Summary: As AI-powered prosthetic limbs become more autonomous, a major hurdle remains: how to make an independent robotic arm feel like a natural part of the user’s body. Researchers used virtual reality to discover that the secret lies in movement speed.
By simulating a prosthetic arm that flexes toward a target automatically, the team found that “moderate” speeds—roughly one second per movement—produced the highest levels of body ownership and usability. Both overly fast and overly slow movements caused the brain to reject the limb as “unsettling” or “foreign.”
This study suggests that for future robotic augmentations to be successful, they must mimic human-like timing rather than just pursuing maximum speed or mechanical efficiency.
Key Facts
- The “Goldilocks” Speed: A movement duration of approximately one second was found to be the ideal speed for maximizing a user’s sense of “owning” the robotic limb.
- The Rejection Threshold: Robotic movements that were too fast (125 ms) or too slow (4 s) significantly lowered the user’s sense of agency and comfort.
- Competence vs. Discomfort: While faster movements were perceived as more “competent,” they also triggered the highest levels of discomfort and felt the least like a natural body part.
- Beyond Prosthetics: These findings have implications for all robotic body augmentations, including wearable “supernumerary” limbs and exoskeletons that operate semi-autonomously.
- The VR Advantage: Virtual reality allowed researchers to safely test advanced prosthetic control schemes that are not yet available in the physical world.
Source: TUT
When AI powered prosthetic arms that move autonomously become widespread, understanding how people feel about them and accept them will be crucial.
In this study, we used virtual reality to simulate a situation in which a participant’s own arm was replaced by a robotic prosthetic arm, and examined how the prosthesis movement speed affects embodiment, including body ownership, the sense of agency, usability, and social impressions of the robot such as competence and discomfort.
We found that both overly fast and overly slow movements reduced body ownership and usability, whereas a moderate speed close to natural human reaching, with a movement duration of about one second, produced the most positive impressions.
When a person loses a hand or arm, prosthetic limbs are essential technologies for maintaining everyday function.
To date, much prosthetics research has focused on control methods that enable the device to move according to the user’s intention, often by using biosignals such as electromyography (EMG) and electroencephalography (EEG), and on improving the accuracy of such control.
Meanwhile, advances in machine learning and AI are making it increasingly realistic that future prostheses will assess the situation and provide assistance through autonomous or semi-autonomous movements.
However, when a body part moves independently of one’s will, people are likely to experience it as “unsettling” or “not part of my body,” creating a major barrier to acceptance.
Addressing this issue, prior work has suggested that even if a limb moves on its own, discomfort can be reduced and acceptance as part of the body can increase when the movement’s goal or intention is understandable.
Building on this idea, Harin Manujaya Hapuarachchi and his colleagues (Hapuarachchi was a doctoral student at the time of the study and is now an Assistant Professor in the School of Informatics at Kochi University of Technology) focused on movement speed.
In virtual reality, we presented an avatar whose left forearm was replaced with a prosthetic limb, and participants performed a reaching task. The prosthetic arm (a virtual forearm) autonomously flexed toward a target, and we systematically varied its movement duration across six levels (125 ms to 4 s).
After each condition, participants rated body ownership, sense of agency, usability (SUS), and social impressions of the robot (RoSAS: competence, warmth, and discomfort).
The results were clear.
• At a moderate speed (movement duration of 1 s), body ownership, agency, and usability were highest.
• In the fastest (125 ms) and slowest (4 s) conditions, body ownership, agency, and usability were significantly lower.
• Perceived competence was higher at moderate to slightly faster speeds, whereas discomfort was highest in the fastest condition. Warmth did not show a clear dependence on speed.
These findings indicate that, in a future where AI enabled prostheses provide autonomous assistance, it is not sufficient to pursue faster and more accurate performance alone. Instead, movement speed should be designed to match what people can readily accept as part of their own body.
The insights may inform not only the design of autonomous prosthetic arms, but also other forms of robotic body augmentation, such as supernumerary robotic limbs, exoskeletons, and wearable robots, that operate as functional extensions of the body.
Looking ahead, we will also examine adaptation and learning through long-term use. People can come to experience familiar tools as if they were part of their body. If a fast and accurate robotic body part is used continuously in daily life, it may become “normal,” feel easier to use, and be more readily embodied.
Finally, using VR is important because it allows researchers to safely simulate prosthetic technologies and control schemes that are not yet widely available, enabling the psychological, acceptance-related, and design requirements to be evaluated in advance.
Funding agency:
This research was supported by JSPS KAKENHI (JP22KK0158), the Murata Science and Education Foundation, JST (JPMJFS121), and MEXT (202334Z302).
Key Questions Answered:
A: If an arm moves too fast, it feels like a tool or a machine rather than “you.” If it moves too slow, it feels sluggish and frustrating. For your brain to accept a robotic limb as part of your body, it needs to behave in a way that matches your internal expectation of human movement.
A: This is called the “Sense of Agency.” The study showed that when the autonomous speed felt “human,” participants actually felt a higher sense of control over the limb, even though it was moving independently.
A: They might be capable of high speeds, but this research suggests they should be designed to move at natural speeds during everyday tasks to ensure the user feels comfortable and “connected” to the device.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this robotics research news
Author: Shino Okazaki
Source: TUT
Contact: Shino Okazaki – TUT
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Movement speed of an autonomous prosthetic limb shapes embodiment, usability and robotic social attributes in virtual reality” by Hapuarachchi, H., Inoue, Y., Shigemasu, H., & Kitazaki, M., Scientific Reports
DOI:10.1038/s41598-026-38977-8
Abstract
Movement speed of an autonomous prosthetic limb shapes embodiment, usability and robotic social attributes in virtual reality
Autonomous robotic prostheses can aid individuals with limb loss regain functionality and near-normal appearance. However, to psychologically integrate such limbs into one’s body image, it is necessary to understand how movement characteristics, such as movement speed, affect the sense of embodiment.
Using a VR simulation, we investigated how the speed of autonomous lower-arm movements affects embodiment and user-perception. Nineteen healthy participants embodied an amputated virtual avatar with a prosthetic lower left-arm moving autonomously at six different speeds following minimum-jerk trajectories.
Participants rated sense of body ownership, sense of agency, usability, competence, warmth, and discomfort after performing a reaching task at each speed. Ownership, sense of agency, and usability were highest for moderate speeds (autonomous movements lasting 1 s), and were significantly lower for both faster (125 ms) and slower (4 s) movements (p < 0.05).
Competence was significantly higher at moderate and moderately fast speeds compared to slower speeds. Discomfort was significantly higher at the fastest speed compared to moderate and slower speeds.
Overall, the results show a tendency of moderate movement speeds being favorable for user perception of autonomous limbs and hint at the existence of an optimal speed or a speed range for enhancing embodiment and usability.
