Space is a microgravity environment, which means there is almost no gravity for muscles and bones to work against. So, when an astronaut wants to move her arm, her arm bones and muscles are not working against gravity like they do on Earth. Because the astronaut does not have to work her muscles against gravity, her muscles can weaken in space. Astronauts have to do special exercises to keep their muscles strong while in space. If they didn’t, their arms and legs would feel like jelly when they landed back on Earth and they might not be able to stand!

Astronauts' bones become thinner and less dense the longer they stay in space. Astronauts on the International Space Station tend to lose about two percent of their body's total bone mass for each month they spend in space. That's because bone is living tissue. Bones are always being broken down by some cells and rebuilt by others. Without gravity and regular exercise, that balance is upset and more bone is lost than is gained. Eventually, a loss of bone density can lead to bone fractures and breaks, especially when they come back to Earth where gravity puts a strain on their weaker skeleton.

Astronaut Terence Hendricks exercising in space. Picture from NASA.