For Space Station Astronauts, Spinal Muscles Shrink after Months in Space

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While astronauts on long space missions do not experience a change in spinal disc height, the muscles supporting the spine weaken, find researchers at University of California San Diego School of Medicine. The study, published October 25 in the research journal Spine, provides new insights into the elevated rates of back pain and disc disease associated with prolonged spaceflight.

"These findings run counter to the current scientific thinking about the effects of microgravity on disc swelling," said Douglas Chang, MD, PhD, associate professor of orthopaedic surgery and chief of physical medicine and rehabilitation service at UC San Diego Health, and first author of the study. “Further studies will be needed to clarify the effects on disc height, and determine whether they contribute to the increase in body height during space missions, and to the increased risk of herniated discs. However, it’s information like this that could provide helpful information needed to support longer space missions, such as a manned mission to Mars."

Six NASA crewmembers were studied before and after spending four to seven months in microgravity on the International Space Station. Each astronaut had magnetic resonance imaging (MRI) scans of their spines before their mission, immediately after their return to Earth and again one to two months later. The data were obtained as part of a NASA-funded research study led by Alan Hargens, PhD, professor of orthopedic surgery at UC San Diego School of Medicine and senior author of the study, and co-author Jeffrey Lotz, PhD, at University of California San Francisco.

The researchers’ 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. Back pain is common during prolonged missions, with more than half of crew members reporting spinal pain. Astronauts are also at increased risk of spinal disc herniation in the months after returning from spaceflight — about four times higher than in matched controls.

Back issues in astronauts are accompanied by a roughly two-inch increase in body height, thought to result from spinal unloading (lack of weight carried by the lower back) and other body changes related to microgravity. The researchers used an image technique that allowed them to estimate lean muscle mass separate from non-lean muscle components.

The MRI scans indicated significant atrophy (weakening) of the paraspinal lean muscle mass during the astronauts’ time in space — that’s the size of the small muscles that connect to the vertebrae and direct the motion of individual bones, helping to support and prevent misalignment of the spine and allowing for core movement. The functional, cross-sectional area of the paraspinal muscles decreased by an average of 19 percent from preflight to immediate post-flight 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 percent preflight to 72 percent immediately post-flight. At follow-up, the ratio recovered by 81 percent, but was still less than the preflight value. In contrast, there was no consistent change in the height of the spinal intervertebral discs.

These findings suggest 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, Chang said.

He also said yoga might be another promising approach, especially for addressing spinal stiffness and reduced mobility. Further study is needed to determine whether new exercise countermeasures can prevent in-flight paraspinal muscle atrophy, improve spinal pain and function and shorten recovery time, and how such exercise might be performed in a microgravity environment with available exercise equipment.

"Above all this science, what I find is the most unique aspect about space research is the inspiration, curiosity and excitement generated in nearly everyone I talk to in terms of overcoming personal challenges, questioning our place in the Universe, and addressing change here at home,” Chang said.

Co-authors of this study also include: Robert M. Healey, Alexander J. Snyder, Dezba G. Coughlin, Jeannie F. Bailey, UC San Diego; Jojo V. Sayson, The Ola Grimsby Institute; Brandon R. Macias, KBRwyle; and Scott E. Parazynski, Arizona State University.

This research was funded, in part, by National Aeronautics and Space Administration (NASA) (grants NNX10AM18G and NNX13AM89G), National Space Biomedical Research Institute (grant NASA NCC 9-58), Environmental and Exercise Section Gravitational Physiology (Pre-Doctoral Award), and American Physiological Society (Excellence in Professional Student Research Travel Award).