Astronauts move in slow motion because their brains are tricked into thinking their bodies have lost all their mass in space.

Brains in microgravity consistently underestimate body weight, which leads to reaching movements that are too weak and too slow. The internal model of the body fails to update correctly even after several weeks of living in space. This persistent bias forces astronauts to put in extra effort to move their limbs through the air. Most people expected the brain to adapt quickly to a weightless environment and recalibrate its force. This failure to adjust means that space missions must account for a permanent drop in physical coordination and speed for long-term travelers. It reveals a fundamental limitation in how the human brain perceives its own physical presence.