The exercise capacity of astronauts may decrease by up to half during long-duration space missions, according to a new study published in the Journal of Applied Physiology.

Scientists from Kansas State University (KSU) and NASA’s Johnson Space Center used data from nine male and female astronauts who had spent six months aboard the International Space Station (ISS).

This data included exercise before and after their time in space. Specifically, the astronauts performed a stationary bike exercise test several months before going to the ISS, and a couple of days after.

And the researchers found a huge reduction in oxygen uptake (how much oxygen is consumed during exercise) after the astronauts had returned. In fact, it was between 30 and 50 percent less, meaning their capacity to do exercise was much reduced.

"It is a dramatic decrease," lead author Carl Ade from KSU said in a statement. "When your cardiovascular function decreases, your aerobic exercise capacity goes down. You can't perform physically challenging activities anymore.

“While earlier studies suggest that this happens because of changes in heart function, our data suggests that there are some things happening at the level of the heart, but also at the level of the microcirculation within capillaries."

To solve the issue for astronauts, it will be important to understand exactly why the oxygen uptake is decreasing, which is not understood at the moment. This will allow solutions such as specific exercise or pharmacological intervention to be devised, which can help astronauts cope with the effects of spaceflight.

The researchers also note their findings could help understand blood vessel function in older patients, or those with heart failure.

"We have seen similar situations happen with heart failure and with aging," Ade said. "If we can better understand what is happening in the astronaut and how to prevent it, then we might be able to do the exact same thing in a patient who is older or who has heart failure."

The results may have some major connotations for long-duration missions to Mars or other destinations. When astronauts get to Mars, they will have spent eight months in weightlessness while traveling there. If their abilities are dramatically reduced on the surface, this will obviously pose some problems.

This isn’t the only issue that astronauts will face while on Mars. They’re also going to have to content with bone and muscle loss, a known issue with long-duration spaceflight.

All of this highlights the importance of proper preparation before attempting a mission to Mars, specifically with our work on the ISS. Before attempting the trek to the Red Planet, NASA is planning to spend a year testing out procedures near the Moon in space.