# Ten astronauts showed mildly weaker heart contractions on day 150 aboard the ISS, and all three ultrasound measures recovered within four days of landing

> Astronauts living aboard the International Space Station showed a mild decline in how strongly part of the heart contracted after about five months in orbit. Measurements taken four days after landing had returned close to their preflight levels, pointing to a fast...

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Published: 2026-08-21T18:55:02+00:00
Categories: News, Space

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Astronauts living aboard the International Space Station showed a mild decline in how strongly part of the heart contracted after about five months in orbit. Measurements taken four days after landing had returned close to their preflight levels, pointing to a fast and largely reversible response to life in microgravity.

The findings come from a [2026 study](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2026.1699925/full) published in **Frontiers in Physiology**. Researchers followed ten astronauts before their missions, on the 150th day of spaceflight and soon after their return to Earth. The team focused on the **left ventricle**, the heart chamber that pumps oxygen-rich blood through the body.

The paper describes the change as modest. "Results indicate a mild reduction in cardiac contractility with spaceflight," the study abstract states. The quick recovery suggests that reduced daily movement and changes in blood volume may explain much of the effect.

## How ultrasound tracked the beating heart

Doctors often use ultrasound to watch the walls and chambers of the heart move. The method sends sound waves into the body and records the echoes that return. A computer builds moving images from those echoes, allowing researchers to see the heart contract and relax.

For the ISS study, the team used a method called **speckle tracking**. Heart ultrasound images contain many small points created by differences within the tissue. Software can follow these speckles from one video frame to the next, much like tracking tiny marks on a moving rubber sheet.

The distance traveled by each speckle provides a measure of heart muscle movement. A longer path usually means that the nearby muscle moved more during the heartbeat. The researchers also counted the tracked points and measured how much of the heart wall showed the greatest movement.

Before launch and after landing, trained sonographers collected the images while each astronaut rested on their back. In orbit, the astronauts collected their own scans with live voice and video guidance from an expert on Earth. Similar remote methods have helped crews perform [advanced ultrasound](https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/International_Space_Station_Benefits_for_Humanity/Advanced_Ultrasound_for_the_Space_Program_and_on_Earth) aboard the station despite limited medical training.

## What changed after 150 days in orbit

On **flight day 150**, the area of heart muscle showing the greatest speckle movement was significantly smaller than it had been before launch. The combined measure based on speckle displacement and the number of tracked points was also lower in orbit than it was four days after landing.

Average speckle movement showed a strong trend toward decline, although the statistical result fell just outside the study's chosen cutoff for significance. Scientists use such cutoffs to judge whether a measured difference is likely to reflect a consistent effect rather than normal variation within a small group.

The software successfully followed about 97 percent of the detected speckles through the cardiac cycle. The number of tracked speckles remained similar across the measurement periods, which gave the researchers more confidence that the movement changes came from the heart images rather than a major loss of tracking quality.

NASA has long used [cardiac ultrasound studies](https://www.nasa.gov/image-article/hrp-integrated-cardiovascular-study/) to examine how long missions influence heart size, workload and blood pumping. Microgravity removes the constant pull that normally shifts blood toward the legs, changing both the heart's load and the way blood spreads through the body.

## Heart muscle movement declined in most astronauts

Seven of the ten astronauts appeared to have lower **mean speckle displacement** on flight day 150. Individual responses varied, so the group average alone could not describe every crew member's experience.

Some astronauts showed a clear drop, while others changed very little. Exercise habits, physical condition before flight and normal differences in heart anatomy could have contributed to that spread. The study did not have enough participants to identify which personal factors best predicted the response.

The measurements captured motion across a two-dimensional view of the ventricle. Hospitals can use more advanced systems to create three-dimensional views, especially when doctors need to locate damaged regions supplied by a blocked artery. The ISS equipment was suited to detecting the broader changes expected during microgravity exposure.

The researchers measured movement through recorded ultrasound video rather than the raw sound-wave signal. Their custom software could therefore process standard two-dimensional recordings from more than one ultrasound machine, an advantage for research aboard a spacecraft where available equipment can change over time.

## Other measures revealed a smaller left ventricle

The ultrasound scans also showed that the amount of blood inside the left ventricle before contraction fell during spaceflight. Scientists call this measurement **end diastolic volume**. A lower value means the chamber held less blood at the point when it was most filled.

**Stroke volume**, the amount of blood pushed out with each heartbeat, was lower on flight day 150 than before launch. The thickness of the heart muscle also declined during the mission. Both chamber volume and muscle thickness returned near their earlier levels after the astronauts came home.

Ejection fraction, which describes the share of blood pumped out of the ventricle during each beat, did not show a statistically significant overall change. A heart can maintain this percentage even when the chamber contains less blood, since the fraction depends on how much blood enters the chamber before the contraction begins.

Similar questions have guided NASA's wider [heart health research](https://www.nasa.gov/missions/station/iss-research/heart-health/) on the space station. Scientists are studying how long missions affect cardiac performance and blood vessels as agencies prepare for crews to spend more time far from Earth.

## Contractility recovered soon after landing

Four days after return, the contractility measurements were close to the values recorded before flight. Speckle displacement increased in nearly every astronaut compared with the in-orbit measurement, although each person followed a slightly different course.

The **rapid recovery** offers an important clue about the cause. Deep changes to heart muscle cells would generally be expected to take longer to reverse. A response that fades within days is more consistent with changes in workload, blood volume and movement after gravity returns.

Landing immediately places new demands on the cardiovascular system. Blood again tends to pool in the lower body and the heart must pump against Earth's gravity. Standing, walking and normal daily activity also raise the heart's workload compared with floating through the station.

The postflight measurement took place around day four, leaving the first hours after landing outside the study's view. Future work with measurements taken sooner and over a longer recovery period could reveal when contractility begins to rise and how quickly each heart measure settles.

## Exercise and fluid shifts may explain the changes

Life in orbit reduces many forms of routine **physical activity**. Astronauts exercise regularly with station equipment, yet floating removes the effort involved in standing upright and walking throughout the day. Muscles and the cardiovascular system therefore receive a different pattern of work.

Past bed-rest experiments offer useful comparisons because long periods spent lying with the head slightly lower than the feet reproduce some effects of spaceflight. Earlier research found changes in heart size and contractility during bed rest, while exercise helped limit several of those changes.

Differences in exercise may also help explain why the ten astronauts responded differently. Training can preserve heart chamber volume and maintain cardiovascular workload. The present study did not provide enough information to connect each astronaut's exercise dose with the size of the contractility change.

**Fluid shifts** provide another possible mechanism. In microgravity, blood and other body fluids move toward the chest and head. The body later reduces fluid volume as it adapts to that new distribution. With less circulating volume, the left ventricle may fill less fully and work under a smaller load.

NASA continues to use [heart and vein scans](https://www.nasa.gov/blogs/spacestation/2025/08/21/ultrasound-scans-on-station-monitor-and-protect-crew-cardiovascular-health/) aboard the ISS to follow cardiovascular changes. Such measurements can help researchers separate the influence of exercise from the effects of fluid movement and the loss of gravity.

## What the small study can reveal

Ten astronauts form a very small study group, a common challenge in human spaceflight research. Space missions involve limited crew numbers, demanding schedules and strict medical privacy. Small samples make it harder to identify subtle effects or differences linked to age, sex and fitness.

Nine participants were men and one was a woman, so the results provide little information about possible sex-related differences. The astronauts were also healthy, carefully selected adults. Their responses may differ from those of older people or individuals with heart conditions.

The study included one main measurement during the six-month mission. Additional scans earlier in flight could show whether the decline begins within days, builds gradually or levels off after the body adapts. More postflight measurements would provide a clearer recovery timeline.

Two ultrasound systems were used for the inflight recordings because the newer device reached the ISS after the first participants had entered the study. The software was designed to analyze ordinary video from either machine and the researchers reported successful tracking across the recordings.

Despite these limits, measurements taken from the same people before, during and after flight offer a valuable comparison. Each astronaut served as their own baseline, reducing some of the variation that would appear if separate groups were compared.

## Heart monitoring for longer space missions

Crews traveling to the Moon or Mars may spend months away from Earth and medical support will become more difficult as distance increases. Communication delays around Mars could prevent doctors on Earth from guiding an ultrasound scan in real time.

Compact ultrasound equipment could give crews a way to watch the heart and blood vessels without carrying a large medical imaging system. Automated software may help astronauts position the probe, capture a useful image and flag changes that deserve closer review.

The ISS result also suggests that several heart measurements should be considered together. Contractility, chamber volume and stroke volume describe related parts of cardiac performance, yet they can change by different amounts. A single percentage may miss an adaptation visible through muscle motion or filling volume.

Researchers will need larger datasets to learn which changes represent healthy adjustment and which could warn of a growing problem. Measurements collected during missions of different lengths may also show whether the mild decline remains stable after six months or continues during a year in orbit.

For **future Moon and Mars missions**, exercise plans may be adjusted with heart imaging data from each crew member. Regular scans could reveal whether a countermeasure is maintaining cardiac workload, while postflight records could improve recovery programs after astronauts return to gravity.