There have been many long duration Russian expeditions, mainly for studying effects of prolonged weightlessness on the human body. Weightlessness is the major problem with living in space. The space station crews have adjusted to living a weightless life without too much difficulty, but their bodies haven’t. Since the human body has evolved on Earth, it is made to work in conditions where gravity pulls on it. This is especially true in case of bones which need gravitational stress to retain their density replenishing lost minerals. During extended spaceflights, all astronauts’ bones have lost calcium and become brittle.
In the early days of manned spaceflight some American experts believed that men can not stay in weightless condition for more than nine months before their bones become so brittle that returning to Earth would be hazardous. This is because their weakened bones would not be able to support their body weight under normal gravity. The experts’ apprehension was unfounded – at least to some extent. Some Russian cosmonauts have remained in space for more than a year. They have stayed healthy on a diet of 3,000 calories a day and plenty of exercise on a cycle unit and treadmill. After coming back to Earth, however, they could not manage to walk properly for some days.