The muscles in the human body, including the heart, weaken in space and NASA is trying to find out how much, a space station scientist said Saturday in Houston.

A study of the International Space Station seeks to determine how much the heart muscle decreases in size over the normal tour on the station, which is six months, and how quickly the decrease occurs.

"This study also will help us determine if there is a risk of abnormal heart rhythms and how significant the risk is in order to develop appropriate countermeasures," said Deborah Harm, international project scientist for the ISS medical program at the Johnson Space Center in Houston.

"At this time it is unknown if heart muscle weakening continues throughout a mission or if it levels off at some point. That's what we want to find out," Harm said.

She said the investigation is intended to determine if the astronauts' exercise program is effective in keeping the heart strong.