Five gravity levels revealed a clear result: people had the greatest difficulty producing smooth, coordinated force with both hands in zero gravity. When some gravitational loading remained, their performance generally stayed closer to the level measured under normal Earth gravity.
A 2026 study in npj Microgravity examined how changing gravity affects two-handed movement and the shared nerve signals reaching muscles in each arm. Researchers led by Deanna M. Kennedy and Ana Diaz-Artiles studied rhythmic hand-force tasks during aircraft maneuvers that briefly produced weightlessness and several levels of partial gravity.
The team, whose members included researchers from Texas A&M University, reported that movement timing held up surprisingly well across the gravity levels. Force became lower and less smooth in weightlessness, especially when each hand had to follow a different rhythm. The journal record lists the paper as published on April 16, 2026, with a version of record dated June 17, 2026.
How parabolic flights recreated five gravity levels
A parabolic flight uses a carefully controlled climb and descent to change the force felt inside an aircraft. During part of each arc, the aircraft and everything inside it fall together. People and equipment then experience a short period of reduced gravity or weightlessness.
For the experiment, the aircraft produced conditions at 0g, 0.25g, 0.5g, 0.75g and 1g. The 1g condition represented ordinary Earth gravity, while 0g produced microgravity. The three levels between them allowed the researchers to examine whether motor control changed step by step as gravitational loading decreased.
Each period lasted only a short time, so participants had to begin their assigned movement quickly. Such flights offer direct exposure to altered gravity without sending an experiment into orbit. A PubMed entry records the experiment as a study of isometric two-handed coordination, meaning the participants produced force while making little large-scale arm movement.
The two-handed force tests
Bimanual coordination is the brain’s ability to control both sides of the body together. Everyday examples include tying shoes and steering a vehicle. Space crews also depend on this ability while using controls, handling tools and completing experiments in confined work areas.
Participants pressed force sensors in repeated rhythms. In the simpler 1:1 task, the left and right hands produced pulses together. During the harder 1:2 pattern, the right hand produced two pulses for every one produced by the left hand. The second task placed greater demands on the brain because each side had to maintain a different pace while remaining linked to the same overall pattern.
The researchers measured the strength and timing of each force pulse. They also recorded muscle activity with electromyography, commonly called EMG. Sensors placed on the skin detected the small electrical changes created when the nervous system activated the muscles.
Several measurements described performance. Mean force showed how strongly participants pressed. Timing measurements showed whether the hands kept the requested rhythm, while force harmonicity described how smoothly the repeated force pulses followed one another. The open-access full paper includes the task design, figures and statistical analyses.
Zero gravity reduced force and smoothness
At 0g, participants produced lower mean force than they did at 1g. Their force pulses also became less smooth, with the strongest difficulty appearing during the complex 1:2 task. The lack of body weight and gravitational loading therefore affected how force was produced, even though the hands could still follow much of the required timing.
Movement timing remained broadly stable across the five conditions. Participants usually kept the 1:1 task near an even ratio and the 1:2 task near its assigned relationship. However, the 1:2 timing ratio at 0g moved farther from the target than it did under the other gravity conditions, showing that weightlessness placed added strain on the hardest coordination pattern.
Gravity supplies a constant stream of information to the nervous system. The inner ear detects motion and orientation, while receptors in muscles and joints report body position and loading. On Earth, the brain has spent a lifetime combining those signals when planning how much force to use.
Weightlessness suddenly changes that familiar flow of information. Muscles no longer support the limbs against the same downward pull and the balance organs in the inner ear receive an unusual motion pattern. The brain must adjust its predictions while the task is already underway, which may help explain the lower and less-smooth force seen during the brief 0g periods.
Partial gravity preserved Earth-like control
Some gravitational loading appeared to support more familiar motor performance. The paper’s abstract states, “Performance under partial gravity (0.25 – 0.75 g) generally trended toward the 1 g condition.” The wording reflects an overall tendency across the measurements rather than a claim that every partial-gravity level produced identical results.
Even a reduced downward load gives muscles, tendons and joints physical resistance. Pressure and stretch receptors can use that resistance to report where a limb is and how hard it is working. Scientists call this body-position sense proprioception and it helps the brain adjust movement without relying entirely on vision.
The tested levels also help place future planetary work in context. Lunar surface gravity is about 0.16g, below the experiment’s lowest partial-gravity level of 0.25g. Mars has about 0.38g, which lies between the 0.25g and 0.5g conditions. The experiment therefore sampled useful parts of the partial-gravity range, while leaving the exact lunar level for future research.
Performance did not change along a perfectly simple line as gravity decreased. Human movement depends on the task, the available sensory information and the time allowed for adaptation. The broad pattern still placed the largest behavioral difficulty at 0g, while partial loading tended to support force control closer to the Earth condition.
Muscle signals weakened in the beta band
Force measurements described what the hands did, while EMG allowed the team to examine how muscle activity in the two arms was linked. The researchers studied matching muscles across the body and looked for activity that rose and fell together at particular frequencies.
An exploratory analysis found lower shared muscle-signal power at 0g than at 1g within the beta band, a frequency range from 13 to 30 hertz. Beta-frequency activity is often connected with the nervous system’s control of steady or coordinated movement. Lower coupling may indicate that common neural input to the two arms became weaker during weightlessness.
The neural result requires caution. It came from an exploratory comparison performed after a mathematical transformation of the data. A more traditional coherence analysis, another method for measuring synchronized activity, found no significant gravity-related differences.
Researchers therefore treated the beta-band result as a clue for future studies rather than final proof of a specific brain mechanism. EMG records electrical activity at the muscles, so it provides an indirect view of commands traveling through the nervous system. Direct brain recordings during a parabolic flight would create major practical challenges.
What the findings mean for Moon and Mars crews
Astronauts regularly perform tasks that require both hands to follow a shared plan. They may operate a spacecraft interface while stabilizing their body, connect equipment during surface work or control a robotic system. Small losses in smoothness can become important when a task demands careful force near fragile hardware.
The results suggest that astronaut training could include complex two-handed force tasks under changing gravity. Training could help crews learn how their movements feel when the usual load disappears or returns. Equipment designers could also consider controls that provide clear physical feedback, allowing the hands to sense pressure and position more easily.
Partial gravity may offer more usable sensory information than weightlessness, yet crews will still need time to adapt after landing. A person traveling to the Moon or Mars would move from Earth gravity into microgravity during the journey, then enter a reduced-gravity surface environment. Each transition changes the forces acting on the body.
The findings could support planning for lunar and Martian missions, especially where crews must begin demanding work soon after arrival. The related NASA Task Book describes research aimed at understanding partial-gravity effects on two-handed tasks that have operational value for spaceflight.
Lessons from altered gravity may also have uses on Earth. Research into shared nerve signals and force control can contribute to rehabilitation work involving movement disorders or recovery from injury. Such applications will require separate clinical studies because the flight experiment examined short-term responses to gravity changes.
Limits of the short flight experiment
Parabolic flights provide only brief exposures to each gravity condition. The nervous system has little time to learn from one arc before the aircraft begins another phase. Astronauts living in orbit or working on a planetary surface would experience altered gravity for days, months or longer, allowing much deeper adaptation.
The participant group was also relatively skilled, according to the researchers. People with less practice may show larger errors, while highly trained astronauts could develop different strategies. Future experiments could compare experience levels and examine whether repeated training changes the size of the gravity effects.
Another limitation comes from the statistical status of the beta-band result. The finding depended on an exploratory analysis and the standard coherence test did not detect a gravity effect. Larger studies with longer recording periods could determine whether reduced beta-frequency coupling appears reliably in weightlessness.
Future work could also test gravity levels closer to the Moon’s 0.16g and Mars’ 0.38g, then use tasks that resemble real spacecraft controls or surface tools. Longer exposures would show whether early coordination problems fade as the nervous system builds new movement predictions.
The present experiment establishes a useful starting point. Two-handed timing remained fairly strong, while force output and smoothness suffered most when gravitational loading disappeared. Partial gravity generally supported behavior closer to Earth performance, giving mission planners a clearer view of how the human nervous system may handle work between 0g and 1g.
