Why Astronauts Exercise Two Hours Every Day in Space or Their Muscles and Bones Fall Apart

Bone density drops at roughly 1 to 2 percent per month in microgravity, a rate that would take a sedentary person on Earth an entire decade to match. NASA researchers tracking astronauts on the International Space Station found that without countermeasures, crew members lose muscle mass at a pace closer to 20 percent in the first two weeks alone. The body is not malfunctioning. It is doing exactly what it was designed to do: stop maintaining structures that aren't being used.

On Earth, every step you take sends a compressive load through your femur, your spine, your heel. Your bones and muscles exist partly because of that constant mechanical conversation with gravity. Remove gravity, and the conversation stops. The skeleton reads the silence as a signal: this mass is no longer needed. Osteoclasts, the cells that break down bone, keep working. Osteoblasts, the cells that build it, slow down. The result is a net loss that, over a six-month ISS mission, can strip 10 to 15 percent of bone density from the lower spine and hip, the load-bearing regions most critical for walking upright.

For Gaganyaan, ISRO's crewed spaceflight programme currently in preparation, this is not a distant problem. Any Indian astronaut, the mission has named Group Captain Shubhanshu Shukla as one of its crew, will face the same physiological arithmetic the moment the rocket clears the atmosphere.

The cardiovascular system takes its own kind of damage, and it works differently from bone loss. In microgravity, fluid shifts toward the head. The heart, sensing more blood volume in the upper body, interprets this as a surplus and begins shedding plasma. Total blood volume drops. The heart itself, a muscle, starts to atrophy from reduced workload, because it no longer has to pump against the resistance that posture and gravity create on Earth.

A 2001 study published in the journal Circulation, tracking astronauts before and after long-duration spaceflight, found measurable reductions in cardiac mass and a significant drop in VO2 max, the body's maximum oxygen uptake capacity, after missions. The heart that returns from orbit is a smaller, less efficient heart. Astronauts who skip or shortchange their in-flight exercise return to Earth with cardiovascular systems that behave like those of much older, sedentary adults.

Three pieces of equipment carry the exercise load on the ISS. The Advanced Resistive Exercise Device, known as ARED, uses vacuum cylinders to simulate free weights, it can generate up to 272 kilograms of resistance in a weightless environment. The Cycle Ergometer with Vibration Isolation and Stabilization, CEVIS, is a stationary bike mounted on dampers so the vibration from pedalling doesn't interfere with sensitive station instruments. The Combined Operational Load-Bearing External Resistance Treadmill, COLBERT, named, with NASA's characteristic deadpan, after the television host Stephen Colbert, lets crew members run while bungee cords pull them toward the belt.

Two hours a day, six days a week. That is the prescription. It is not a wellness recommendation. It is the minimum load the body needs to slow the loss to a rate that rehabilitation on Earth can reverse.

The return to Earth is the real test. Astronauts who have spent six months aboard the ISS routinely cannot walk unassisted off the Soyuz or Crew Dragon capsule. They are carried. Their vestibular system, the inner-ear balance apparatus, has recalibrated to a world without gravitational orientation, and the ground feels wrong. Their blood pressure drops when they stand because their cardiovascular system has forgotten how to push blood upward against gravity. Their legs, despite two hours of daily exercise, are weaker than when they left.

Scott Kelly, who completed a 340-day mission aboard the ISS in 2016, described the return as feeling like his skin was on fire. Every point of contact with a surface, a chair, a bed, a floor, registered as pain because his nervous system had spent nearly a year processing a world with no contact pressure. Recovery took months. His twin brother Mark, who remained on Earth as a control subject in NASA's Twin Study, showed none of these changes.

The Twin Study, published in Science in 2019, documented changes in gene expression, gut microbiome composition, cognitive performance, and telomere length in Scott Kelly compared to Mark. Some changes reversed within months of return. Others had not fully resolved at the time of publication. Spaceflight does not merely weaken the body. It rewrites parts of it.

The exercise regimen cannot prevent all of this. What it does is compress the damage into a range that is survivable and reversible. Astronauts who exercise consistently during a six-month mission typically recover bone density over one to three years post-flight. Those who don't face deficits that take longer and, in some skeletal regions, may never fully close.

Bed rest studies, where healthy volunteers lie flat for weeks at a time to simulate microgravity, produce nearly identical results to spaceflight. Bone loss, muscle atrophy, cardiovascular deconditioning, fluid shifts. The space station is an extreme version of the same process that happens to any body deprived of mechanical load: a hospitalised patient, a person with a sedentary desk job, an elderly person who stops walking. The astronaut's two-hour prescription is not exotic. It is a demonstration, in the most controlled environment humans have ever created, of what the body requires to remain the body.

The deeper fact is this: the two hours are not building fitness. They are maintaining baseline. Every kilogram of resistance on the ARED, every kilometre on the COLBERT treadmill, is a message sent to a skeleton that would otherwise conclude it is no longer needed. The skeleton listens to load. It has no other language. Shubhanshu Shukla, preparing for a mission that will take an Indian crew to orbit for the first time under the Gaganyaan programme, will spend those two hours not becoming stronger, but staying human.