The human body in space is a marvel of adaptation and resilience, and one of the most striking changes that occurs is the temporary increase in height experienced by astronauts. This phenomenon, which has been well-documented, is a result of the absence of gravity's compressive force on the spine. But what many people don't realize is that this seemingly simple observation has far-reaching implications for space exploration and beyond. In my opinion, the story of spinal elongation in microgravity is a fascinating example of how our bodies respond to extreme environments, and it raises important questions about the long-term effects of space travel on human health.
The Measure of a taller Astronaut
The fact that astronauts return from space missions measurably taller than when they left is not a surprise to those in the field. On average, astronauts gain one to three percent of their standing height, which translates to roughly five centimeters for someone of average height. This increase is a result of the absence of gravity's compressive force on the spine, which causes the intervertebral discs to swell and the spine to lengthen. What is less widely appreciated is the impact of this phenomenon on spacecraft design and the challenges it poses for astronauts upon returning to Earth.
The Science Behind the Measurements
Early data on spinal elongation came from awkward and improvisational methods. During the Apollo-Soyuz Test Project in 1975, crew members' heights were recorded with their feet against the docking module hatch and their bodies supine along the control and display panel. These measurements showed increases of up to three percent within about two days, after which the height held steady. On the Skylab 4 mission, one crew member gained 3.8 centimeters by the end of the mission, with the increase plateauing at around day 29.
The Disc Explanation and Its Limitations
The standard explanation for spinal elongation is the swelling of the intervertebral discs in the absence of compressive load. However, imaging studies have complicated this explanation. A study by Douglas Chang and colleagues at the University of California San Diego found that lean paraspinal muscle content fell from 86 percent of total muscle cross-sectional area to 72 percent immediately after a six-month ISS mission, with about two-thirds of that loss recovered within 46 days. Lumbar disc heights showed no appreciable difference at any point measured, suggesting that the discs may not be the primary cause of spinal elongation.
The Role of Hardware in Spinal Elongation
Spinal elongation became a design constraint before it became a research question. The need to accommodate the increase in height has led to the addition of an allowance of 2.54 centimeters to spacesuit torso length as standard. Seat layout is another critical factor, with the tight squeeze of Orion's four-person configuration limiting headroom for the lower pair. This has led to seated height rising by roughly two to six percent, a larger proportional change than the figures quoted for standing height.
The Clinical Interest in the Reload
The clinical interest in spinal elongation lies in the reload, or the process of returning to Earth. The Skylab 4 crew member whose height was tracked reported back pain on landing day, associated with a herniated disc. This raises important questions about the long-term effects of spinal elongation on astronaut health and the potential for injury upon returning to Earth.
The Open Question
The open question is whether an exercise countermeasure aimed specifically at the lumbar paraspinal muscles can be performed usefully with the equipment available on orbit, and whether it shortens recovery. This is a critical question for Artemis surface operations and any Mars-class transit, as the reload would happen a long way from a rehabilitation team.
In my opinion, the story of spinal elongation in microgravity is a fascinating example of how our bodies respond to extreme environments. It raises important questions about the long-term effects of space travel on human health and the potential for injury upon returning to Earth. As we continue to push the boundaries of space exploration, it is crucial to understand the physiological changes that occur in microgravity and their implications for astronaut health and safety.
