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when you and I walk around on the earth

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Earth's gravity plus our body weight

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pulls us down to the earth and creates a

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force that's felt by our bones and

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muscles. the cells in our bones and

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muscles sense this force and use it as a

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stimulus to maintain bone and muscle

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health. in spaceflight there is an

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absence of gravity and the stimulus is

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absent. to reintroduce this we have to

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have them exercised on specialized

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equipment so we have to mechanically

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introduce the loading that's associated

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with Earth's gravity. we do this by

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having the astronauts wear a harness and

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be connected to a treadmill through

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bungee cords when they run on a

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treadmill on the International Space

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Station and we use specialized