Astronaut Spine Lengthening: How Space Travel Changes Height

Astronauts returning from extended missions in orbit frequently find themselves taller than when they departed, a phenomenon attributed to the effects of microgravity on the human spine. This temporary elongation, which can reach as much as five centimeters, typically resolves within days of re-acclimation to Earth's gravity as the spinal column is compressed by body weight. While this height gain is a known factor in the spaceflight community, its implications extend beyond mere physiological curiosity, significantly influencing the design of crucial equipment like spacecraft seats and spacesuits.

Early measurements of astronaut stature in space were often improvisational. During the 1975 Apollo-Soyuz Test Project, a nine-day mission, crewmembers' heights were recorded by having them stand with their feet against a docking module hatch while lying supine along a control panel, with a colleague reading a measurement decal. These early readings indicated height increases of up to three percent within approximately two days, after which the stature remained stable, according to a summary in the journal *Applied Ergonomics*. Later missions, such as the 84-day Skylab 4 mission, saw more precise measurements, with one astronaut gaining 3.8 centimeters by the mission's end, a gain that appeared to plateau around day 29. Reviews of existing research confirm similar gains, with older reports suggesting increases as large as seven centimeters. These figures are notably larger than the typical overnight height increase of about one percent, underscoring the substantial impact of prolonged unloading of the spine in microgravity.

Spinal Changes Beyond Disc Swelling

The standard explanation for this spinal lengthening has centered on the swelling of intervertebral discs due to the absence of compressive load. However, recent imaging studies have introduced a more complex picture. An MRI study published in *Spine* in December 2016 by Douglas Chang and colleagues at the University of California San Diego examined six crewmembers before and after six-month missions aboard the International Space Station (ISS). The study found a decrease in lean paraspinal muscle content from 86 percent of total muscle cross-sectional area to 72 percent immediately after flight, with only partial recovery observed 46 days later. Crucially, lumbar disc heights showed no significant difference at any measured point post-flight. While these are post-flight scans and cannot directly detail in-orbit changes, they suggest that disc swelling might not be the sole or primary factor. Reviews for the European Space Agency have noted that increased disc swelling during spaceflight has not been directly demonstrated, relying instead on inferred data from height gains and bed-rest studies. NASA's own research on seated height experiments also points to a second contributing factor: the straightening of the spine's natural curve in the absence of gravity, resulting in a longer, straighter column.

The phenomenon of spinal elongation has historically been a practical design constraint before becoming a focus of scientific inquiry. Astronauts have reported difficulties donning spacesuits after long durations in orbit, necessitating standard adjustments to suit torso lengths, often around 2.54 centimeters. The seating arrangements within spacecraft also pose a challenge. For instance, the Orion spacecraft's stacked seating configuration for four crewmembers limits headroom for those in lower positions, creating a critical clearance issue. The *Applied Ergonomics* paper highlighted that this clearance can be as little as 6.6 centimeters, based on a sample of 29 crewmembers from ISS and Shuttle flights. An earlier NASA abstract on similar work reported an in-flight increase in seated height of roughly two to six percent, a more significant proportional change than standing height increases. This seated height measurement is particularly relevant, as it directly impacts whether an astronaut's helmet will clear the spacecraft's internal structures, making an astronaut unable to fit into their seat upon return a serious re-entry concern.

The reintegration to Earth's gravity is where the most significant health implications arise. The Skylab 4 astronaut who experienced a 3.8-centimeter height gain also reported back pain upon landing, linked to a herniated disc. Research by Smith Johnston and colleagues, published in *Aviation, Space, and Environmental Medicine* in 2010, indicated a notably higher rate of herniated discs among astronauts compared to matched controls—a 4.3 times greater incidence. This analysis drew from NASA's Longitudinal Study of Astronaut Health, covering a vast period from 1959 to 2006, predominantly reflecting data from the Apollo and Shuttle eras. This historical context is important, as it differentiates findings from different mission types. Subsequent reviews suggest that Apollo and Shuttle crews experienced a higher incidence of disc injury than those on the ISS and Mir. This difference is partly attributed to landing procedures: Shuttle crews landed seated and were able to walk away, whereas Soyuz crews were often carried out and spent longer periods in a recumbent position. A significant number of post-flight herniations documented in Johnston's data occurred within a week to a year of landing, highlighting the critical window for injury prevention. Current research, such as the work by Green and Scott, emphasizes the need for more contemporary reports on in-flight and post-flight back injuries to accurately assess the current risk under modern exercise regimens. Routine measurement of stature, once common, is no longer standard practice aboard the ISS, occurring mainly for suit fit before spacewalks. This gap in data is why researchers advocate for regular in-flight measurements, pre- and post-flight imaging, and extended injury tracking for at least two years post-landing. The central unanswered question, as raised by Chang's group, concerns the effectiveness of targeted exercise countermeasures for lumbar paraspinal muscles using available orbital equipment and whether such exercises can accelerate recovery. Addressing this is crucial not only for future long-duration missions, such as those planned for Artemis surface operations and potential Mars transits, but also for ensuring astronaut well-being, particularly when rehabilitation teams are not readily accessible.