What happens to your spine in space

What changes and why

On Earth, gravity squeezes the soft discs between the bones of the spine every time we sit or stand. Lying down at night lets them take up water again, which is why most people are about 1.5 cm taller in the morning than in the evening [1]. In orbit, crews are in continuous free fall (microgravity), so that daily squeeze never comes. The discs may keep swelling, the spine’s natural curves flatten, and the muscles and ligaments that normally hold us upright have much less work to do [1].

• The spine gets longer. The U.S. space agency, the National Aeronautics and Space Administration (NASA), finds that astronauts’ height rises by about 3% over the first 3 to 4 days in space, almost all of it from the spine [2]. For a 180 cm adult, that would be about 5 cm. NASA’s landing brief describes a spine about 6% longer when crews come home [3]. The European Space Agency (ESA) reports growth of up to 7 cm [4].
• The back often hurts at first. In U.S. medical records of 728 astronauts going back to the earliest programs (mostly short missions), 52% had “space adaptation back pain”. For 86% of them it was mild; 11% had moderate and 3% severe pain [5]. NASA defines this pain as starting within the first few days of a mission [6].
• The deep back muscles shrink. Magnetic resonance imaging (MRI) of six NASA crewmembers before and after about 6 months on the International Space Station showed the lean (working) part of the lower-back muscles falling from 86% to 72% of the muscle area [7]. In a second group of six, five lost about 20% of working muscle area in two key back muscles, the lower-back curve flattened by 11%, and the middle of the lower back bent less when crew moved by themselves [8].
• The discs themselves looked much the same. In those small studies, disc height [7] and disc water content [8] did not change in a consistent way after 6 months. The researchers concluded that loss of the small stabilizing muscle (multifidus), more than disc swelling, was linked to the stiffer, flatter lower back [8].

Timeline: days to a Mars mission

1. First daysHours to about 4 days

   Height rises by about 3% over the first 3 to 4 days [2]. Back pain, usually mild, starts within the first few days [6][5].

2. WeeksGround studies

   There are few spine measurements from the first weeks of flight. In a 60-day bed rest study in women (lying slightly head-down, a ground model of weightlessness), lower-back muscle area fell by about 11% without exercise and about 4% with exercise [9].

3. About 6 monthsTypical station stay

   Lower-back muscles are smaller and the lower back is flatter and stiffer at landing [7][8], and the spine is about 6% longer [3].

4. Mars-length missionsAbout 2 to 3.4 years (730–1,224 days)

   Nobody has yet flown that long. NASA says data are limited for missions longer than the usual 4 to 6 months [10]. Whether back muscles keep shrinking or level off over years, and what that means for working on Mars without a recovery team, is not known.

5. Back on EarthLanding and after

   Height returns to normal soon after landing [2]. The risk of a slipped disc appears highest right after flight [11].

What crews do about it

• Easing early back pain. In the records study, curling into a fetal (knees-to-chest) position helped 91% of those who tried it; pain medicines and exercise each helped 85% [5].
• Daily exercise with spinal loading. Station crews train on a treadmill, a cycle and a heavy resistance machine, the Advanced Resistive Exercise Device, which provides up to 600 pound-force of resistance [12]. On the ground, treadmill and resistance exercise designed to load the spine during bed rest cut the loss of lower-back muscle (about 4% versus 11%) and of back-extension strength (about 4% versus 17%) [9].
• An experimental “skinsuit”. ESA has developed a tight, tailor-made suit that squeezes the body from shoulders to feet, a little like gravity [4]. Skinsuits have been tried by astronauts on the station, and the newest version gives around 20% loading, slightly more than lunar gravity. Early, not yet published ground results suggest it reduces spine lengthening; it remains a research tool, not routine equipment [1].
• Planned reconditioning. NASA requires an individual reconditioning program for every crewmember, starting as they leave the spacecraft [13].

After landing and recovery

• Height returns to the preflight value in a short time once gravity is back [2].
• Back muscles recover only partly at first: about 68% of the lost lean muscle had come back by about 1–2 months after landing (scans at 33–67 days), still below preflight levels [7].
• Slipped discs. In NASA’s long-term astronaut health records, a herniated (“slipped”) disc was 4.3 times more common in 321 U.S. astronauts than in 983 matched people who had not flown. Neck discs made up a larger share in astronauts (18 of 44 herniations) than in the comparison group (3 of 35), and the risk appeared highest immediately after flight [11]. This is a single observational study.
• Who gets symptoms. In the 6-astronaut imaging study, only the two crewmembers who already had marked irregularities of the vertebral end plates before flight developed back symptoms afterwards, including chronic low back pain or a disc herniation [8].
• Getting back to normal. Reconditioning begins at landing; the structured program, about 2 hours a day, runs about 45 days from the day after landing and can be extended [14][12]. In the first week crews are usually advised not to drive or do physically demanding tasks; most return to normal activities within about 45 days [3].

For anyone, on Earth or after a flight, back pain together with bladder or bowel changes or numbness in the saddle area is a medical emergency [15].

What it means for clinicians on Earth

These points are general education for clinicians, not advice about any individual patient. A person who has flown should have their own flight surgeon (the physician responsible for crew health); coordinate with them.

• Ask about spaceflight. In back or neck pain, a recent flight matters: the higher disc-herniation risk seen in astronauts appeared greatest soon after landing, and neck herniations were relatively common [11].

• Assess as you would anyone else. Use the usual assessment for low back pain and sciatica. Without red flags or severe or progressive neurological deficits, imaging is not routinely needed [16][17].

• Never miss cauda equina syndrome (compression of the nerve roots at the bottom of the spinal canal). Back and/or leg pain with any of the following, starting recently (within about 2 weeks), needs same-day emergency evaluation, ideally at an emergency department that can do an MRI scan straight away [18][15]:

  • new difficulty starting to pass urine, or reduced feeling of urine flow;
  • urinary retention or incontinence;
  • numbness or altered feeling around the anus, genitals or buttocks (the “saddle” area);
  • loss of the feeling of a full rectum, or bowel leakage;
  • new sexual dysfunction;
  • severe or worsening weakness in both legs.

  A bladder scan alone must not be used to rule it out [18]. New pain down both legs, or one-sided sciatica that spreads to both legs, is a warning sign that needs urgent review [18].

• Remember the bones. Hip and spine bone density historically fell about 1–1.5% per month in flight, and about 0.3–0.8% per month with current resistance exercise, varying widely [19]. Keep this in mind when judging new back pain or a fall after a long mission.

• Respect deconditioning. Back muscles are smaller and only partly recovered weeks after landing [7], so heavy lifting early on deserves caution while reconditioning is still under way [12].

• Short flights count too. Spine lengthening and back pain begin within days [2][6], so they apply to spaceflight participants on short orbital trips as well as to career astronauts.

For a worked example, see the case file back pain and leg numbness three weeks after landing.