When astronauts come back down to Earth, it can take a moment to adjust to the return of gravity.
A viral video of NASA astronaut Tom Marshburn cursing gravity as he tries to place a pen mid-air was actually a parody. But it's not too far off what really happens to astronauts after prolonged time in space, as a new study almost 20 years in the making explains.
Researchers from the Catholic University of Louvain in Belgium and the Basque Foundation for Science in Spain compared the way astronauts gripped and moved objects when they were in the microgravity of space versus when they were on Earth.
The study involved two female and nine male astronauts, who stayed for at least five months aboard the International Space Station (ISS).
Even after months of weightlessness, they found, the "imprint of gravity remains visible" in the way the astronauts manipulate objects, suggesting it takes the human brain quite some time to reprogram that muscle memory.
On Earth, the main reason we grip objects is to ensure they don't fall. Meanwhile, in the microgravity environment of outer space, objects won't fall even if we let them go: The purpose of gripping them is usually to move them through space, rather than to hold them aloft.
To test these effects, the astronauts completed a series of tasks to measure their grip, movement, and ability to hold objects without slipping, when interacting with an object designed for the purpose. They repeated the tasks multiple times before, during, and after their ISS journey.
In the first task, the astronauts held the object between their right thumb and forefinger, and moved their arm up and down (while holding the object) in time to a metronome, or sometimes without.
In the second task, the same object was fixed in an upright position on a platform in front of the participant, who gripped the object between thumb and forefinger while sliding their grasp up and down.
This allowed the researchers to measure the astronauts' sense of the minimum friction force required to hold an object without slipping.
Arm movements were slower in microgravity (unless the metronome was there to keep time), and they were more symmetrical: The forces involved in lifting the object up or down were similar.
But it appeared that even after several months in orbit, the astronauts hadn't fully adjusted to their new weightless environment, applying far greater hand grip than is needed in space, anticipating a fight against gravity when holding or moving objects.
Though the astronauts didn't seem to adapt their grip much in space, their return to Earth revealed something had indeed shifted, as they slowly adapted back to gravity's pull.
"Interestingly, some astronauts reported orally that the object felt heavier than they expected," the authors note.
"The robust grip-load force coupling acquired through years of learning on Earth can thus be disrupted after sufficient time spent in weightlessness."
After just one day, the astronaut's movements in the first task returned to the usual state of asymmetry: That is, more force needs to be applied to lift an object than to lower it. Their bodies quickly adjusted back to their home environment, but their brains still made some incorrect predictions about objects' mass.
Related: Astronauts Return to Earth With Lasting Brain Changes
"The gradual and incomplete adjustments when passing from one gravitational context to the other underlines the predictive nature of the neural processes underlying these behaviors," the authors explain.
The research was published in The Journal of Neuroscience.