AI-driven prosthetic arms may feel most like part of the body when their autonomous reaching motion lasts about one second, a virtual reality experiment reported in *Scientific Reports* suggests. In the study, that mid-range speed produced the highest ratings of body ownership, sense of control and usability, while very fast and very slow movements reduced acceptance and increased discomfort.
A study in Scientific Reports examined how the movement speed of an autonomous prosthetic arm affects whether people experience it as “part of me” and how positively they evaluate it.
The paper—"Movement speed of an autonomous prosthetic limb shapes embodiment, usability and robotic social attributes in virtual reality"—was authored by Harin Hapuarachchi, Yasuyuki Inoue, Hiroaki Shigemasu and Michiteru Kitazaki and published on Feb. 7, 2026. The research used a virtual reality (VR) setup in which participants embodied an avatar whose left lower arm was replaced by a prosthetic limb that moved on its own during a reaching task.
In the experiment, the virtual prosthetic autonomously flexed toward a target along a minimum-jerk trajectory, with the duration of the movement varied across six speed conditions ranging from 125 milliseconds to 4 seconds. After each condition, participants rated multiple measures commonly used in embodiment and human-robot interaction research: sense of body ownership, sense of agency, perceived usability using the System Usability Scale (SUS), and social impressions using the Robotic Social Attributes Scale (RoSAS), which includes competence, warmth and discomfort.
Across measures tied to embodiment and practical acceptance, the study found a consistent “middle-speed” advantage. Ownership, agency and usability ratings were highest when the movement took about 1 second, and were significantly lower at both extremes—the fastest condition (125 milliseconds) and the slowest (4 seconds). The fastest movement also produced the highest discomfort ratings. Perceived competence was rated higher at moderate to moderately fast speeds than at slower speeds, while warmth did not show a clear dependence on speed.
The findings add to ongoing efforts to design prosthetic devices that may include autonomous or semi-autonomous assistance—systems that can move without continuous user input in order to help with everyday actions. Such autonomy could improve functionality, but the results suggest that designers may need to tune movement timing to match what users readily accept as human-like, rather than prioritizing speed alone.
The researchers said the implications could extend beyond prosthetic arms to other technologies that function as body extensions—such as exoskeletons and wearable robots—where movement that feels “off” may undermine comfort and acceptance. They also pointed to VR as a way to evaluate user perceptions early and safely, and noted that future research could test whether longer-term exposure changes how people perceive different movement speeds.
The work was supported by Japanese research funding programs and foundations, including JSPS KAKENHI, JST and MEXT, as well as the Murata Science and Education Foundation.
