Lack of gravity causes you to use less work to move your muscles.
Explanation:
If on Earth your arm weighs 3 pounds, the muscle will only have to move 3 pounds of force. For example, if you were to go on the moon, your arm would have the same mass but there would be less gravity pulling it dow. So now your muscles only need to pull 1 pound. Over extended periods, you would be working less, thus not using your muscles to fullest potential and in result you would get weaker.
Because astronauts work in a weightless environment, very little muscle contraction is needed to support their bodies or move around. Without regular use and exercise our muscles weaken and deteriorate. It's a process called atrophy.
