International Space Station Expedition 46 Commander Scott Kelly during a spacewalk, Dec. 21, 2015 Reuters/NASA/Scott Kelly

For astronauts who have undertaken lengthy sojourns in the low-gravity environment of space, back pain is a serious health concern. According to NASA, lower back pain is nearly 70 percent more prevalent in space than on Earth, but despite this, the exact cause of the malady has been hard to ascertain.

Until now.

According to a NASA-sanctioned study published Tuesday in the journal Spine, astronauts’ back pain is caused by wasting of paraspinal muscles — that connect the vertebrae and control their movement — rather than swelling of spinal disks due to lack of gravity as previously thought.

The study, which compared MRIs and stress test data of six NASA astronauts before and after they spent several months on the International Space Station, found that the cross-sectional area of the paraspinal muscles decreased by an average of 19 percent between preflight and immediate post-flight scans. Two months after the flight, only about two-thirds of the reduction had reversed.

In addition, the ratio of the cross-sectional area of the paraspinal muscles relative to the total paraspinal cross-sectional area also decreased drastically — to 72 percent immediately postflight from 86 percent preflight. This ratio later recovered by 81 percent, remaining below the pre-flight value.

“These findings run counter to the current scientific thinking about the effects of microgravity on disc swelling,” lead author Douglas Chang, an associate professor of orthopaedic surgery at the University of California, San Diego, said in a statement. “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.”

Using current technology, the journey to Mars would take anything between six to eight months. And, when the first humans do land on the red planet, they would need to be in the best shape possible, given that they would be burdened with the onerous task of starting a colony from scratch.

The good news is that researchers believe that there are certain “core-strengthening” exercises — similar to the ones recommended for patients on Earth suffering from back pain — that can prove effective in preventing, or at least reducing, the effects of spaceflight on spinal muscles.

“If you could do yoga in space that might really help the astronauts,” co-author Alan Hargens, also from the University of California, San Diego, told the Guardian. “We need to reproduce the normal daily loading activity on the spine in space.”