New research shows the weird ways zero gravity messes with your body awareness

A new study published in Experimental Brain Research provides evidence that human position sense — our ability to detect where our limbs are without looking — is disrupted under conditions of altered gravity. But not all types of position sense appear to respond in the same way. Researchers found that weightlessness during parabolic flights affected some forms of proprioceptive judgment while leaving others relatively stable, suggesting that the nervous system may rely on more than one type of position sense.

Proprioception — sometimes called the “sixth sense” — is our internal sense of body position and movement. It plays a vital role in tasks like walking, reaching, or even just knowing where your arm is when your eyes are closed. This sense arises from signals sent by receptors in muscles and joints, along with input from the inner ear structures responsible for balance and acceleration.

Gravity is a constant force shaping our movement and posture on Earth. When gravity changes, as it does during parabolic flight (which briefly creates states of microgravity and hypergravity), the mechanical forces on joints and muscles change too. Prior studies have suggested that these changes can alter proprioceptive signals, but questions remain about which components of position sense are most sensitive to gravity shifts.

The new study led by Uwe Proske (emeritus professor of physiology at Monash University) and Bernhard Weber (senior researcher at the German Aerospace Center) set out to test whether different methods of measuring position sense — each thought to rely on slightly different mechanisms — are equally affected by gravity. By doing so, the researchers hoped to clarify the neural processes behind proprioception and shed light on how the brain constructs a sense of limb position under novel physical conditions.

“At one point or another, all of us have probably wondered: what would it be like to travel into outer space? That dream has come closer to reality in recent years, as space travel becomes commercialized. But, as is widely known, there are aspects of space travel that can be hazardous,” Proske and Weber told PsyPost.

“We’ve been studying people’s ability to know where their limbs are, even when they’re not looking at them. This is known as position sense, and it’s currently believed to be generated by sensors in our muscles called muscle spindles. We’ve identified different ways to measure position sense and categorized them into three main methods. The question we posed was whether all of these methods rely on signals from muscle spindles. We tested that idea in two ways. One approach involved altering the sensitivity of muscle spindles through muscle contraction or stretch to see whether that affected position sense. The other involved making measurements during parabolic flight.”

“That led us to conclude that muscle spindles don’t consistently play a major role across the different measurement methods,” the researcher continued. “It’s been known for some time that changes in gravity can interfere with muscle spindle responses. That observation led us to examine position sense during parabolic flight. During these maneuvers, gravitational force doubles during the initial climb, followed by a brief weightless phase as the aircraft traces a parabolic arc. When the plane descends and pulls up again, gravitational force doubles once more. We were especially interested in the drop in gravity, which resembles the conditions experienced in a space capsule.”

“We had read reports of astronauts waking up in the dark aboard a spacecraft and seeing what looks like a glowing dial floating in the air above them. Only after switching on a light do they realize it’s the face of their wristwatch — still on their own arm. In low gravity, without visual cues, they had completely lost awareness of their arm’s position. This kind of disorientation can become a problem for astronauts, especially when they’re required to operate complex machinery — sometimes without being able to see their hands.”

“When the chance came to take part in a series of parabolic flights, we decided to make measurements using all three position sense methods. We asked: are all three methods affected in the same way, or do they show measurable differences? We hoped that a deeper understanding of how position sense behaves in low-gravity environments might help us find ways to address the position sense issues reported by astronauts.”

The researchers used three commonly used methods to measure position sense: two-arm matching, one-arm pointing, and one-arm repositioning. Each method involves estimating the angle of an elbow joint without visual feedback, but they differ in how participants use memory, vision, and internal feedback.

Two-arm matching requires participants to align one arm with the other based solely on proprioception. One-arm pointing involves using one arm to point to where the hidden reference arm is believed to be. One-arm repositioning, in contrast, involves memorizing the angle of a moved arm and then trying to reproduce it later from memory.

The experiment took place during parabolic flights that produced short phases of microgravity (0G), hypergravity (1.8G), and normal gravity (1G). Twelve healthy adults completed position sense tasks during these different gravitational conditions while seated in a specially designed apparatus fixed inside an aircraft.

Each participant performed tasks using their dominant or non-dominant arm in a randomized design. To prevent visual cues, blindfolds or eye patches were used depending on the task. Participants were strapped into the setup for stabilization and guided through each trial using audio instructions.

Across the group, the researchers found a consistent pattern. When position sense was measured using two-arm matching or one-arm pointing, gravity had a noticeable effect. In hypergravity, errors increased: participants tended to overshoot, perceiving their arms as being more extended than they actually were. In microgravity, errors decreased — and in the case of two-arm matching, the decrease was statistically significant.

In contrast, one-arm repositioning errors remained largely unchanged across gravity conditions. Regardless of whether participants were in microgravity or hypergravity, their ability to reproduce a remembered elbow position did not differ significantly from their performance in normal gravity.

These results suggest that position sense, as measured by matching and pointing, is influenced by gravity. In these tasks, proprioceptive signals likely depend on inputs from sensors that are affected by joint torque, which changes with gravity. Repositioning, by contrast, appears to rely more on memory and central processing rather than ongoing signals from muscle receptors. This may explain its relative stability across gravity conditions.

“Interestingly, we found that two of the methods were significantly disrupted by the microgravity conditions likely to be experienced in a space capsule,” Proske and Weber said. “We assumed this was due to the known effect of low gravity on muscle spindle responses. But, as often happens in scientific research, we encountered one completely unexpected result. The third method of measuring position sense was entirely unaffected by changes in gravity!”

“After testing several different possibilities, we were ultimately led to the conclusion that this method is governed by memory mechanisms in the brain, completely independent of what the muscle spindles are doing. That’s a surprising conclusion. The currently accepted view is that position sense is generated by nerve signals from sensors in the body. That idea will have to be revised. We believe this finding may have broader implications for how we think about the brain’s role in shaping our sensory experiences.”

One limitation of the study is the relatively small sample size — a common constraint in parabolic flight research due to cost and logistics. In addition, while repositioning performance appeared stable, the researchers acknowledge that the task may still be influenced by central sensory processing that was not directly measured in this study.

Future research is planned to examine whether modifying joint torque artificially — for example, using elastic bands — could shift perceived position even in weightless conditions. Such manipulations might help further distinguish between the contributions of peripheral and central mechanisms in position sense. The researchers also raise broader questions about how the brain integrates peripheral sensory input with centrally stored spatial information.

“The bigger question we faced was: why do we have more than one type of position sense?” the researchers told PsyPost. “Right now, we don’t have a straightforward answer to that. When it comes to the loss of position sense reported by astronauts, we assume it involves situations where gravity changes disrupt the function of muscle spindles. But what exactly causes that disruption? In earlier work, we proposed that a key factor in generating position sense is the torque at the joint the limb moves around — in our case, the elbow.”

“In low gravity, torque is expected to decrease, and we hypothesized that this reduces the sensitivity of the muscle spindles, which in turn weakens position sense. If we’re right, then increasing torque at the elbow in low gravity could restore spindle sensitivity and help recover position sense. We’re suggesting that if astronauts wore elastic suits stiff enough to increase joint torque, it might help preserve their sense of limb position. That’s the direction our research is now heading.”

The study, “Disturbances in human position sense during alterations in gravity: a parabolic flight experiment,” was authored by Bernhard M. Weber, Michael Panzirsch, Benedikt Pleintinger, Martin Stelzer, Stella Arand, Christian Schöttler, Ralph Bayer, Annette Hagengruber, and Uwe Proske.