Why do astronauts have to work out on the International Space Station?

When an astronaut spends a prolonged amount of time in space, he or she experiences the effects of microgravity and remains weightless during the whole trip. Instead of staying anchored to the floor like we do on Earth, astronauts float around much like swimmers do underwater, and they have to hold on to something if they want to stay stable.

Researchers have found that after spending weeks or months in a weightless environment, astronauts lose a significant amount of bone mineral density (BMD). The loss of BMD in the spine, neck and pelvis is about 1.0 to 1.6 percent per month, while cortical bone, the heavy, outer part of the bone found around the whole body and legs, experiences a loss of about 0.3 to 0.4 percent per month. For comparison, a healthy adult on Earth loses 3 percent of cortical bone structure over the course of a decade -- an astronaut could lose that much in less than a year in space.

The result of this bone loss is weakened bones that are more prone to fracturing upon returning to Earth. What's more, even after several years, the astronaut won't have recovered the same bone density he or she had before launch.

So why does something like this happen in space? Astronauts experience bone loss for the same reason chronically bedridden patients do: Their entire skeletons don't bear any weight. They go through a period called skeletal unloading, in which bones lose the ability to make new bone cells and replace old ones. The movement of important minerals such as calcium and phosphorus also slows down.

Although experts aren't sure exactly why this happens in microgravity, Dr. Roger K. Long, an endocrinology research fellow performing research for the National Space Biomedical Research Institute (NSBRI) is currently looking for this specific answer. He and his mentor, Dr. Daniel B. Bikle, believe there are three substances at play when astronauts undergo bone loss: insulin-like growth factor (IGF-1), a chemical produced in the bones that causes bones and cartilage to grow; IGF-1 receptor, which is found inside bone cells and allows them to react to IGF-1; and beta-3 intergrin, a protein that helps the IGF-1 receptor function. The researchers believe that during weightlessness, the body produces less beta-3 integrin, which makes it harder for the IGF-1 receptor to relay any messages from IGF-1 to the bone cells and tell them what to do. The result should be a decrease in bone production and an increase in bone loss.

What exercises do astronauts perform to reduce the risk of bone loss? And can they take any medicine to help? To learn more about the techniques and equipment used in space, read the next page.

There are three basic equipment devices astronauts use during spaceflights.

The treadmill on the International Space Station, formally called the Treadmill Vibration Isolation System (TVIS), is just like any other one on Earth, except it isn't connected to the station at all. It simply hovers around like the astronauts. This has three advantages: The weight of the station itself is less, there's a reduction in vibrations and the treadmill moves with the astronaut. Crew members still have to wear a harness and attach themselves to the treadmill; otherwise, their feet will simply push the machine away from them if they attempted to do any running.

Astronauts also use the Cycle Ergometer with Vibration Isolation System (CEVIS), which is essentially a mechanical bicycle. The CEVIS is actually bolted to the floor of the ISS, and astronauts strap their shoes into buckles and wear seat belts to hold themselves down. Finally, the Resistive Exercise Device (RED) is a weight-lifting device that simulates gravity. Both the CEVIS and RED help build muscle and prevent muscle atrophy, another condition astronauts and bedridden patients experience after long periods of inactivity.

Even with lots of time put aside for exercise, astronauts still suffer from small amounts of bone loss. This poses a problem if we ever want people to stay for prolonged periods of time on someplace like the moon, where there's much less gravity. Since astronauts only stay in space for a few weeks or months at a time, we don't know if bone loss eventually tapers off and stops, or if it keeps on happening.

Scientists are thinking of new ways to reverse bone loss. Vibrating plates that astronauts stand on for 10 to 20 minutes a day while working, for instance, may mimic the sensation of bearing weight and decrease the amount of bone loss during space flight. NASA researchers have also suggested rotating entire shuttles or stations to create a significant gravitational force or designing large centrifuges to overcome bone loss [source: Houston Chronicle].

Astronauts also pay close attention to their diets and take dietary calcium supplements and other medications such as biophosphonates and potassium citrate, but this doesn't necessarily solve anything -- the root of the problem is still the lack of gravity [source: Dartmouth News].

Studies on how astronauts live in space and attempt to counteract bone loss can also benefit life here on Earth. The European Space Agency (ESA), for instance, is carefully monitoring and researching astronaut activity on the ISS -- it's worked with the Institute for Biomedical Engineering and Scanco Medical to design a special scanner that creates high-quality, 3-D images of bone structures for studying and measuring bone growth [source: ESA]. Their findings could help both astronauts in space and patients suffering from osteoporosis on Earth. Even though the causes behind osteoporosis and astronaut bone loss are different -- the former happens through hormonal changes, the latter through suppression of weight -- the treatments may be similar.

For lots more information on living in space, see the next page.