Why can't astronauts walk after landing?

In weightlessness, the load that legs, hips and spine constantly work against on the ground disappears. The body economises: the large extensor muscles lose mass and strength, weight-bearing bones lose minerals, and the heart no longer needs to pump blood upward. Fluid distribution also changes: part of the blood volume shifts toward the upper body and the total volume decreases. This is not an illness but a sensible adaptation to an environment without up or down, and it is precisely that adaptation that becomes a problem on return.

When the capsule lands, full Earth gravity hits this reconfigured system all at once. At first, the circulatory system cannot maintain blood pressure either sitting or standing: dizziness, rapid heartbeat, blurred vision. That is why returning astronauts are carried out of the hatch, seated and given medical attention, rather than standing up in front of the cameras. On top of this comes the inner ear: the balance organs have gone months without any "down" signal, and now send information that does not match what the eyes see. The ground seems to sway, and turning the head causes nausea.

Rehabilitation begins the same day as landing and follows a fixed programme: first balance and coordination, then strength, and finally endurance. Balance usually recovers within a few days; muscles and the circulatory system need weeks, and bone recovery takes the longest. To make sure some of this recovery starts early, crews train daily in orbit on a treadmill, an exercise bike and a resistance device. Training does not prevent the deterioration, but it keeps it within a range the body can recover from.