Exercise is not optional on the International Space Station. It is a medical prescription, scheduled like any other part of the crew's working day. In weightlessness, the human body starts to dismantle itself: muscles weaken because they no longer have to fight gravity, bones lose density because nothing loads them, and the heart shrinks slightly because it no longer has to pump blood uphill. Without countermeasures, an astronaut can lose 1 to 2 percent of their bone density per month in the hips and spine, and significant muscle strength in a matter of weeks.

The routine is serious and structured. Crews typically exercise for around two hours a day, six days a week, splitting the time between cardiovascular work and resistance training. It is not a casual gym session squeezed in when convenient; it is blocked into the daily timeline alongside science experiments and station maintenance, and flight surgeons on the ground monitor the results. For a six-month mission, that adds up to hundreds of hours of exercise, and it is one of the main reasons astronauts can walk off a returning spacecraft instead of being carried off it.

The treadmill: running without gravity

The station's treadmill, known as T2, looks familiar but works very differently from the one in your gym. There is no gravity to hold a runner down, so astronauts wear a harness attached to bungee cords that pulls them toward the belt with a force roughly equivalent to most of their body weight. Without the harness, a runner would simply float away with the first stride. The bungees can be adjusted, and crews typically run with a loading of around 70 to 80 percent of their body weight.

Running in a harness is harder than it sounds. The cords pull unevenly and can chafe, so astronauts wear padded belts and learn to tolerate a certain amount of discomfort. Sessions usually last 30 to 40 minutes, including intervals and steady running, and the treadmill records speed, distance and heart rate for the flight surgeons. It is also one of the few times in the day when the crew gets to do something that feels physically normal, and many astronauts say the run is the best part of their day.

The cycle and the resistance machine

The second machine is a stationary cycle ergometer, a bike with no wheels, called CEVIS. It provides the low-impact cardiovascular half of the programme, and it doubles as a medical device: during periodic fitness tests, the crew pedal against a set resistance while doctors on the ground watch their heart rate and oxygen use. Cycling is gentler on the knees than running and gives the schedule flexibility, since it can be done on days when the treadmill is busy or under maintenance.

The third machine, and arguably the most important, is the Advanced Resistive Exercise Device, or ARED. It uses vacuum cylinders and flywheels to simulate weightlifting, providing up to around 270 kilograms of resistance for exercises such as squats, deadlifts, bench presses and calf raises. ARED is the main defence against muscle and bone loss, because only heavy loading signals the body to keep its skeleton strong. Before ARED arrived, astronauts returned from long missions noticeably weaker; with it, crews now come home in far better shape, which is essential preparation for future missions to the Moon and Mars, where astronauts will need to work on the surface the day they land.

What exercise cannot fix

Exercise slows the damage, but it does not stop it completely. Even with two hours a day, astronauts still lose some bone density and muscle mass on long missions, and the losses are greater in some people than in others. The cardiovascular system also adapts to weightlessness in ways exercise cannot fully prevent, such as a shift of fluid toward the head that causes puffy faces, stuffy noses and, in some crew members, changes in vision. Researchers are studying these effects closely, because a Mars mission could last two to three years, far longer than a typical six-month station stay.

Recovery on Earth takes time too. Returning astronauts usually need weeks of rehabilitation to rebuild strength and balance, and bone density can take a year or more to return to pre-flight levels, if it fully does. That is why exercise research is one of the most important programmes on the space station: every lesson learned about keeping a human body healthy in space brings a crewed Mars landing a little closer. The treadmill, the bike and the weights rack may look mundane, but they are among the most critical pieces of equipment in orbit.

Frequently Asked Questions

Q: How many hours a day do astronauts exercise on the ISS?

A: Around two hours a day, six days a week, split between cardiovascular exercise on the treadmill and bike, and resistance training on the ARED machine.

Q: Why do astronauts lose bone density in space?

A: Bones stay strong because gravity loads them every time you stand, walk or lift. In weightlessness that loading disappears, so the body starts removing bone mineral it no longer thinks it needs.

Q: Can astronauts skip exercise for a day?

A: Occasionally, if the schedule demands it, but the programme is treated as a medical requirement rather than a hobby, and missed sessions are made up rather than dropped.