Exercise in Space: What Artemis II Reveals About the Human Body

When the Artemis II crew left Earth, they also left behind something we almost never think about: Resistance!

No gravity pulling them down.

No ground pushing back.

No body weight to lift.

At first, that sounds like a dream.

But for the human body, it’s a problem.

Because without resistance, the body starts to adapt—and not in a way that helps when you come home.

The Artemis II crew. Image credit: NASA

How Do You Exercise When Nothing Has Weight?

This is where things get fascinating.

Astronauts can’t just pick up dumbbells or go for a run. In microgravity, weights don’t weigh anything—and if you try to run, you’ll just float away.

So instead, exercise in space is engineered.

NASA astronauts Bob Hines and Kjell Lindgren work out on the Advanced Resistive Exercise Device (ARED). Video by NASA

Strength Training (ARED):

Astronauts use the Advanced Resistive Exercise Device (ARED), which can simulate loads of up to ~272 kg. This allows them to perform movements you’d recognise—squats, deadlifts, heel raises, and presses—at intensities approaching 6-repetition maximum.

Despite this, strength losses still occur.

Across long-duration missions (~4–6 months), astronauts lose approximately 8–17% of isokinetic strength (force a muscle produces), even with regular resistance training [1].

Running (with a harness):

Running in space requires astronauts to be strapped to a treadmill using a harness system that pulls them down toward the belt, often at forces approaching body weight.

This is critical for bone health, as impact loading is one of the main drivers of bone maintenance.

Without it, bone mineral density declines at approximately 1–1.5% per month in key weight-bearing regions [2].

Flywheel training

One of the more innovative approaches to space exercise is flywheel training.

Instead of relying on weight, a flywheel device uses inertia. Astronauts pull on a strap or cable to spin a wheel, and as the wheel slows down, it pulls back—creating resistance in both the lifting and lowering phases of movement.

This has a few key advantages:

• It doesn’t rely on gravity

• It allows for high force production

• It naturally emphasises eccentric loading

That last point is particularly important.

Eccentric (lowering) contractions are known to be highly effective for maintaining muscle mass and strength—something that is especially valuable in an environment where muscle atrophy occurs rapidly.

This type of training has been explored as a way to improve efficiency and reduce reliance on large equipment, which becomes increasingly important for future missions like Artemis II and beyond [2].

Artemis II pilot Victor Glover spotted using flywheel to exercise onboard Orion capsule. Video by CBSNews.

And How Much Do They Do?

A lot!

Astronauts complete approximately ~2 hours per day, equating to ~600 minutes per week of structured exercise [4].

NASA refers to this not as a workout, but as a countermeasure—a strategy to offset the physiological effects of microgravity.

What Actually Happens to the Body in Space?

Muscles get Weaker (and smaller):

Without gravity, muscles—particularly in the lower limbs and trunk—are underloaded.

Over time:

• Strength declines by 8–17%

• Muscle mass decreases

• Postural muscles are most affected [1]

Fitness Drops:

Following long-duration missions:

• VO₂max decreases by ~15–20%

• Cardiac output decreases by ~7–8%

• Oxygen extraction declines by ~9% [3]

Bones Lose Density:

Bone requires load to maintain strength.

In microgravity:

• Bone density declines at ~1–1.5% per month

• This far exceeds typical age-related changes on Earth [2]

Even the Cells Change

At a molecular level:

• Muscle experiences increased nitrosative stress

• Proteins involved in energy production and contraction are altered

• Mitochondrial function is disrupted [5]

And not everyone responds the sam: e

In a cohort of astronauts:

• Changes ranged from +5% to −30%

• Up to 17% may experience performance-limiting declines [4]

JAXA (Japan Aerospace Exploration Agency) astronaut Satoshi Furukawa pedals on the upgraded CEVIS system. Image by NASA.

Rethinking exercise: Doing more with less

The NASA Sprint Study compared traditional daily training with a high-intensity, lower-volume program, where astronauts completed resistance training 3 days per week and aerobic sessions on alternating days.

Despite doing less overall exercise, the results were similar.

Across long-duration missions:

• VO₂peak decreased by ~6% in both groups

• Strength and muscle function showed comparable changes

• Bone density declined at similar rates

In other words, more exercise didn’t necessarily lead to better outcomes.

The key difference was efficiency.

The high-intensity group achieved these results with less total training time, highlighting a shift toward smarter, more targeted exercise for future missions like Artemis II [6].

What Artemis II is Teaching us

Artemis II is pushing us toward longer missions with tighter constraints.

That means:

• Less space for equipment

• Less time for training

• Greater need for efficiency

Take away gravity, and the body adapts.

Add it back, and suddenly everything matters again—strength, balance, coordination, control.

Space doesn’t just challenge the limits of human exploration.

It quietly tests the limits of the human body itself.

References:

1. English KL et al. Isokinetic strength changes following long-duration spaceflight on the ISS. 2015.

2. Scott JPR et al. Optimization of Exercise Countermeasures for Human Space Flight. 2019.

3. Ade CJ et al. Decreases in maximal oxygen uptake following long-duration spaceflight. 2017.

4. Scott JM et al. Effects of exercise countermeasures on multisystem function. 2023.

5. Blottner D et al. Nitrosative stress in astronaut skeletal muscle. 2024.

6. English KL et al. High intensity training during spaceflight: NASA Sprint Study. 2020.