How do astronauts exercise in space?

The environment of the International Space Station isn’t exactly hospitable to the human body. Thanks to microgravity, astronauts experience a variety of health and physical changes while living in space — some of which they can counteract through daily exercise and other activities. But the space environment also exposes astronauts to other elements that cannot necessarily be mitigated.

Our bodies aren’t built for space; they’re built for a planet a lot like our own. Human beings have evolved here on Earth over millennia, so our bodies have adapted to excel in a gravity environment under the protection of our planet’s atmosphere. In low Earth orbit, however, those ubiquitous elements are taken away, and the body’s various systems adapt accordingly.

Perhaps the biggest change astronauts experience is bone and muscle loss. Humans on Earth work out these systems every day, simply by moving and standing against gravity. But without gravity to work against, the bones lose mineral density and the muscles risk atrophying. It’s something astronauts are consistently trying to prevent from happening. “We try to minimize it as much as possible,” says Bob Tweedy, the countermeasures systems instructor at NASA’s Johnson Space Center. To do that, astronauts on the station work out six out of seven days a week for 2.5 hours each day.

The International Space Station is equipped with three machines designed to give astronauts that full-body workout: a bicycle, a treadmill, and a weightlifting machine called ARED, for Advanced Resistive Exercise Device. Each machine is specially designed for space, since normal gym equipment would be useless in microgravity. Lifting weights, for instance, wouldn’t do much in space since dumbbells wouldn’t weigh anything. So instead, the ARED machine utilizes two canisters that create small vacuums that astronauts can pull against with a long bar. This allows them to do squats, bench presses, dead lifts, and more.

Similarly, the station’s treadmill is no ordinary running machine. Astronauts have to be strapped into it with a harness and bungee chords, otherwise they would float away and never actually get a workout. A stationary bicycle is also available for strengthening astronauts’ legs, though it has no seat (since your butt wouldn’t sit on it anyway). Instead, astronauts grip handles and sit up against a back pad to stay stationary. Practicing with this equipment on Earth, it’s hard to get a full grasp of what they will feel like in space, since gravity is ever present.

But it’s not just muscles and bones that astronauts have to worry about either. People also experience something call fluid shift in space. Without gravity pulling your bodily fluids downward, they shift up into the chest and head, causing issues with the circulatory system and even changes to vision. Most of these changes are temporary, though, and typically go away once astronauts return to Earth.

However, there are harmful elements that astronauts are exposed to just simply by being outside of Earth’s atmosphere — the biggest of which is space radiation. Energetic particles from the Sun and outside our Solar Systems can travel through materials and skin, doing damage to the body over time. Fortunately, astronauts on the ISS are still shielded from a significant amount of space radiation thanks to Earth’s magnetic field, which acts like a barrier around our planet. However, their exposure to radiation is monitored over the course of their careers, and NASA institutes lifetime limits for its astronauts so they don’t experience too much.

Space radiation is going to be a concern for those traveling into deeper space, though, since astronauts will be outside Earth’s magnetic field. “If you spend most of your time on the space station, you're exposed to a different amount and quality of radiation than you would be, say, if you went into deep space,” says Peter Guida, the liason biologist at NASA’s Space Radiation Lab. “You might be able to do — these numbers are arbitrary — 10 space stations missions versus one deep space mission. It really depends, but those things are carefully measured.”

NASA studies the effects of space radiation at a special lab at the Brookhaven National Laboratory in Long Island. There, Guida and other scientists use a particle accelerator to create simulated space radiation and see how it affects biological samples. “Instead of bringing the samples up to the radiation, we bring the radiation down to the samples,” he says. It’s research that is helping NASA create better shielding that can mitigate radiation in space, potentially allowing astronauts to travel deeper into space than ever before.

Update December 23, 2019 10:00AM ET: This article was originally published on August 29, 2017 and has been updated to include video.