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. Using virtual reality, researchers discovered that the secret lies in the speed of movement.
By simulating a prosthetic hand that automatically bends toward a target, the researchers found that a “moderate” speed (about 1 second per movement) produced the highest level of body ownership and ease of use. Movements that are too fast or too slow cause the brain to reject the limb as “unsettling” or “foreign.”
This research suggests that to be successful, future robot augmentations will need to mimic human-like timing, rather than simply pursuing maximum speed or mechanical efficiency.
important facts
- “Goldilocks” speed: We found that a motion time of approximately 1 second was the ideal speed to maximize the user’s sense of “ownership” of the robot’s limbs.
- Rejection threshold: If the robot moves too fast (125 ms) or too slow (4 s), the user’s sense of agency and comfort is significantly reduced.
- Ability and discomfort: Faster movements are perceived as more “competent”, but they also cause the highest level of discomfort and do not feel like a natural part of the body.
- Beyond prosthetics: These findings have implications for all robotic body augmentations, including wearable “extra” limbs and exoskeletons that operate semi-autonomously.
- Advantages of VR: Virtual reality has enabled researchers to safely test advanced prosthetic control schemes not yet available in the physical world.
sauce: Tutankhamun
As autonomous prosthetic arms equipped with AI become more widespread, it will be important to understand how people feel about and accept them.
In this study, we used virtual reality to simulate a situation in which a participant’s own arm was replaced by a robot’s prosthetic hand, and examined how the speed of the prosthetic hand’s movement affects bodily expressions such as a sense of ownership of the body, a sense of independence, ease of use, and social impressions such as the robot’s competence and discomfort.
We found that while movements that are both too fast and too slow reduce body ownership and ease of use, moderate speeds, close to humans’ natural reach, i.e. movement times of about 1 second, produce the most positive impressions.
When a person loses a hand or arm, a prosthetic leg is an essential technology to maintain daily functioning.
Much of the prosthesis research to date has focused on control methods that allow devices to move according to the user’s intentions, often 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 viable for future prosthetics to assess the situation and provide assistance through autonomous or semi-autonomous movement.
However, when a part of the body moves independently of one’s will, people feel that it is “unsettling” or “not part of their own body,” which creates a major barrier to acceptance.
To address this issue, previous research suggests that even when limbs move on their own, understanding the purpose or intent of the movements can reduce discomfort and increase acceptance as part of the body.
Based 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 at Kochi University of Technology’s School of Informatics) focused on movement speed.
In virtual reality, we presented an avatar with the left forearm replaced by a prosthetic leg, and participants performed a reaching task. The prosthetic hand (virtual forearm) autonomously flexed toward the target, and its movement time was systematically varied over six levels (from 125 ms to 4 s).
After each condition, participants rated their sense of body ownership, sense of agency, ease of use (SUS), and social impressions of the robot (RoSAS: competence, warmth, discomfort).
The results were clear.
• Moderate speed (1 second travel time) had the highest levels of body ownership, agency, and ease of use.
• Body ownership, agency, and usability were significantly reduced in the fastest (125 ms) and slowest (4 s) conditions.
• Cognitive performance is higher at moderate to slightly faster speeds, but discomfort is highest at the fastest speeds. Warmth was not clearly dependent on speed.
These findings demonstrate that in a future where AI-enabled prosthetics provide autonomous assistance, simply pursuing faster and more accurate performance will not be enough. Instead, travel speeds should be designed for speeds that people can easily accept as part of their bodies.
This insight could inform not only the design of autonomous prosthetic hands, but also other forms of robotic body augmentation that act as functional extensions of the body, such as supernumerary robotic limbs, exoskeletons, and wearable robots.
In the future, we will also consider adaptation and learning through long-term use. People will be able to feel that familiar tools are part of their own bodies. If fast and accurate robot body parts are used continuously in daily life, they may become “normal”, more convenient to use, and more easily embodied.
Finally, the use of VR is important because it allows researchers to safely simulate prosthetic techniques and control schemes that are not yet widely available, allowing for a preliminary assessment of psychological, acceptance-related, and design requirements.
Funding agency:
This research was supported by JSPS KAKENHI (JP22KK0158), Murata Science Education Foundation, JST (JPMJFS121), and Ministry of Education, Culture, Sports, Science and Technology (202334Z302).
Answers to key questions:
answer: If your arm moves too quickly, it will feel more like a tool or a machine than “you.” If you move too slowly, you’ll feel sluggish and frustrated. For the brain to accept a robot’s limbs as part of its body, it must behave in a way that matches its internal expectations for human movement.
answer: This is called a “sense of subjectivity.” This study showed that when autonomous speed was felt to be “human-like,” participants actually felt a greater sense of control over their limbs, even if the limbs were moving independently.
answer: It could be faster, but this research suggests it needs to be faster. designed It operates at natural speeds during daily tasks, allowing users to feel comfortable and “connected” to their devices.
Editorial note:
- This article was edited by the editors of Neuroscience News.
- Journal articles were reviewed in full text.
- Additional context added by staff.
About this robot research news
author: Shino Okazaki
sauce: Tutankhamun
contact: Shino Okazaki – TUT
image: Image credited to Neuroscience News
Original research: Open access.
“Autonomous prosthetic leg locomotion speed shapes embodiment, usability, and robot social attributes in virtual reality” Hapuarachchi, H., Inoue, Y., Shigemasu, H., Kitazaki, M., authors scientific report
DOI:10.1038/s41598-026-38977-8
abstract
Autonomous prosthetic leg locomotion speed shapes embodiment, usability, and robot social attributes in virtual reality
Autonomous robotic prosthetics can help amputees regain function and a near-normal appearance. However, psychologically integrating such limbs into body image requires an understanding of how movement characteristics, such as movement speed, influence the sense of embodiment.
Using VR simulation, we investigated how the speed of autonomous lower arm movements affects physicality and user perception. Nineteen healthy participants embodied an amputated virtual avatar in which the lower left arm of the prosthesis moved autonomously at six different speeds following a minimum jerk trajectory.
After performing the reaching task at each speed, participants rated their sense of body ownership, sense of agency, ease of use, competence, warmth, and discomfort. Ownership, agency, and ease of use were highest for moderate speeds (autonomous movements lasting 1 s) and significantly lower for both fast (125 ms) and slow movements (4 s) (p< 0.05).
Capacity was significantly higher at medium and medium speeds compared to low speeds. Discomfort was significantly greater at the fastest speeds compared to medium and lower speeds.
Overall, the results show a trend in which moderate movement speeds are favorable for users’ perception of autonomous limbs, suggesting the existence of an optimal speed or speed range to enhance embodiment and ease of use.
