How Astronauts Keep Muscle and Bone in Orbit: Jessica Meir, the ARED, and the Science of Strength Training on the ISS

Table of Contents

1. Key Highlights
2. Introduction
3. Why Gravity—or Its Absence—Dictates Bone and Muscle Health
4. How the ARED Recreates “Weight” in Weightless Conditions
5. Daily Training on the ISS: Structure and Priorities
6. Why Squats and Other Multi-Joint Lifts Matter in Orbit
7. Complementary In-Flight Devices: Treadmills and Cycle Ergometers
8. Evidence from Research: How Effective Are Current Countermeasures?
9. Biomechanics and Cellular Mechanisms: How Loading Preserves Tissue
10. Nutrition, Supplements, and Sleep: Supporting the Exercise Prescription
11. Rehabilitation and Reconditioning After Return
12. Implications for Future Missions: Moon, Mars, and Beyond
13. Translating Space Physiology to Earth: Clinical and Public Health Lessons
14. Jessica Meir: Communicating Science and Strength from Orbit
15. Practical Takeaways for Athletes, Clinicians, and Advocates
16. The Limits of Technology and the Human Variable
17. Looking Ahead: Research Gaps and Priorities
18. FAQ

Key Highlights

• NASA astronauts counter rapid muscle and bone loss in microgravity with daily, structured exercise—centered on the Advanced Resistive Exercise Device (ARED)—to replicate the loads of gravity-dependent movements like squats and deadlifts.
• Long-duration missions cause measurable decline: roughly 15% overall muscle mass loss and 1.7–2.5% bone density loss per month without countermeasures, making in-flight resistance training and recovery plans essential for crew health and mission success.

Introduction

Floating hundreds of kilometers above Earth, astronauts live in a world without the constant pull of gravity that shapes nearly every movement and stress on human bones and muscles. Without that daily mechanical loading, the body begins to remodel itself in ways that undermine strength, balance, and bone integrity. Protecting crewmembers from those changes is a core part of modern human spaceflight—and one of the most visible pieces of hardware inside the International Space Station (ISS) for that purpose is the Advanced Resistive Exercise Device, or ARED.

Jessica Meir, who shares her life on the ISS with nearly 350,000 Instagram followers as “Astro Jessica,” demonstrates that resistance training remains fundamental in orbit: “to make sure that we can maintain our bone density, and our muscle mass,” she told followers while performing squats aboard the station. Her social videos illustrate a practical truth backed by decades of research: mechanical loading, replicated through specialized equipment and disciplined daily routines, is the single most effective in-flight strategy to preserve musculoskeletal health.

This article explains why that work matters, how the ARED functions, how daily routines are structured aboard the ISS, and what these lessons mean for long-duration missions to the Moon and Mars—and for life on Earth.

Why Gravity—or Its Absence—Dictates Bone and Muscle Health

Gravity provides continuous mechanical stimulus. On Earth, walking, standing, and even sitting apply forces through bones and muscles that maintain tissue mass and structure. Remove that steady load and the body adapts quickly.

Physiological responses to microgravity:

• Muscle atrophy: Antigravity muscles—primarily the lower-limb extensors such as the quadriceps, calves and gluteals, and spinal postural muscles—undergo the most pronounced atrophy because they no longer support the body against gravity. Studies show astronauts can lose roughly 15% of overall muscle mass on long missions, with reductions in lower-body mass reaching around 30% without adequate countermeasures.
• Bone density loss: Bone responds to mechanical strain by remodeling—bone formation is stimulated by load, while lack of load favors resorption. In microgravity, bone mineral density can decline at rates of 1.7–2.5% per month depending on which skeletal site is measured. A typical six-month ISS mission could therefore produce bone loss of approximately 10–15% at vulnerable locations such as the hip and lumbar spine.
• Cardiovascular deconditioning: Fluid shifts and reduced orthostatic stress lead to changes in blood volume and cardiovascular fitness, compounding post-flight functional challenges.

Consequences on return to Earth include weakened posture, reduced ability to tolerate upright posture, greater risk of vertebral and hip fractures if bone loss is severe, and prolonged rehabilitation. Protecting musculoskeletal integrity during flight is therefore both a crew health priority and a mission-enabling requirement.

How the ARED Recreates “Weight” in Weightless Conditions

Free weights rely on gravity to generate load. In microgravity, an alternative mechanism must produce resistance that simulates the pattern, magnitude, and variability of terrestrial loading. The ARED fulfills that role.

Core design principles of ARED:

• Variable resistance mechanics: ARED uses a system of vacuum tubes and flywheels to produce resistance that behaves much like free weights. Flywheels store rotational energy; when the user applies force, the system resists by accelerating the flywheel. The vacuum environment in some components reduces friction and permits clean, smooth loading and eccentric control.
• Multiple configurations: The device supports barbell-style lifts and attachments for different setups, enabling traditional multi-joint movements—squats, deadlifts, bench press, rows, and more—so astronauts can train the same movement patterns they use on Earth.
• High resistance capacity: ARED is engineered to deliver resistance levels comparable to heavy terrestrial lifts, enabling high-load, low-repetition strength protocols that are particularly effective at stimulating bone formation.
• Stabilization and safety: The machine anchors to the station structure and incorporates harnesses or braces where needed to help crewmembers stabilize their bodies during lifts—critical in an environment where pushing off incorrectly can send a person drifting.

By allowing loads and movement patterns similar to Earth-based resistance training, ARED preserves the mechanical stimuli bones and muscles require to maintain mass and architecture.

Daily Training on the ISS: Structure and Priorities

Exercise aboard the ISS is not optional recreation—it is a scheduled, mission-critical task. NASA and its international partners structure workouts to balance resistance training, cardiovascular conditioning, and recovery.

Typical in-flight exercise profile:

• Frequency: Astronauts typically exercise roughly two hours per day on average, though exact daily schedules vary with mission demands.
• Resistance training: A daily or near-daily session of resistance exercise on ARED focuses on large, compound lifts that load the spine and lower limbs—squats, deadlifts, and bench presses. These movements create high strain rates and magnitudes known to stimulate bone remodeling.
• Aerobic training: Treadmills (with harness systems) and cycle ergometers (such as CEVIS) provide cardiovascular stimulus and maintain aerobic capacity, which influences orthostatic tolerance on return to gravity.
• Flexibility and core work: Exercises to preserve spinal health and mobility are integrated to mitigate injury risk and support functional performance.

Jessica Meir’s approach mirrors this strategy. Her posts show compound lifts—“Squats, deadlifts, bench press, you name it, you can do it here on the ARED”—underscoring the system’s versatility. She emphasizes squats in particular, which load the hips, femurs and spine—sites most susceptible to microgravity-induced loss.

Scheduling and monitoring:

• Workouts are planned with ground support input and personalized based on crewmember fitness, injury history, and mission timeline.
• Data from wearables and the exercise devices themselves inform adjustments. Load, repetitions, and heart rate are monitored to ensure training intensity and volume remain effective.

Consistency is essential: missing bouts or reducing intensity correlates with larger declines in muscle mass and bone density.

Why Squats and Other Multi-Joint Lifts Matter in Orbit

Mechanical loading of bone is most effective when it is dynamic, high magnitude, and involves multi-axis stresses. Squats and deadlifts meet these criteria.

Physiological reasons multi-joint lifts are prioritized:

• Load distribution: Squats and deadlifts place compressive and bending loads across hip, femur, pelvis, and lumbar vertebrae—stimulating bone cells across multiple skeletal sites.
• Muscle recruitment: These lifts recruit large muscle groups simultaneously, promoting greater metabolic and neuromuscular stimulus than isolated movements.
• Bone mechanotransduction: Bone responds to strain amplitude and rate. Heavy, controlled lifts produce high strain rates that drive osteoblast activity and bone formation signaling pathways.
• Functional carryover: The ability to stand, walk, and perform work after landing depends on the same muscle groups trained by squats and deadlifts.

In Meir’s demonstration—wide-stance sumo squats on the ARED—she targets hip abductors and extensors in a way that mimics heavy ground-based lifting. That specificity helps preserve the structural and functional properties crewmembers need on return.

Complementary In-Flight Devices: Treadmills and Cycle Ergometers

ARED is the cornerstone of in-flight resistance training, but a complete countermeasure program requires cardiovascular and impact-like loading too.

Treadmills:

• A harness-and-bungee system holds the astronaut on a treadmill to simulate body weight. This provides impact and vertical load patterns important for bone maintenance, particularly in the lower limb.
• Running and walking with simulated load elicit ground reaction forces, which complement the compressive loads from resistance training.

Cycle ergometers:

• Devices such as the Cycle Ergometer with Vibration Isolation and Stabilization (CEVIS) deliver high-quality aerobic sessions without producing large station-wide vibrations.
• Cycling preserves cardiac fitness and leg endurance, and is often used for interval training protocols to maintain VO2 max.

Together, these devices provide a multimodal stimulus: resistance for bone and muscle, impact loading to mimic walking and running forces, and aerobic conditioning for cardiovascular resilience.

Evidence from Research: How Effective Are Current Countermeasures?

Decades of human spaceflight and dedicated research programs have quantified the impact of exercise on in-flight musculoskeletal outcomes. The results show meaningful but incomplete protection.

What the data indicate:

• Resistance training with ARED, combined with aerobic exercise and nutritional strategies, substantially reduces the rate of muscle and bone loss compared with missions where such countermeasures were unavailable or minimal.
• Some astronauts who adhere closely to exercise prescriptions maintain near-preflight muscle strength and bone density in certain regions, but variability exists between individuals and skeletal sites.
• Even with exercise, some degree of bone loss occurs in most crew on long-duration flights. Recovery post-flight is possible, but full restoration of bone mass can take months to years and may be incomplete at some sites.

Ground analogs reinforce these findings:

• Bed rest studies, which remove weight-bearing as a model of microgravity, show bone loss and muscle atrophy patterns similar to spaceflight. They also confirm that resistance and impact-like exercise attenuate those declines.
• Short-term countermeasure trials using lower-body negative pressure, artificial gravity exposure (centrifugation), and novel resistive devices yield promising results, but operational constraints have limited widespread adoption of some technologies.

The current strategy—consistent, fairly high-volume exercise with devices like ARED—is evidence-based and effective at reducing risk, though not a perfect replacement for gravity.

Biomechanics and Cellular Mechanisms: How Loading Preserves Tissue

Understanding the link from mechanical load to cellular response clarifies why exercise regimes are designed as they are.

Mechanotransduction in bone:

• Osteocytes detect mechanical strain and convert it into biochemical signals. Reduced loading lowers osteocyte signaling for bone formation and increases signaling for resorption.
• High-strain, dynamic loading—especially at high rates—stimulates osteoblast recruitment and bone matrix deposition.
• Absence of load shifts the balance toward osteoclast-mediated resorption, thinning trabecular networks and reducing cortical thickness.

Muscle adaptations:

• Reduced contractile load leads to decreased protein synthesis, increased proteolysis, and a shift in muscle fiber size and composition—particularly atrophy of type I and II fibers depending on the activity pattern.
• Resistance training promotes hypertrophy through signaling pathways (e.g., mTOR activation) that increase muscle protein synthesis and satellite cell activity.

Inflammation and endocrine factors:

• Microgravity alters hormonal milieu—changes in sex steroids, vitamin D metabolism, and other factors influence bone turnover.
• Exercise modulates systemic factors, including myokines and anti-inflammatory effects, which can indirectly support bone health.

These mechanisms explain why heavy, compound, and dynamic loading has more osteogenic and myogenic effect than low-intensity or isolated modalities.

Nutrition, Supplements, and Sleep: Supporting the Exercise Prescription

Exercise is the central countermeasure, but supporting systems matter. Nutrition and recovery amplify training effects and help mitigate bone and muscle loss.

Key nutritional considerations:

• Protein intake: Adequate dietary protein supports muscle protein synthesis. Astronauts’ diets are tailored to supply sufficient protein distributed across meals to optimize synthesis.
• Calcium and vitamin D: Bone health depends on calcium availability and vitamin D-mediated absorption. Spaceflight can alter vitamin D status due to lack of sunlight exposure, so monitoring and supplementation are common.
• Energy balance: Underconsumption reduces the anabolic response to exercise. Ensuring adequate caloric intake preserves muscle and supports training.
• Anti-inflammatory and omega-3 fatty acids: Some studies suggest omega-3s modulate muscle mass and bone turnover favorably, though evidence is evolving.

Sleep and recovery:

• Quality sleep and adequate rest are necessary for hormonal recovery and tissue repair. Spaceflight can disrupt circadian rhythms, so sleep management is part of the countermeasure suite.

Combining training with targeted nutrition and recovery strategies produces a synergistic effect that outperforms any single intervention.

Rehabilitation and Reconditioning After Return

Landing is the beginning of a rigorous recovery period. After months in microgravity, muscles, bones, and the cardiovascular system require staged rehabilitation.

Post-flight priorities:

• Early-phase reconditioning focuses on restoring upright tolerance and basic mobility. Progressive standing, gait training, and graded load-bearing help the cardiovascular and postural systems reacclimate.
• Strengthening programs scale in intensity as tolerance increases. Physical therapists emphasize lower-limb and trunk strengthening that mirrors the deconditioning pattern.
• Bone recovery is slower. Imaging and biochemical markers track progress, and return-to-duty decisions consider residual deficits, especially if astronauts will perform high-impact tasks or EVAs.

Historical examples:

• Crewmembers from the early Space Shuttle era, when exercise equipment was limited on long flights, commonly required extended rehabilitation on return.
• Recent crews who maintain rigorous in-flight regimens tend to have shorter initial rehab timelines, though individualized programs remain necessary.

The rehabilitation phase confirms a fundamental principle: preserving but not fully restoring bone in flight shifts the burden of recovery back to intensive, long-term therapy on Earth.

Implications for Future Missions: Moon, Mars, and Beyond

Lunar sorties and Mars expeditions will challenge current countermeasures in new ways—longer transit times, partial gravity environments, and mission architectures that limit equipment mass and volume.

Key challenges:

• Duration: A Mars mission may require many months of transit, increasing cumulative risk for bone and muscle loss even with current exercise approaches.
• Partial gravity: The Moon’s one-sixth gravity and Mars’s roughly one-third gravity provide some mechanical loading, but their sufficiency for maintaining health over months remains unproven.
• Mass, volume, and power constraints: ARED is a large piece of equipment well-suited for ISS but less feasible for constrained lunar landers or transit vehicles.

Emerging solutions and research directions:

• Artificial gravity: Short-radius centrifuges or rotating sections of a spacecraft could provide intermittent gravity exposure. Research is exploring optimal gravity levels and exposure durations to protect musculoskeletal and cardiovascular systems.
• Compact resistive systems: Engineers are developing lighter, more compact resistive devices—flywheel-based systems, elastic resistance optimized with variable-stiffness materials, and hydraulics—that aim to produce osteogenic loads within tight mass and power budgets.
• Pharmacological adjuncts: Bisphosphonates and other anti-resorptive agents have been tested as supplements to exercise; they reduce bone resorption but come with tradeoffs and are not standalone solutions.
• Personalized countermeasures: Individual variability in response to microgravity suggests a future of tailored regimens based on genetic, biomarker, and activity-pattern data.

All future mission planning must integrate robust, reliable countermeasures into spacecraft design and habitability concepts to ensure crew readiness for planetary operations and safe return.

Translating Space Physiology to Earth: Clinical and Public Health Lessons

Research developed for astronauts has direct relevance on Earth, particularly for aging populations and patients with prolonged immobility.

Applications include:

• Bedrest and rehabilitation: Protocols used to mitigate deconditioning in space inform strategies for hospital patients confined to bed, helping to preserve muscle and bone and shorten recovery times.
• Osteoporosis prevention: High-strain exercise interventions, appropriately modified, inform community programs to reduce fracture risk in older adults. The space program’s focus on measuring bone responses provides rigorous data on which activities elicit the greatest osteogenic response.
• Portable resistance technology: Flywheel systems and compact resistance devices developed for constrained environments have been adopted by clinics and sports teams for rehabilitation and training where heavy free weights are impractical.

Jessica Meir’s public demonstrations serve a dual purpose: they demystify astronaut life and highlight the resonance of these practices for anyone concerned about long-term musculoskeletal health.

Jessica Meir: Communicating Science and Strength from Orbit

Jessica Meir’s social media presence bridges technical reality and public curiosity. Her posts—showing the ARED in action, explaining why she trains daily, and floating to the window to watch city lights after a workout—combine outreach with accurate depiction of the daily work that preserves crew health.

Why that communication matters:

• Public engagement: Demonstrations of disciplined in-flight training humanize astronauts and provide visible proof of the physical demands of spaceflight.
• Recruitment and inspiration: Visibility of science-based exercise and the hardware involved raises interest in engineering, medicine, and human physiology careers.
• Health literacy: Meir’s straightforward explanations—“to make sure that we can maintain our bone density, and our muscle mass”—translate technical imperatives into personal health behaviors that resonate with athletes, clinicians, and the public.

Her content also underscores the international cooperation embedded in the ISS: systems like ARED result from decades of research across agencies and industries to make sustainable human presence in orbit possible.

Practical Takeaways for Athletes, Clinicians, and Advocates

While the constraints of spaceflight differ from daily life on Earth, the underlying principles of load-induced adaptation apply broadly. Practical lessons include:

For athletes:

• Prioritize multi-joint, heavy resistance training to build systemic strength and to maintain bone density.
• Balance resistance with appropriate impact or plyometric work to stimulate bone where running or jumping are possible.
• Maintain consistency; intermittent high-intensity efforts do not substitute for regular loading when bone and muscle preservation are goals.

For clinicians and rehabilitation professionals:

• Design interventions that apply dynamic, multi-directional loads early in immobilization recovery when safe to do so.
• Consider compact, flywheel-based devices for patients who cannot use heavy free weights but require high-strain stimuli.
• Monitor nutrition, sleep, and systemic inflammation alongside mechanical interventions for an integrated recovery plan.

For public-health advocates:

• Promote strength training across the lifespan as a primary prevention strategy for sarcopenia and osteoporosis.
• Invest in community infrastructure—accessible gyms, safe outdoor spaces, and clinician education—to enable regular resistance and impact activities.

The space program’s emphasis on disciplined, instrumented training offers a model for evidence-based, outcome-driven fitness and recovery strategies.

The Limits of Technology and the Human Variable

Equipment like ARED changes what’s possible in orbit, but technology alone cannot ensure outcomes. Human factors—motivation, adherence, preexisting health, and mission workload—determine the real-world effectiveness of countermeasures.

Operational realities aboard the ISS:

• Crew schedules are dense. Exercise sessions must compete with scientific tasks, maintenance, and EVA preparations.
• Illnesses, injuries, and mission contingencies sometimes force schedule changes that reduce exercise frequency or intensity.
• Psychological factors influence adherence; monotony or stress can reduce engagement over long missions.

NASA and partner agencies thus emphasize not only hardware and protocols but also behavior-support systems: variety in training, incentives for adherence, and psychological support to maintain consistency. Jessica Meir’s visible enthusiasm and normalizing of daily exercise contribute to this cultural approach.

Looking Ahead: Research Gaps and Priorities

To make crewed exploration beyond low Earth orbit safe and sustainable, several research priorities stand out:

• Optimal loading regimens: Defining the precise amplitudes, rates, and frequencies of loading required to maintain different skeletal sites over months in partial or zero gravity.
• Artificial gravity parameters: Determining the minimal effective gravity level and exposure schedule that meaningfully preserves musculoskeletal and cardiovascular systems, and engineering compact, safe centrifugation systems.
• Compact, deployable countermeasures: Creating multi-functional resistance devices that fit constrained habitats yet deliver osteogenic stimuli.
• Pharmacological adjuncts and biomarkers: Developing safe adjunct therapies and sensitive biomarkers to personalize countermeasure intensity and to detect early tissue loss.
• Long-term recovery dynamics: Mapping how bone microarchitecture and muscle quality recover over years after extended missions and what interventions accelerate full functional restoration.

Progress in these areas will shape the feasibility of human missions to Mars and prolonged habitation of cislunar space.

FAQ

Q: What is the ARED and how does it differ from a regular gym machine? A: The Advanced Resistive Exercise Device (ARED) is a specialized resistance-training system aboard the ISS that reproduces free-weight-like loads using mechanical components such as flywheels and vacuum-reduced friction systems. Unlike typical Earth gym machines that rely on gravity-based weight stacks, ARED generates resistance through rotational inertia and cable systems, allowing astronauts to perform barbell-style multi-joint lifts in microgravity.

Q: How often do astronauts exercise on the ISS? A: Exercise is a daily, scheduled activity. Astronauts typically devote roughly two hours per day to combined resistance, aerobic, and flexibility training, though precise times vary depending on mission tasks and individual programs.

Q: Why are squats emphasized by astronauts like Jessica Meir? A: Squats load the hips, femurs, pelvis, and lumbar spine—areas most susceptible to microgravity-induced bone and muscle loss. Multi-joint lifts such as squats and deadlifts produce the high-strain, high-rate loading that stimulates bone formation and preserves muscular strength and function.

Q: Can exercise fully prevent bone loss in space? A: Exercise substantially reduces bone loss compared with no countermeasures but does not always fully prevent it. Even with rigorous in-flight training, some astronauts experience measurable bone loss at certain sites. Combined strategies—exercise, nutrition, and potentially pharmacological agents—improve protection, but further research is needed for complete prevention on very long missions.

Q: What happens to astronauts when they return to Earth after a long mission? A: Many require a structured rehabilitation program to rebuild cardiovascular tolerance, balance, muscle strength, and coordination. Bone recovery occurs more slowly and may take months to years; physical therapy focuses on progressive loading and functional training.

Q: Are space-exercise practices relevant to people on Earth? A: Absolutely. The principles—regular heavy resistance training, impact loading for bone, adequate nutrition, and consistent recovery—apply to athletic training, clinical rehabilitation, and public health strategies to prevent sarcopenia and osteoporosis. Technologies developed for space, such as compact flywheel devices, also have clinical and fitness applications on Earth.

Q: How does nutrition interact with exercise in space? A: Adequate protein, calcium, and vitamin D intake support the anabolic and bone-preserving effects of exercise. Energy balance matters: insufficient calorie intake diminishes gains from resistance training. Sleep and recovery further influence hormonal and repair pathways.

Q: Will future missions to the Moon or Mars require different exercise systems? A: Likely yes. ARED is large and power-hungry, so transit vehicles and planetary habitats with strict mass, volume, and power budgets will need more compact solutions—either smaller high-resistance devices, intermittent artificial gravity, or a combination. Research is ongoing to identify feasible and effective countermeasures for those missions.

Q: Can drugs replace exercise for bone preservation in space? A: Pharmacological agents, such as anti-resorptives, can reduce bone loss but do not replicate the full spectrum of benefits from mechanical loading. Drugs may serve as adjuncts—especially where exercise capacity is limited—but exercise remains central to musculoskeletal health.

Q: How does Jessica Meir’s outreach help the science? A: Her posts illustrate lived experience, raise public awareness of the physiological challenges of spaceflight, and highlight the importance of consistent training. Public engagement supports funding, recruitment, and broader interest in translational applications of space research.

Preserving muscle and bone in orbit is a technical and human challenge. Equipment like ARED, disciplined daily routines, tailored nutrition and recovery, and ongoing research together enable astronauts to live and work in microgravity while reducing the physiological toll. Jessica Meir’s demonstrations aboard the ISS capture that blend of engineering, science, and personal discipline: rigorous training, purposeful hardware, and the view that follows—a reminder of why humanity invests in the hard science of staying strong off the planet.