How does a weightless man run on the spot while hurtling through space at 17,150mph? As the countdown continues to the London Marathon – taking place on 24 April – ESA astronaut Tim Peake will perhaps have more reason to be nervous, as he prepares to run his version on the International Space Station (ISS). QinetiQ's Steven Hens explains how technology makes it possible.

"Preparing for a marathon on Earth takes a great deal of dedication, but in microgravity an astronaut must train just as intensively to simply maintain a basic level of fitness," said Hens in a press release.

"Whether an astronaut is running a marathon or flying to Mars, having the means to train is vital, but the microgravity environment poses unique problems. If Tim were to use a normal treadmill in space, he would push off with his foot to take his first step of the race, but leave the treadmill and keep travelling in that direction until he crashed into something.

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"To prevent this, Tim will be tethered to the treadmill using elastic bungees, known as a Subject Loading System (SLS), which will pull him back down with every step," said Hens.

So, why should we spare a thought for Tim? Firstly, he has to train longer and harder to achieve the same results as those who have the luxury of gravity. Secondly, the bungees don't behave exactly like gravity – "the higher Tim's stride, the harder they will pull him down, which can cause extra stress and discomfort," added Hens.

In the video above, the first clip shows QinetiQ's next generation of SLS being tested on a "vertical treadmill", currently stationed at the DLR Institute of Aerospace Medicine in Cologne, Germany. The elastic bungees have been replaced with a sophisticated set of gears and pistons, which are adjustable and can maintain a consistent force throughout the stride that is equivalent to gravity on Earth.

The subject lies parallel to the ground and the SLS pulls him or her horizontally on to the treadmill to simulate its effect, allowing them to examine the safety and performance of the equipment, and the user's response to it. However, the true test of the technology is how it performs in weightlessness – so they took it to the skies on board a parabolic flight, as you can see in the second clip.

Experienced test subjects verified that, compared to bungees, the new technology creates a running feeling closer to that experienced on Earth. Unfortunately for Tim, the next generation of SLS will not be delivered to the ISS before his marathon. Perhaps it's best not to mention it to him until afterwards.

NASA