Returning astronauts report extreme difficulty moving their arms and legs right after coming back, reason why they call it “the second birth.”
Long-term space missions can reduce muscle mass and strength, again caused by reduced gravity and again affecting largely the legs. Like the opposite of wearing a weighted exercise vest, low gravity means less strain to your muscles causing them to weaken.
The human body is designed to work in Earth’s gravity, which means that when astronauts return to Earth, their bodies have to readjust to the planet’s gravitational force.