Spine postural change elicits localized skin structural deformation of the trunk dorsum in vivo

As the most superficial organ, the skin is the most accessible sensory system interfacing one's body and environment. With changes in posture, it is possible that the skin may undergo large deformations resulting in changes in its structural properties. The purpose of the current work was to determine the influence of spine posture on measures of trunk dorsum skin deformation, thickness and hardness in vivo. 28 young and healthy individuals were assessed while holding three static, supported spine postures (prone neutral, spine extension, and spine flexion). Skin stretch deformation was measured across each posture using an 11×9 dot matrix of 3D kinematic markers affixed to the skin of the back. Skin thickness (epidermis + dermis) was quantified using ultrasound images obtained from specific spinal levels (L4, L2, T12 and T10). Skin hardness was measured at the same specific spinal levels using a handheld Shore durometer. During the spine extension posture it was observed that the skin of the trunk dorsum relaxed/retracted on average by 12% to become both ~17% thicker and ~39% softer compared to the neutral prone posture. During the spine flexion posture the skin of the trunk dorsum stretched on average by 38% to become both ~19% thinner and ~106% harder. Additionally, in the spine flexion posture it was clear that the majority of the skin deformation occurred within the lumbar region, while deformation was more evenly distributed in the extension posture. Results also suggest that stretch distributions could be in large part determined by the intervertebral distribution of the overall spine angle, as well as in part due to the local hypodermal/subcutaneous fat and fascia distribution. With such large structural deformations of the skin of the trunk dorsum, it is suggested that corresponding changes in skin tactile sensitivity (e.g. perceptual thresholds of force necessary to elicit a sensation of touch) may be present, and should be investigated through future work. These data will aid in the development of computer models to investigate skin structural deformation (e.g. finite element models or kinematic models that track skeletal structures by accounting for soft tissue artifacts) as well as further applications pertaining to skin sensitivity, and tactile biofeedback across various body postures.