Long-flight astronauts have atrophy of the muscles supporting the spine — and they don’t return to normal even after several weeks back on Earth, a NASA study shows.

Among many space-related health changes, NASA astronauts frequently report back pain while in orbit and when they return to Earth, which physicians believe is related to changes in the astronauts’ intervertebral discs. The discs create a cushion between vertebrae in a person’s spine, and changes to their shape and size can affect the spinal column and back.

“NASA crewmembers have a 4.3 times higher risk of herniated intervertebral discs, compared to the general and military aviator populations,” explained lead author Dr. Douglas Chang, from the University of California, San Diego, and co-authors.

“The highest risk occurs during the first year after a mission. Microgravity exposure during long-duration spaceflights results in about 2 inches (5 cm) lengthening of body height, spinal pain, and skeletal deconditioning.”

Six NASA astronauts were studied before and after spending four to seven months in microgravity conditions on the International Space Station.

Each astronaut had magnetic resonance imaging (MRI) scans of the spine before their mission, immediately post-flight and then 33 to 67 days after landing.

The goal was to understand factors affecting lumbar spine strength and low back pain during long-duration spaceflight, as well as the spine’s response after returning to Earth gravity.

Dr. Chang and his colleagues used an image ‘thresh-holding’ technique to estimate lean muscle separated from non-lean muscle components.

The MRI scans indicated significant atrophy of the paraspinal lean muscle mass — which plays a critical role in spinal support and movement — during the astronauts’ time in space.

The lean muscle, or ‘functional,’ cross-sectional area of the lumbar paraspinal muscles decreased by an average of 19% from preflight to immediate postflight scans.

A month or two later, only about two-thirds of the reduction had recovered.

There was an even more dramatic reduction in the functional cross-sectional area of the paraspinal muscles relative to total paraspinal cross-sectional area.

The ratio of lean muscle decreased from 86% preflight to 72% immediately postflight.

At follow-up, the ratio recovered to 81%, but was still less than the preflight value.

In contrast, there was no consistent change in the height of the spinal intervertebral discs.

“These measurements run counter to previous hypotheses about the effects of microgravity on disc swelling,” the study authors said.

“Further studies will be needed to clarify the effects on disc height, and whether they contribute to the increase in body height during space missions, and to the increased risk of herniated disc disease.”

“Meanwhile, the finding of paraspinal muscle atrophy suggests possible preventive steps to reduce the spinal effects of spaceflight.”

“For instance, core-strengthening exercises, like those recommended for patients with back pain on Earth, might be a useful addition to the astronaut exercise training program. Yoga might be another promising approach, especially for addressing spinal stiffness and reduced mobility.”

The findings were published online this week in the journal Spine.

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D.G. Chang et al. Lumbar Spine Paraspinal Muscle and Intervertebral Disc Height Changes in Astronauts after Long-duration Spaceflight on the International Space Station. Spine, published online October 25, 2016; doi: 10.1097/BRS.0000000000001873