Astronauts aboard the International Space Station (ISS) or those on long duration space flights, such as journeys to Mars or deep space are likely to have a 30 and 50 per cent decline in exercise capacity because of a decrease in the way oxygen moves through the body, say researchers.

The findings showed that astronauts' heart and small blood vessels are not as effective at transporting oxygen to the working muscle during space flights."It is a dramatic decrease...when your cardiovascular function decreases, your aerobic exercise capacity goes down. You can't perform physically challenging activities anymore," said Carl Ade, Assistant Professor at Kansas State University in the US.

"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," Ade added.For the study, detailed in the Journal of Applied Physiology, the team used NASA's Johnson Space Center data on nine male and female astronauts who spent about six months aboard the ISS. 

The astronauts showed a 30 to 50 per cent decrease in maximal oxygen uptake -- the maximum rate of oxygen that is consumed during exercise and shows the cardio respiratory health of a person.The researchers believe that the findings can also help understand blood vessel function in older patients or patients with heart failure."We have seen similar situations happen with heart failure and with ageing. 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," Ade said.