# Prosthesis Embodiment in Lower Extremity Limb Loss: A Narrative Review
## 4 Results
### Proprioceptive Clues Through Haptic Feedback  
Muscle vibration can simulate proprioception by stimulating stretch receptors that detect changes in muscle length and tension [44]. One study incorporated vibrational sensory feedback into a myoelectric motor-controlled prosthesis. Although this study had a small sample size, it showed promising results of vibrational haptic feedback improving the satisfaction of lower leg prostheses [45]. Muscle spindles are stretch receptors that are vital for proprioception, but there are a limited number of studies that have investigated how stretch receptors affect prosthesis embodiment [46]. Skin stretch over a vibrated muscle improves proprioceptive signals [19]. One study showed that in people with below-knee amputations (BKAs), stretching the skin along with vibrating the muscle of the remaining portion of the limb produced stronger and more uniform movements [47]. Another study asked participants to move a cursor a certain distance without being able to visualize the cursor. They then analyzed changes in accuracy with and without haptic, vibrational, or skin stretch feedback, and they found fewer errors due to increased proprioception when skin stretch was applied [48]. Although the prospect of skin stretch is promising for improving prosthesis use, there are some limitations to consider. Ref. [48] identifies the following potential constraints:  
* The area of skin present to attach a stretch apparatus cannot be too small. * Different areas of the skin have different properties, such as the presence of hair or elasticity of the skin. * Different skin characteristics between individuals, such as wrinkles, create variations in sensory perception. * Shear forces can cause an undesirable tissue breakdown. The lack of vibrotactile feedback also makes it difficult for users to navigate stairs, a challenging task that many lower limb prosthesis users avoid. Nearly 33% do not use stairs at all, and only 21% can handle stairs without a handful, both of these low proportions due to complex demands on balance, motor control, limb symmetry, and foot placement awareness [49]. Research has shown promising results using a haptic thigh band that transmits tactile feedback from the prosthetic foot, improving foot placement accuracy during stair climbing by 17.5% in people with BKAs [49]. One user noted that artificial \begin{table}
\begin{tabular}{c c} \hline
**Elicited Sensations** & **Modality** \\ \hline “My toes are pushing down against a surface” & Proprioceptive \\ “My big toe wanted to move, but there was not enough [room] to do it” & Tactile \\ “Very light touch, on top of the big toe” & Tactile \\ “Like a piece of hair, or piece of grass rubbed against my calf” & Tactile \\ “It was in the ankle, made my foot go down” & Proprioceptive \\ “Felt like a pressure in the heel” & Tactile \\ “It was like pressure on one side of my ankle” & Tactile \\ “Bottom of my foot was waking up” & Tactile \\ \hline \end{tabular}
\end{table} 
Table 2: The possible elicited sensations from signals generated by C-FINE devices, as perceived by subjects in [43]. Note that the boundary between proprioception and cutaneous sensation can be quite gray, and proprioceptive modalities can also be generated. These descriptions are robust and stable over time. feedback served as \"a secondary layer of confirmation, supporting what [they] already suspected about the position of their foot on the stair edge.\" Despite providing feedback that differs from natural foot sensations in both location and type, this noninvasive solution helps the prosthesis function more like a natural limb. Although not definitively proven to increase embodiment scores, haptic feedback can enhance proprioception and confidence in patients with BKAs as they use their prosthetic devices when ascending stairs. These studies highlight how vibrational and skin-stretch feedback can enhance proprioceptive signals, bridging sensory gaps between prosthetic devices and the user's internal body schema.