# Nine cosmonauts returned from missions averaging 247 days with a 14 percent loss in the small heart muscles that control the mitral valve

> Deep inside the heart, two small muscles help keep blood moving in the right direction. After long missions aboard the International Space Station, those muscles were about 14 percent smaller in a group of cosmonauts, even though the heart continued to pump...

Canonical URL: https://www.argo.net/nine-cosmonauts-returned-from-missions-averaging-247-days-with-a-14-percent-loss-in-the-small-heart-muscles-that-control-the-mitral-valve/
Byline: ARGO.net Editorial Team
Published: 2026-08-21T04:00:02+00:00
Categories: Explainer, Space

![Astronaut cardiac ultrasound](https://www.argo.net/wp-content/uploads/2026/08/astronaut_cardiac_ultrasound_1200x675.jpg)

Deep inside the heart, two small muscles help keep blood moving in the right direction. After long missions aboard the International Space Station, those muscles were about 14 percent smaller in a group of cosmonauts, even though the heart continued to pump a normal amount of blood with each beat.

The finding comes from a [study in npj Microgravity](https://www.nature.com/articles/s41526-025-00531-7), published on November 12, 2025. Researchers used magnetic resonance imaging before and after spaceflight to examine the **papillary muscles**, which sit inside the heart's main pumping chamber and help control the valve between its left chambers.

The scans point to a highly local response to life in orbit. The papillary muscles lost mass while the wider heart muscle remained stable and may have grown slightly. Blood volumes and major pumping measures were also preserved, suggesting that separate parts of the heart can respond differently to months of reduced gravitational loading.

## MRI scans reveal 14 percent muscle loss

Researchers studied nine male cosmonauts who spent an average of 247 days in space. Six completed missions lasting about six months, while three remained in orbit for roughly one year. Their flights took place during International Space Station Expeditions 63 through 69, which ran between early 2020 and late 2023.

Each cosmonaut received **cardiac MRI** scans before launch and after landing. The preflight examinations took place 45 to 60 days before departure. Follow-up imaging was carried out an average of six days after return, allowing the researchers to compare the heart's structure across the mission.

The study found that papillary muscle mass fell from an average of 10.1 grams before flight to 8.7 grams afterward. The paper's abstract reports that "LV papillary muscle mass decreased by 14%." The result reached statistical significance, which means the measured decline was unlikely to be explained by random variation within the small group. The study details are also listed in the [PubMed record](https://pubmed.ncbi.nlm.nih.gov/41224777/).

Meanwhile, the mass of the wider left ventricular wall showed a possible increase, although that result did not cross the study's threshold for statistical significance. The ratio of papillary muscle mass to total left ventricular mass also decreased. Together, the measurements support a **region-specific cardiac response** in which nearby muscle tissues follow different paths during spaceflight.

## How papillary muscles support the mitral valve

The **mitral valve** sits between the left atrium, which receives oxygen-rich blood from the lungs and the left ventricle, which sends that blood around the body. Two thin flaps open as blood enters the ventricle and close when the ventricle contracts.

Papillary muscles rise from the inner wall of the ventricle. Tough cords connect them to the valve flaps, much like ropes holding a flexible panel in position. As pressure rises during a heartbeat, the muscles tighten the cords and help prevent the flaps from bending too far back toward the atrium.

The muscles operate under an unusual loading cycle. Their electrical activation comes later than activation in parts of the ventricular wall and their contraction continues through much of the pumping phase. They also experience high tension while stretched. Months of reduced loading in orbit could therefore affect them differently from the thicker muscle that forms the outer ventricular wall.

Muscle cells in the papillary structures tend to run along their length. The researchers propose that this layout may increase their sensitivity when gravity no longer pulls along the body's usual head-to-foot direction. Reduced tension could lower the signals that tell cells to maintain contractile proteins, gradually allowing the tissue to shrink.

## Microgravity changes the heart's geometry

Life in **microgravity** changes how fluid moves through the body. On Earth, gravity draws blood toward the legs. In orbit, more fluid shifts toward the chest and head and the body later reduces its total plasma volume. The heart also loses part of the steady mechanical load created by gravity.

The MRI results suggest that the left ventricle became slightly shorter and more spherical after flight. Its average length decreased from 101.3 millimeters to 99.2 millimeters, while its sphericity index increased. The volume inside the chamber remained broadly stable, so the main change involved shape rather than capacity.

Researchers describe the process as **geometric remodeling**. A rounder ventricle may alter the direction and strength of forces passing through the ventricular wall, papillary muscles and mitral valve. Lower strain on the papillary muscles could reduce local mechanical stimulation, which may help explain their loss of mass. The measurements and statistical results are available in the open-access [full paper](https://pmc.ncbi.nlm.nih.gov/articles/PMC12612046/).

The mitral valve opening also became wider when measured in a four-chamber MRI view. Its average diameter increased from 34.8 millimeters before flight to 36.8 millimeters afterward. A second viewing angle did not show a significant change, so dedicated valve imaging will be needed to define the alteration more clearly.

## Exercise preserves the heart's pumping function

Astronauts and cosmonauts exercise during long missions to limit the loss of muscle and bone. Their programs usually combine cycling, treadmill sessions and resistance training. Such **exercise countermeasures** also place a repeated demand on the cardiovascular system, helping the heart retain its ability to pump blood.

The cosmonauts in this study maintained stable left ventricular volumes. Stroke volume, which measures the blood pushed out during one beat, showed no significant change. The percentage of blood ejected from the chamber also remained stable, as did measurements of how strongly the ventricular wall shortened.

Heart rate increased from an average of 51 beats per minute before flight to 59 beats per minute after flight. Because each beat moved a similar volume of blood at a faster rate, total cardiac output rose from about 5.2 liters per minute to 6 liters per minute. The rise may reflect the body's adjustment after returning to Earth.

Preserved **ejection fraction** and wall strain suggest that the ventricle continued to work effectively despite its altered geometry. Earlier bed-rest experiments, which recreate some effects of microgravity on Earth, have also found that regular exercise can preserve ventricular mass and function. Hydration measures may contribute by restoring plasma volume around landing.

## Possible risks after astronauts return to gravity

Papillary muscle loss could influence how well the mitral valve closes when full gravity returns. A wider valve opening combined with smaller supporting muscles may change the balance of tension across the valve flaps. The available scans did not provide the specialized views needed for a full diagnosis of valve leakage or prolapse.

Doctors already know from clinical research that changes in papillary muscle size and position can contribute to mitral regurgitation, a condition in which some blood moves backward through the valve. Valve widening can also accompany poor closure. The spaceflight study establishes a reason to monitor these features while stopping short of showing that the cosmonauts developed valve disease.

Timing may become especially important during landings. After months in orbit, the heart must quickly adjust to Earth's gravity while the body is also dealing with reduced blood volume. Future crews traveling to the Moon or Mars would face partial gravity, followed eventually by another change when they return home.

According to the researchers, no related clinical problems have been reported among astronaut crews after spaceflight. The observed changes may represent a temporary adaptation that recovers after landing. Longer follow-up is needed to learn how rapidly the papillary muscles regain mass and whether the valve opening returns to its earlier size.

## Nine cosmonauts provide an early result

The study's greatest limit is its small group of **nine male cosmonauts**. Human spaceflight research often relies on small samples because only a limited number of people complete long orbital missions and medical experiments must fit around demanding flight schedules.

A group of nine can reveal a clear change when the effect is consistent, although it cannot capture the full range of responses expected across age groups, sexes and health backgrounds. Individual exercise habits may also have influenced the results. Cosmonauts can differ in how closely they follow assigned workouts and how strongly their bodies respond.

The first postflight MRI took place about six days after landing. Cardiovascular recovery begins quickly once gravity returns, so some changes may already have weakened by the time scanning began. Measurements made within hours of landing could reveal larger shifts or additional effects that disappear during the first week.

Water and salt supplements used around landing may have affected tissue hydration and heart measurements. MRI scanner differences also added a technical challenge, although the researchers tested their analysis methods for consistency. Blood pressure readings gathered near the scan dates showed no meaningful preflight-to-postflight change.

## Future missions need targeted heart scans

Future studies will require **targeted MRI protocols** designed to inspect the mitral valve, its supporting cords and both papillary muscles in greater detail. Imaging shortly after landing, followed by scans over several months, could reveal whether the loss reverses and how fast recovery occurs.

Larger groups should include a wider range of astronauts and mission lengths. Researchers could also compare exercise schedules, hydration practices and individual workloads. Such comparisons may identify which countermeasures best protect the small structures inside the heart as well as the larger ventricular wall.

Functional tests could examine whether reduced papillary muscle mass changes contraction speed or tension. Tissue mapping may help detect shifts in water content and the material surrounding heart cells. The complete paper is also preserved in the [DLR research archive](https://elib.dlr.de/219731/1/Tordeur-et-al-Tank-2025-s41526-025-00531-7.pdf).

Long missions to the Moon and Mars will expose crews to several gravity levels over months or years. Careful tracking of **astronaut heart health** can show whether the observed remodeling remains a harmless adaptation or creates a weakness during return to stronger gravity. The first human measurements now give mission doctors a specific part of the heart to watch. Broader mission context is available from [NASA's Human Research Program](https://www.nasa.gov/hrp/).