Human Bodies in Space

On earth, gravity pulls down on us. It keeps the bones close together at their joints. But in space, there is no gravity pulling down on the bones and joints. The joints of astronauts spread out a little more in space, and fluid goes between the joints in the spine. This makes the astronauts about two or three inches taller in space than they are on Earth! They do not stay tall forever. Once astronauts come back to Earth, they shrink back to normal height because of the gravity.

Have you seen athletes who train with weights and barbells? Athletes lift weights so their muscles will get stronger by lifting up against something heavy. On Earth, our bones and muscles have to work against gravity to lift our arm over our head, to sit up straight after lying down, to bend our knees to start walking, and even to hold our heads up! In a way, working against gravity to lift our arm is exercise that helps to keep our muscles strong and in shape.