RESOLVING THE LEDGER
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Add as preferred source It's common to see images of astronauts on the International Space Station exercising, running on special treadmills and pedaling cycles designed for tight quarters.
Besides breaking the monotony of long missions, these workouts are important because astronauts who spend extended periods in space lose muscle mass and strength , since they don't have to deal with gravity to move their bodies and push, pull or lift things.
The effects of zero gravity on the body's most important muscle—the heart—are less clear, but new research from the University of Chicago and the University of Nebraska shows encouraging signs that heart muscle cells aren't negatively affected by space travel.
The researchers analyzed heart cells from five mice that spent 38.5 days on the space station, looking specifically at their sarcomeres, the molecular motors inside the cells that make the heart contract. The contracting force of these motors in mice from the space station was just as strong as that of control mice that stayed on the ground.
Molecular testing also showed no major differences in the proteins that made up the sarcomere between the space mice and their earthbound controls, suggesting that heart cells would continue functioning well during a long trip to space. This is promising news as NASA considers longer missions back to the moon and potentially to Mars.
"There are a lot of things in common between cardiac and skeletal muscle, so we thought that we would see some decrease in heart function from space travel," said UChicago associate professor Jonathan Kirk, a co-senior author of the paper, which was published in the journal npj Microgravity . "But in the end, we're pretty happy that this is the result we found. It doesn't give us something else to dig into scientifically, but it's obviously wonderful news for astronauts in the space program that the heart is going to be okay in space."
Kirk's lab studies cardiovascular disease, particularly what affects the heart's ability to function as a mechanical pump to circulate blood. His team has developed a wide variety of tests to study heart function in frozen tissues, giving them more flexibility to study samples after the fact—perhaps samples collected for a different purpose—without having to rely on fresh tissue collected on the spot.
In August 2023, Kirk gave a presentation at the University of Nebraska that included some Star Wars references to make it more relatable to the audience. After the talk, Pooneh Bagher, an associate professor of cellular and integrative physiology at Nebraska and co-senior author of the new paper, approached him with a proposition: How would he like to study mice from space?
She had access to frozen heart tissue from mice that had been on the space station, left over from a project by scientists at Baylor University.
"I said, "Absolutely, that sounds awesome,'" Kirk said. "It was a perfect fit, and that's exactly why we go to seminars and have in-person conversations with our peers."
Even though this study used a small sample of mice that had been in space for a relatively short period in human terms, Kirk said it's still extremely useful because mice have accelerated life spans.
Their hearts beat up to 600 times a minute—compared with the human range of 60 to 100 beats per minute—so 38.5 days in space for a mouse is more like 7.5 to 10 months for a human heart.
"If there's going to be a problem with the heart, you're going to see it in the sarcomeres first. So, the nice thing about this is that with the biophysical assays of how this little engine inside the heart works, we are able to gauge very early, before a person starts to feel sick or a mouse looks sick, whether or not there was something at the molecular level starting to go wrong," Kirk said.
The researchers did see some traces of inflammation in the heart cells, and Kirk said they would like to study mice that had been in space longer to see if that increases over time. They would also like to study tissues that were collected while on the space station instead of from animals that returned to Earth, to potentially rule out the effects of stress from the trip back home.
As someone who drops Star Wars references into his academic presentations, Kirk said the appeal of studying the effects of space travel on biology is irresistible.
"Space science is fascinating because it lets you look at science from a totally new angle. Everything is different, and therefore it's a tool to understand our biological system under entirely new conditions from anything else we can do in the lab," he said.
"Sometimes you find something that opens a whole new area of research, and sometimes you just confirm that, yeah, astronauts will be fine. Let's go to Mars."
Henry M. Gong et al, Space travel does not significantly impact cardiac sarcomere function but does induce immune-related proteomic changes, npj Microgravity (2026). DOI: 10.1038/s41526-026-00636-7
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