According to one study, astronauts who spend several months on the International Space Station (ISS) are likely to have significant reductions in the size and density of their spinal muscles after returning to Earth. Some changes in muscle composition are still present for up to four years after a long-duration space flight, researchers at the Massachusetts Institute of Technology (MIT) in the United States said. Like NASA’s plans for future missions to Mars and beyond, these results can be used to guide future measures to mitigate declines in trunk muscle morphology and associated functional deficits, they said.
“Spaceflight-induced changes in paraspinal muscle morphology may contribute to commonly reported back pain in astronauts,” said Katelyn Burkhart of MIT.
The study, published in the journal Spine, analyzed computed tomography (CT) scans of the lumbar (lower) spine in 17 astronauts and cosmonauts who flew missions on the ISS. The scans obtained before and after the missions were analyzed to determine changes in the size and composition of the paraspinal muscles. The average time in space was six months. When running up and down the spine, the paraspinal muscles play a key role in the movement and posture of the spine.
Previous studies have found a reduction in paraspinal muscle mass after prolonged time in space, suggesting that muscle atrophy can occur without the resistance provided by gravity. CT scans showed reductions in the size of the paraspinal muscles after space flight. For individual muscles, muscle size decreased from 4.6 to 8.8 percent. On follow-up scans one year later, size returned to at least normal for all muscles.
The scans also showed significant increases in the amount of fatty tissue present in the paraspinal muscles. Consequently, the astronauts’ muscle density, which is inversely related to fat content, decreased by 5.9 to 8.8 percent. For most muscles, composition returned to normal within a year. However, for two muscles, the square and psoas muscles, the fat content remained above pre-flight values, even two to four years after the astronaut returned from space. These muscles, which connect the spine to the pelvis, are found along the spine. In comparison, the paraspinal muscles located behind the spine returned to normal size and density.
Changes in muscle size and composition varied between individuals. For some muscles, changes in size were related, at least in part, to the amount and type of exercise the astronauts performed while in zero gravity: resistance exercise or cycling. Exercise in flight does not appear to affect changes in muscle density, the researchers said. Previous studies of astronauts have linked spaceflight to muscle atrophy, especially of the muscles that maintain upright posture and stability on Earth in normal gravity.
Many astronauts experience low back pain during and immediately after space missions, and they appear to be at increased risk for spinal disc herniation. The new study is the first to measure changes in the size and density of individual paraspinal muscles. The results show that muscle size returns to normal after Earth’s recovery, but that some changes in muscle composition, particularly increased fat infiltration, can persist for at least a few years. Some of the changes in the paraspinal muscle appear to be affected by exercise, suggesting possible approaches to prevent the adverse effects of prolonged space flight on spinal health and function.