the brain, drops when the astronauts are in a standing position, sometimes resulting in fainting. Additionally, neuromuscular control is disrupted in weightlessness, due to changes in forces acting on moving limbs and on the vestibular system of the inner ear. Exercise serves as a countermeasure to many of these changes, reducing (though not eliminating) the chance of such problems arising upon the return to earth.
Exercise Countermeasures used during Spaceflight
Weights and dumbbells are useless when exercising in a weightless environment.
Although astronauts experience nearly the same pull of gravity during spaceflight as they do on earth, orbital flight puts them into "free-fall" or weightlessness. Thus, space exercise equipment is not designed to simulate effects of gravity, but the effects of weight. For resistance exercise simulating the effects of weight, astronauts use devices which exert force using elastic components. As a countermeasure to the headward shift of blood and other fluids, exercise is sometimes performed in a device which provides negative pressure to the lower extremities. Essentially, lower body negative pressure (LBNP) pulls blood into the legs, so that the heart must work harder to pump blood in the other direction, and so that valves in the inner veins of the legs which help blood to move "up" do not deteriorate in their function. Aerobic exercise is performed on stationary bicycle devices or on treadmills to which the astronaut is held with bungee cords.
Exercise in Space Flight with Artificial Gravity
Artificial gravity using rotation of a small centrifuge can produce vertigo.
Various devices and exercise techniques are under development for use in combination with artificial gravity. By rotating an astronaut in a centrifuge, his or her weight can be restored, completely or in part, while in space. However, unless rotated around a large circumference, such centrifuges produce forces on moving extremities and on the vestibular apparatus of the inner ear. In the latter case, this leads to vertigo and nausea if the head is moved during rotation.
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