Local postural changes elicit extensive and diverse skin stretch around joints, on the trunk and the face

Skin stretch, induced by bodily movements, offers a potential source of information about the conformation of the body that can be transmitted to the brain via stretch-sensitive mechanoreceptive neurons. While previous studies have primarily focused on skin stretch directly at joints, here we investigate the extent and complexity of natural skin stretch across various body regions, including the face and trunk. We used a quad-camera set-up to image large ink-based speckle patterns stamped on participants' skin and calculated the resulting stretch patterns on a millimetre scale during a range of natural poses. We observed that skin stretch associated with joint movement extends far beyond the joint itself, with knee flexion inducing stretch on the upper thigh. Large and uniform stretch patterns were found across the trunk, covering considerable portions of the skin. The face exhibited highly complex and non-uniform stretch patterns, potentially contributing to our capacity to control fine facial movements in the absence of traditional proprioceptors. Importantly, all regions demonstrated skin stretch in excess of mechanoreceptive thresholds, suggesting that behaviourally relevant skin stretch can occur anywhere on the body. These signals might provide the brain with valuable information about body state and conformation, potentially supplementing or even surpassing the capabilities of traditional proprioception.