Target Pillar: Astronauts, Training, and Life in Orbit
Last Updated: August 15, 2026 Author: Sarah Mitchell
Introduction
Without gravity constantly loading the skeleton and muscles, the human body treats bone and muscle mass as expensive tissue it no longer needs to maintain — and it starts dismantling both almost immediately. A 2026 narrative review synthesizing three decades of spaceflight research puts the scale of the problem starkly: bone mineral density losses (without exercise) of 0.5–2% per month at weight-bearing sites mean that a six-month ISS mission can produce skeletal deficits comparable to a full decade of postmenopausal bone loss on Earth.[^1] This isn’t a minor side effect of spaceflight — it’s one of the central physiological problems standing between current low Earth orbit missions and future multi-year journeys to Mars.
Why Bones and Muscles Deteriorate in Microgravity
On Earth, bone is in constant equilibrium between two processes: resorption (breaking down old bone tissue) and formation (building new bone tissue), a balance maintained partly through the mechanical loading gravity places on the skeleton every time you stand, walk, or lift something. Remove that loading, and the balance breaks. Research using high-resolution imaging (HR-pQCT) has confirmed that in microgravity, bone resorption outpaces formation by roughly threefold during flight — the body keeps breaking bone down at a normal or accelerated rate while building far less new bone to replace it.[^1]
Muscle follows a parallel but distinct path. A 2026 systematic review across NASA, ESA, and Roscosmos research found that prolonged microgravity exposure produces muscle strength reductions of 20–30%, concentrated particularly in “antigravity muscles” — the muscle groups whose primary job on Earth is fighting gravity to keep the body upright, like the calves, quadriceps, and lower back.[^2] These are precisely the muscles that do essentially nothing in microgravity, since there’s no body weight to support.
ARED: The Machine Built to Fight Back
NASA’s primary countermeasure is the Advanced Resistive Exercise Device, or ARED — described by NASA as astronauts’ weightlifting machine in orbit, using a piston and flywheel system to simulate the resistance of free weights without relying on gravity to create that resistance.[^3] Astronauts use ARED for squats, deadlifts, and presses targeting exactly the lower-extremity muscle groups that lose the most mass and strength in microgravity.[^3]
ARED replaced an earlier, less effective generation of resistance equipment, and the improvement is well documented in peer-reviewed research. A NASA-funded study comparing outcomes before and after ARED’s introduction found that ARED reduced hip bone mineral density loss by approximately 40% compared to aerobic-only exercise protocols — a substantial, measurable improvement, though the same research noted that deficits at the proximal femur (upper thigh bone near the hip) still persist even with ARED use, meaning the countermeasure clearly helps without being a complete solution.[^1]
Alongside ARED, ISS crews use T2, a second-generation treadmill, and CEVIS, a stationary cycling machine with vibration isolation to keep exercise from disturbing the station’s sensitive microgravity research environment.[^4] Together, these three machines form the full onboard gym — and current crews average roughly two hours of exercise daily using this equipment.[^3]
The Discovery That Changed the Exercise Approach
Early ISS exercise protocols emphasized long-duration, low-intensity work — astronauts spent up to 10 hours per week exercising, running on treadmills at low velocity and lifting light loads for extended periods. Despite that substantial time investment, crews continued losing meaningful muscle mass and bone density regardless.[^4]
That outcome mirrored a broader shift already underway in exercise science on Earth, where growing evidence showed that high-intensity, lower-volume training was often more effective for maintaining fitness than longer, gentler routines. NASA tested this directly in orbit through the Integrated Resistance and Aerobic Training Study, known as Sprint, which directly compared low-intensity/high-volume against high-intensity/low-volume workout protocols during actual spaceflight.[^4]
Published results showed Sprint’s high-intensity approach either reduced or eliminated losses in several measures — including knee extensor strength and agility test performance — compared to the control protocol, though some losses (like reduced peak oxygen uptake, or VO2peak) still occurred in both groups to a similar degree.[^5] This research meaningfully reshaped how current ISS exercise protocols are structured, favoring intensity over sheer time spent exercising.
Beyond Exercise: Drugs and Monitoring
Exercise alone hasn’t fully solved the problem, so NASA has also investigated pharmaceutical countermeasures. Bisphosphonates — a class of drugs already used on Earth to treat osteoporosis by slowing bone resorption — have shown real promise in combination with ARED. A NASA and JAXA-funded study found that combining the bisphosphonate alendronate with ARED helped preserve group-average bone mineral density more effectively than ARED alone, though researchers noted it remains unclear how much of that benefit comes from the drug specifically versus the exercise it was paired with.[^6]
Separately, NASA has also investigated myostatin inhibitors — a drug class that may prevent both bone and muscle loss — in rodent studies aboard the ISS as part of ongoing efforts to find countermeasures that could benefit long-duration astronauts and, potentially, osteoporosis patients on Earth.[^7]
Astronauts also undergo routine bone density scans before and after spaceflight, letting researchers directly track how much resistive exercise is actually preserving bone tissue for each individual crew member, rather than relying purely on population-level research averages.[^7] One further, less commonly discussed effect: astronaut spines actually straighten out and elongate in microgravity, since there’s no longer gravitational compression pulling the vertebrae and spinal discs together the way there is on Earth.[^7]
Why This Matters More for Mars Than the ISS
The exercise research happening on the ISS isn’t primarily about six-month missions — it’s preparation for missions that will last far longer. Current ISS bone loss research explicitly frames the problem in terms of future Moon and Mars missions, where astronauts will spend far more time in transit and in reduced gravity, with far less opportunity for resupply, faster rescue in an emergency, or rapid medical intervention if bone or muscle deterioration becomes severe.[^4]
A 2026 review noted that even with ARED, bone repair after return to Earth concentrates mostly within the first six months post-flight, while deficits in trabecular bone (the spongy, load-bearing inner bone structure) can persist even two years later despite standard bone-density scans appearing to show full recovery — a gap between what a routine scan shows and what’s actually happening at a structural level that researchers flag as a genuine concern for multi-year exploration missions.[^1]
Frequently Asked Questions
How much bone density do astronauts actually lose in space?
Research indicates losses of roughly 0.5–2% of bone mineral density per month at weight-bearing sites like the hip and femur, meaning a six-month ISS mission can produce bone loss comparable to a decade of postmenopausal osteoporosis-related loss on Earth.
Does bone density fully recover after returning to Earth?
Partially, and the recovery pattern is more complex than a single number suggests. Most measurable bone repair happens within the first six months after landing, but detailed imaging has found some deficits, particularly in trabecular (spongy) bone structure, can persist for two years or more even when standard bone scans appear to show a full return to normal.
Why do astronauts need two hours of exercise daily instead of a shorter routine?
NASA’s own research history shows this wasn’t the original approach — earlier long-duration, low-intensity routines of up to 10 hours weekly still failed to prevent significant bone and muscle loss. Current protocols reflect the finding that intensity, not just total time spent exercising, is the more important variable for preserving musculoskeletal health in microgravity.
Can medication alone replace exercise for astronauts?
No — current research treats bisphosphonates and other drug candidates as potential complements to exercise, not replacements for it. Studies combining medication with ARED use have shown promising results, but the countermeasure protocols NASA actually uses still center on resistive exercise as the primary tool.
Is this research relevant to people who aren’t astronauts?
Yes, directly — NASA has explicitly noted that its microgravity bone-loss research overlaps significantly with osteoporosis research on Earth, and drugs or exercise protocols developed or refined for astronauts have real potential to benefit bone-loss patients who will never leave the planet.
Sources
- International Journal of Innovative Technologies in Social Science — Microgravity and Skeletal Deconditioning: A Narrative Review
- Journal of Advanced Health Care — The Weight of Lightness: The Role of Physical Exercise in Musculoskeletal Deconditioning in Space
- NASA — Train Like an Astronaut: ARED
- NASA — Astronaut Exercise: Science in Space, May 2024
- PubMed Central — High Intensity Training During Spaceflight: Results from the NASA Sprint Study
- ScienceDirect — Resistive Exercise in Astronauts on Prolonged Spaceflights Provides Partial Protection Against Bone Loss
- NASA — Counteracting Bone and Muscle Loss in Microgravity
Note on methodology: figures above are drawn from peer-reviewed spaceflight research and NASA’s published mission and research pages as of mid-2026. Countermeasure protocols and equipment continue to evolve as new research findings inform NASA’s exercise and pharmaceutical strategies — consult NASA’s Human Research Program directly for current protocols.