When NASA astronaut Frank Rubio returned from his record-breaking 371-day mission aboard the International Space Station in September 2023, he could not walk unassisted off the Soyuz capsule and later described a burning ache in the soles of his feet and a deep pain low in his back that lingered for weeks. His spine, no longer floating, was suddenly being asked to carry roughly half his body weight through vertebrae that had spent a year expanding in the absence of gravity.
The pain is predictable. Flight surgeons expect it. Long-duration crew members commonly report some version of it within the first days after touchdown.
What is happening inside the returning body is a small physiological drama that begins the instant the parachute opens over the Kazakh steppe or the SpaceX capsule splashes into the Gulf.
The spine that grew in orbit
In microgravity, the intervertebral discs — the gel-filled cushions between each vertebra — swell. On Earth, gravity presses down on the spinal column all day, squeezing water out of the discs; a night of lying flat lets them rehydrate slightly, which is why almost everyone is a few millimetres taller in the morning than at bedtime.
Take gravity away entirely for six months and the effect compounds. Astronauts on the ISS routinely grow several centimetres in orbit. Scott Kelly gained height during his year in space and lost it again within days of landing.
The lengthening feels harmless in flight. On the ground, it is the source of the problem.
Why the soles of the feet burn
Feet in orbit have almost nothing to do. Crew members glide with their toes, hook themselves under handrails, and lose the thick calluses that Earth-dwellers build up on their heels and forefeet. The skin on the soles becomes almost baby-soft — cosmonauts have described peeling entire layers off after long missions.
When that newly delicate skin meets a full body’s worth of weight for the first time in months, the sensation is close to walking on bruises. Add the fact that the vestibular system is still recalibrating and the returning astronaut is often gripping objects and stepping down as if the world were much heavier than it actually is — a bias documented in recent Scientific American reporting on how astronaut brains fail to fully adapt to microgravity — and every footfall arrives harder than intended.
The 72-hour window
The lower back pain follows a different curve. It tends to peak between day two and day four after landing, when the discs are recompressing under load faster than the surrounding muscles and ligaments can adjust.
The paraspinal muscles — the long ropes of tissue that flank the spine and hold it upright — atrophy measurably during long-duration flight. A weakened muscular corset now has to stabilise a spine whose discs are still oversized.
The result is a set of symptoms that overlap significantly with the musculoskeletal injury patterns documented in military recruits during initial entry training — the same combination of decompressed connective tissue and undertrained stabilising muscles suddenly asked to bear repetitive load.
Herniation risk is not theoretical
Returning astronauts face elevated risk of herniated discs, with the risk concentrated in the first year post-flight and especially in the cervical spine.
The mechanism is straightforward. A disc that has been swollen and unloaded for months is more vulnerable when suddenly compressed. Bend to lift a suitcase off a baggage carousel three weeks after landing and the nucleus of a lumbar disc can push through the annulus in a way it would not have before flight.
This is why flight surgeons impose a strict no-lifting rule for weeks after return, why crew members are helped into reclining seats, and why the first press conference after landing is always sitting down.
Two hours a day on the treadmill was not enough
ISS crew exercise several hours per day, multiple days a week, on three devices: the T2 treadmill, the CEVIS cycle ergometer, and the Advanced Resistive Exercise Device, or ARED, which uses vacuum cylinders to simulate free weights up to about 600 pounds.
ARED was installed specifically because its predecessor could not preserve bone and muscle adequately. Even with it, crew members still lose bone mineral density in weight-bearing regions like the hips and lower spine, and paraspinal muscle loss continues despite the daily routine.
The exercise program slows the deconditioning. It does not prevent it.
The Artemis problem
The pain-on-return phenomenon is about to matter more, not less. When Artemis II carried four astronauts on a roughly 10-day flight around the Moon in April 2026, reporting on their return drew on what longer-duration crews had already described: ESA astronaut Andreas Mogensen, recalling a roughly 200-day stint aboard the ISS during SpaceX Crew-7, said that with his eyes closed it was “almost impossible to walk in a straight line” once he was back on the ground. Ten days is short. A Mars transit will be roughly nine months each way.
An astronaut stepping onto the Martian surface after nine months in transit will be doing so in 0.38 g — lighter than Earth but heavier than the ISS by an infinite factor. There will be no ground crew to carry them out of the capsule. They will need to walk, kneel, climb, and lift equipment on a spine that has just spent nine months expanding.
What flight surgeons see at the hatch
The Direct Return protocol that NASA and Roscosmos use for landed crew is unusually cautious. Astronauts are extracted from the capsule feet-first, kept horizontal, wrapped in blankets, and passed to a medical team within minutes.
They are not asked to stand. Blood pressure is checked immediately because orthostatic intolerance — the inability to keep blood from pooling in the legs when upright — is common after long-duration flight. The cardiovascular system, like the spine, has forgotten what gravity does.
Some crew members vomit within the first hour. Some cannot track moving objects with their eyes without triggering vertigo. Mike Fincke, a veteran of four ISS missions, experienced a sudden inability to speak during dinner five months into one of his stays — a reminder that the nervous system’s responses to prolonged microgravity are still being catalogued.
The brain is part of this too
Recent MRI work has shown that astronaut brains physically shift upward inside the skull during long missions, with cerebrospinal fluid redistributing and the pituitary gland deforming. The changes take months to reverse.
That fluid shift is the same headward flow that causes the puffy-face, chicken-legs look of orbiting crew, and it is part of why the spine unloads so completely: without gravity pulling fluid and tissue toward the feet, everything above the waist expands slightly and everything below shrinks.
On landing, the flow reverses. Legs swell. Faces gaunten. The lumbar spine, now under load again, complains loudly.
Research published in 2026 suggests the brain-body coupling involved in these fluid shifts is more mechanical, and more intimate, than earlier models assumed.
Recovery has a shape
Astronauts typically stand within hours of landing but walk unsteadily for days. Height returns to normal within about ten days as the discs recompress. Balance and gaze stabilisation recover within several weeks.
Bone density is slower. It can take years to rebuild what was lost during a six-month flight, and some regions never fully recover. Muscle mass rebuilds faster, provided the crew member follows the post-flight reconditioning protocol that begins the day after landing.
The pain in the soles fades as the skin thickens again. The lower back settles once the paraspinal muscles remember their job.
Rubio’s first steps
Frank Rubio said in his first post-flight briefing that he had not intended to break the American duration record — he had been scheduled for six months, and a Soyuz coolant leak turned his stay into a year. He described the return as the hardest part of the mission, harder than the launch, harder than the year in orbit.
The soles of his feet hurt. His back hurt. His inner ear insisted the floor was tilting when it was not. And somewhere in his lumbar spine, five discs that had spent a year at rest were being asked, all at once, to remember what half a body weighs.
The astronauts who will one day step onto Mars will feel a lighter version of the same thing — and they will feel it alone.
The post Returning astronauts describe pain in the soles of the feet and lower back within days of landing — the spine suddenly forced to support half the body’s weight again after months of weightlessness appeared first on Space Daily.