Indian-American astronaut Sunita Williams, who plans to run the 26.2 mile Boston Marathon on board the International Space Station on April 16, finds it a challenging exercise.

The 41-year-old, who is undergoing intense training schedule in space and has so far worked up to 15-mile training runs, finds it challenging due to a harness she will have to wear to keep her from floating off her treadmill when she runs the marathon from inside the ISS.

"It really is kind of a torture device," says ISS commander Michael Lopez-Alegria. The harness causes shoulder and hip pain even when worn for shorter runs. Williams says she plans to take regular breaks during the race to give her shoulders and hips a chance to rest.

Sunita, an accomplished marathoner, has been training for the marathon for months while serving a six-month stint as a flight engineer on board the ISS. She runs at least four times a week, 2 longer runs and 2 shorter runs.

Although the race starts at 10 am EDT (US time) on Earth, Williams' race may not coincide exactly with the race on the ground because of her sleep schedule.

But mission control is working to match the events as closely as possible. However, if marathon officials factor in that the ISS is moving at about 27,000 kilometres per hour,

Williams will technically travel the race distance in record time of 5.4 seconds.

The Boston Athletic Association has issued Williams bib number 14,000. The bib has been sent electronically to NASA, which has forwarded it to Williams. She's a Needham, Mass., native and says her reason for running the marathon is simple.

"I would like to encourage kids to start making physical fitness part of their daily lives. I thought a big goal like a marathon would help get this message out there."

Regular exercise is essential to maintaining bone density while in space for astronauts.

"In microgravity, both of these things start to go away because we don't use our legs to walk around and don't need the bones and muscles to hold us up under the force of gravity," Sunita said.

For two years, Steve Hart, who has been Sunita's flight surgeon, says "There are specific challenges to staying healthy while in space. Sunita wants to make fitness the hallmark of her expedition stay. She wants to educate and motivate others about being physically fit in general."

Sunita qualified for the marathon when she ran a 3:29:57 in the Houston Marathon last year. Her biggest challenge running in space will be staying harnessed to a specially designed treadmill with bungee cords.

She says running on the TVIS which stands for Treadmill Vibration Isolation System can sometimes be uncomfortable. The machinery puts a strain on the runner's hips and shoulders.