AI prosthetic arms feel most natural at moderate speed, study reveals why timing matters

The exact speed that makes an AI prosthetic arm feel like your own

As artificial intelligence powered prosthetic arms become more common, understanding how people respond to them will be essential. Acceptance depends not only on how well these devices function, but also on how natural they feel. In this study, researchers used virtual reality to create the illusion that a participant's own arm had been replaced with a robotic prosthetic. They then tested how different movement speeds influenced embodiment, including body ownership, sense of agency, usability, and social impressions such as competence and discomfort. The findings showed a clear pattern. When the prosthetic arm moved too quickly or too slowly, participants felt less connected to it and rated it as less usable. However, when the arm moved at a moderate pace similar to natural human reaching, taking about one second to complete the motion, participants reported the strongest sense that the arm felt like part of their own body. From User Controlled Prosthetics to Autonomous AI For individuals who lose a hand or arm, prosthetic limbs are critical tools for daily life. Much of the research in this field has focused on helping devices respond accurately to a user's intentions. This often involves detecting biosignals such as electromyography (EMG) and electroencephalography (EEG) and translating them into movement. At the same time, rapid progress in machine learning and AI is making it possible for future prosthetics to assist users by moving on their own in certain situations. These autonomous or semi autonomous systems could anticipate needs and provide support automatically. Yet when a limb begins moving independently, it can feel "unsettling" or "not part of my body." That reaction presents a major challenge for widespread adoption. Virtual Reality Study Tests Speed and Embodiment Earlier research suggests that people are more comfortable with autonomous movement when they understand the goal behind it. Building on this idea, Harin Manujaya Hapuarachchi and 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) explored whether movement speed plays a role in acceptance. In a virtual reality environment, participants saw an avatar whose left forearm had been replaced with a prosthetic limb. They were asked to complete a reaching task while the virtual prosthetic arm moved on its own toward a target. The researchers adjusted the duration of each movement across six different speeds (125 ms to 4 s). After each trial, participants evaluated how much the arm felt like their own, how much control they felt, how usable it seemed (SUS), and their impressions of the robot using a standardized scale (RoSAS: competence, warmth, and discomfort). In short, simply making a prosthetic arm faster does not make it better. Matching the timing of natural human movement appears to be far more important for helping users feel that the device truly belongs to them. Designing Human Like Robotic Body Augmentation These insights suggest that future AI enabled prostheses should prioritize human compatible timing rather than speed alone. Designers may need to tune movement patterns so they align with what the brain expects from a natural limb. The implications extend beyond prosthetic arms. Other technologies that function as extensions of the body, including supernumerary robotic limbs, exoskeletons, and wearable robots, could also benefit from movement that mirrors natural human rhythm. Researchers also plan to explore how long term use changes perception. People often begin to experience frequently used tools as if they were part of their body. With continued daily use, even a fast and highly capable robotic limb may start to feel "normal," easier to operate, and more fully embodied. Virtual reality plays a key role in this research. It allows scientists to test emerging prosthetic technologies and control systems in a safe and controlled setting before they are widely available. This approach makes it possible to evaluate psychological responses, user acceptance, and design considerations early in development.

How much can an autonomous robotic arm feel like part of the body?

When AI-powered prosthetic arms that move autonomously become widespread, understanding how people feel about them and accept them will be crucial. In a study appearing in Scientific Reports, scientists 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. The researchers 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. Shifting from control to comfort 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, they 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 the researchers 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. Design lessons for future prosthetics 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, the team 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. According to the team, 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.