We always know where our hands and fingers are, even when we aren't looking at them or when our eyes are closed: this is proprioception, the subconscious ability to feel the body's position and movement in space, the unique "sixth sense" that muscle and tendon receptors offer. New research published in the Journal of the Royal Society Interface illustrates the role of skin stretching, which crosses the joints during active voluntary movement, in helping us grasp the location of our limbs. Previous research had looked into the issue under non-physiological settings including anesthesia and passive stimulation. Today, a study team from the Piaggio Center and the Department of Information Engineering at the University of Pisa, the Italian Institute of Technology (IIT), and the University of Rome Tor Vergata tackled the issue under more realistic voluntary movement conditions. To do this, researchers created TWIST (Tactile Wearable Interface for Skin Stretch), a non-invasive wearable device that enhances the natural stretch of the skin around the proximal interphalangeal (PIP) joint, the index finger's primary joint. Throughout the experiment, participants were required to replicate with one hand the position they imagined the other to be in, first with their bare hand and then while wearing the device. "Increasing the skin stretch led participants to behave as if their finger were more flexed than it actually was", the investigators state. "They subsequently compensated for this altered perception by maintaining a slightly more extended finger, which is direct evidence that the nervous system actively considers skin deformation when estimating body posture". Similar methodologies could potentially facilitate stroke rehabilitation, offer more intuitive sensory feedback to individuals who use robotic prosthetics, or enhance the naturalness of interaction in virtual reality and teleoperation, where the transmission of information about the limb's posture is as critical as the transmission of touch or force. However, there is much more, as the researchers clarify: "Understanding the physical and neural principles that govern proprioception is not merely a scientific objective; it is an enabling condition for the next generation of machines that are capable of perceiving and controlling their bodies in the world. From wearable devices to humanoid robots, any system that aspires to move and interact with the efficiency of the biological body requires engineering solutions that are rooted in this knowledge."