The way we perceive our own bodies in motion is fundamental to learning new skills, from mastering a sport to perfecting a dance routine. But a new study reveals this process unfolds differently for individuals adapting to robotic prosthetic limbs, challenging long-held assumptions about body image and motor learning. Researchers have found that people learning to walk with a prosthetic leg consistently misjudge their gait, and that this misjudgment evolves in unexpected ways as they gain experience.
“When people first start walking with a prosthetic leg, they think their bodies are moving more awkwardly than they really are,” explains Helen Huang, corresponding author of the study and the Jackson Family Distinguished Professor of Biomedical Engineering at North Carolina State University and the University of North Carolina at Chapel Hill. “With practice, as their performance improves, people still do a poor job of assessing how their bodies move, but they are inaccurate in a very different way.” This research, published February 17 in the open access journal PNAS Nexus, marks the first to specifically examine this phenomenon in individuals using lower-limb robotic prosthetics and could lead to improvements in prosthetic design and rehabilitation strategies.
The Evolving Perception of Movement
Our “body image” – our internal understanding of our body’s structure and how it moves – profoundly influences our physical actions. When learning a new skill, there’s often a disconnect between our perceived movement and our actual movement. Over time, with practice and feedback, these align, leading to improved performance. However, the study suggests this alignment process is disrupted when a prosthetic limb is introduced. Researchers wanted to understand how, and if, people integrate a robotic prosthetic into their existing body image, and whether this integration correlated with improved performance.
To investigate, the research team recruited nine able-bodied participants. Over four days, each participant walked on a treadmill using a robotic prosthetic attached to a leg, with the knee bent at a right angle. The goal was to walk as quickly as possible without using the handrails. After each practice session, participants were presented with computer animations depicting various walking gaits and asked to select the animation that most closely resembled their own recent performance.
Initial Misconceptions and Growing Confidence
The results revealed a fascinating pattern. Initially, participants consistently underestimated the smoothness of their gait, perceiving it as more awkward and unbalanced than it actually was. However, as the four-day study progressed and their walking performance improved, their self-assessment shifted. By the end of the experiment, participants tended to *overestimate* the fluidity and naturalness of their gait. Despite this change, they remained inaccurate in their assessment, albeit with a newfound confidence.
“The performance of all participants did improve significantly over those four days,” Huang noted. “However, the participants were all still inaccurate at assessing the way their own bodies moved – just in a more confident way.” This suggests that while individuals adapt to using the prosthetic, their internal representation of their movement doesn’t necessarily become more accurate, but rather shifts in a predictable direction.
Focus on the Torso, Not the Device
Interestingly, the researchers discovered that participants primarily focused on the position of their torso when evaluating their gait. They paid comparatively little attention to the behavior of the prosthetic device itself. This finding points to a potential limitation in how individuals perceive and process information related to the prosthetic limb.
“One reason for What we have is likely because they are receiving very little direct feedback about the behavior of the device – they can’t observe themselves moving,” Huang explained. This lack of visual or sensory feedback hinders their ability to accurately calibrate their body image and gait. The study suggests that providing users with more comprehensive feedback – visual cues, for example – could significantly improve their performance and their perception of movement.
Addressing Overconfidence and Future Directions
The research also highlights the importance of addressing the overconfidence that can develop as individuals become more accustomed to using a prosthetic. “If you already think you’re doing great, you’re less likely to put in the work necessary to get better – even if there is significant room for improvement,” Huang said. Developing methods to provide users with a more objective assessment of their movement could encourage continued effort and optimization.
The study was co-authored by I-Chieh Lee, a former research assistant professor in the Lampe Joint Department, Huan Min, a Ph.D. Student at NC State, and Ming Liu, a research associate professor in the Lampe Joint Department. The work was supported by the National Institutes of Health (grant R01HD110519) and the National Science Foundation (grant 2211739).
As robotic prosthetics become increasingly sophisticated, understanding the interplay between the device, the user’s body, and their perception of movement will be crucial for maximizing functionality and improving quality of life. Future research will likely focus on developing more effective feedback mechanisms and training protocols to help individuals achieve a more accurate and nuanced understanding of their movement with these advanced technologies. The next step for the research team is to investigate these feedback mechanisms in a larger and more diverse group of participants, including individuals with amputations.
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