Physical properties of human lips: experimental and theoretical analysis.

The lip stiffness characteristics in the vertical plane of a white population sample, aged 18-26, were studied. The physical properties of the lip system were analyzed and a mathematical model was derived to approximate this system. The instrumentation consisted of a force-transducer bar, a head holder and a three-directional micrometer adjustment. Four specific contact areas were used. Individual lower lip is stiffer than upper lip and the corresponding male lip is stiffer than the female lip. The force-displacement relationship approximates a linear function at small displacements (less than 1 mm) and is a second order function at larger displacements. A mechanical T-spring model approximates the system to an accuracy of about 96%.