When artificial intelligence-powered prosthetic arms that move on their own become more common, it will be important to understand how users perceive and accept these devices. A recent study used virtual reality to simulate the experience of having a robotic prosthetic arm replace a participant's real arm. The research focused on how the speed of the prosthesis' movement affects factors such as body ownership, sense of agency, usability, and social impressions like competence and discomfort.
The study found that both very fast and very slow movements led to reduced feelings of body ownership and usability. In contrast, a moderate speed similar to natural human reaching—about one second in duration—produced the most positive reactions from participants.
Prosthetic limbs are crucial for people who have lost a hand or arm. Traditionally, research has aimed at improving control methods so that users can direct the device using signals from their bodies, such as electromyography (EMG) or electroencephalography (EEG). With advances in machine learning and AI, future prostheses may increasingly use autonomous or semi-autonomous movements based on situational assessment. However, when a limb moves without conscious control by the user, it can feel "unsettling" or "not part of my body," which poses challenges for acceptance.
Previous studies suggested that if an autonomous limb’s purpose is clear to the user, discomfort decreases and acceptance increases. Building on this idea, Harin Manujaya Hapuarachchi—who was a doctoral student during the study and is now an Assistant Professor at Kochi University of Technology—and his colleagues investigated whether movement speed plays a role in embodiment.
In their experiment using virtual reality, participants saw an avatar with its left forearm replaced by a prosthetic limb. The virtual arm flexed toward targets autonomously at six different speeds ranging from 125 milliseconds to four seconds per movement. After each trial, participants rated their sense of body ownership over the limb, sense of agency (control), usability (using SUS scores), and social impressions including competence, warmth, and discomfort.
Key findings included:
• Body ownership, agency, and usability were highest at moderate speeds (one-second duration).
• These measures dropped significantly at both fastest (125 ms) and slowest (4 s) speeds.
• Perceived competence was higher at moderate or slightly faster speeds; discomfort peaked at the fastest speed.
• Warmth did not depend strongly on speed.
According to Hapuarachchi's team: "At a moderate speed (movement duration of 1 s), body ownership, agency, and usability were highest." They added: "In the fastest (125 ms) and slowest (4 s) conditions, body ownership, agency, and usability were significantly lower."
These results suggest that for AI-enabled prosthetics offering autonomous assistance in daily life settings—not just technical performance but also movement timing should be designed around what feels natural to users.
The insights could influence not only future designs for autonomous prosthetic arms but also other wearable robotics like exoskeletons or supernumerary limbs intended as functional extensions of the human body.
Researchers plan further studies into adaptation over time since long-term use might help users become accustomed even to initially unfamiliar robotic movements. They note: "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."
Using virtual reality allowed researchers to safely test new types of control schemes before they are widely available in real-world devices.
