Researchers at Toyohashi University of Technology have examined how the movement speed of autonomous robotic prosthetic arms affects users’ sense of embodiment, usability, and social perception. The peer-reviewed study used virtual reality to simulate a scenario in which a participant’s left forearm was replaced by an autonomous prosthetic limb that moved independently toward a target at varying speeds.
Prosthetic limb research has traditionally focused on enabling devices to respond directly to a user’s intention, often through biosignals such as electromyography and electroencephalography. Advances in artificial intelligence and machine learning are expanding the potential for prosthetic devices to operate autonomously or semi-autonomously, assessing situations and initiating movement without direct user input. However, when a body part moves independently of conscious intention, it may be perceived as external to the body, potentially limiting user acceptance.
The research team, led by Harin Manujaya Hapuarachchi, who conducted the work as a doctoral student and is now an assistant professor at Kochi University of Technology, investigated whether movement speed influences the extent to which an autonomous prosthetic limb is experienced as part of the body. In the virtual environment, participants embodied an avatar whose forearm was replaced with a robotic prosthesis. The device autonomously flexed toward a visual target, with movement durations systematically varied across six conditions ranging from 125 milliseconds to four seconds.
After each condition, participants evaluated their sense of body ownership, agency, and usability using the System Usability Scale. They also assessed social impressions of the robotic limb using the Robotic Social Attributes Scale, measuring perceived competence, warmth, and discomfort.
The findings indicated that a moderate movement duration of approximately one second, similar to natural human reaching speed, produced the highest ratings for body ownership, sense of agency, and usability. Both the fastest condition, at 125 milliseconds, and the slowest condition, at four seconds, were associated with significantly lower ratings across these measures.
Perceived competence was rated higher at moderate to slightly faster speeds, while discomfort was most pronounced in the fastest condition. Perceptions of warmth did not show a consistent relationship with movement speed.
The results suggest that optimizing autonomous prosthetic arms for speed and technical accuracy alone may not ensure user acceptance. Instead, aligning movement characteristics with natural human timing may enhance the extent to which such devices are experienced as integrated parts of the body.
The researchers indicated that the findings may have implications beyond prosthetic arms, including other forms of robotic body augmentation such as supernumerary robotic limbs, exoskeletons, and wearable robotic systems. Future work will examine how prolonged use and adaptation may influence embodiment and usability over time.
Photo credit: Toyohashi University of Technology
