Why do Astronauts who stay in space for a long time lose muscle and bone mass?

There is growing evidence that exercise can help astronauts recover from muscle and bone losses and limit such losses.

In the near absence of gravity, bone mass decreases in the load-bearing regions of the skeleton, and muscles shrink when not used. But Michael Foale, who was on the Russian space station Mir for four and a half months, said that he had been able to recover most of the bone density and all of the muscle he lost on Mir through weight lifting, running, and

Muscle recovery took about six months; bone rebuilding, to up to 99 percent of his preflight levels, took a year.

After earlier long flights, astronauts often had trouble recovering. On the 184-day Skylab 4 mission in 1974, astronauts lost 4.5 percent to 7 percent of their bone mass and were still down about that much 5 to 7 years later.

When both Shannon Lucid and Norman Thagard spent long periods aboard Mir, he lost up to 11.7 percent of his bone mineral and 17.5 pounds of overall muscle and weight, while she maintained her weight and lost less muscle mass and bone mineral than he did, even though she was in space 73 days longer.

Dr. Lucid exercised more; she put in hundreds of hours on Mir’s treadmill and stationary bicycle.