U.S. researchers have found that skeletal muscles of astronauts deteriorates so much during extended weightlessness and greatly reduces their capacity to do physical tasks.

The findings of the team of Robert Fitts of Marquette University in Milwaukee, Wisconsin raises the safety risk of going to Mars or elsewhere in the universe.

The study, published in the online version of the Journal of Physiology, involved the biopsy of muscles cells in the calf of nine astronauts before and after staying for 180 days at the orbiting International Space Station (ISS).

The researchers explained that the muscle of a 30- to 50-year-old astronaut becomes that of an 80-year-old. In this condition, traveling to Mars would be dangerous as the astronauts would fatigue more rapidly and would hardly perform routine work in a space suit, according to Fitts, chair and professor of Biological Sciences at Marquette.

The return to Earth would be even more dangerous because the astronauts would then be so weak to act in case of an emergency landing. U.S. space agency NASA estimates that a Mars mission will take three years, 10 months to reach Mars, one year stay on the Red Planet and another 10 months to return to Earth.

The study also found that crew members with the biggest muscles showed the greatest decline so being physically fit at the start of a space journey is not helpful. Moreover, in-flight exercises have no effect in keeping muscles from deteriorating in space.

Fitts recommended the development of better methods of protecting muscle and bone while in zero gravity.