Normal force in natural active touch correlates with fingertip stiffness.

Through tactile sensations, we perceive the mechanical interactions between our body and the external world, which enable us to explore its physical properties such as shape, texture, hardness, and temperature. When we casually touch objects, we perceive their textures even without performing any specific task. Individuals employ different strategies when exploring an object with or without an explicit task. This study examined individual differences in baseline motion patterns. Skin characteristics significantly influence tactile perception and friction coefficients. Thus, it can be inferred that skin properties are also related to motion characteristics. This study focuses on skin stiffness and investigates its relationship with normal force during natural stroking of a textured surface. The experimental results for thirty participants aged 18-25 years showed a significant correlation between the normal force used and skin stiffness of the fingertip. This indicates that individuals with stiffer skin tend to apply a larger normal force, while those with softer skin apply a smaller one. We also revealed that fingertip skin stiffness varies more widely than finger size, and finger size does not correlate with the normal force. Therefore, we conclude that skin stiffness is a key factor affecting the normal force applied during natural touch.