The lines of non-extension are the curves on the surface of the body along which the skin does not stretch during body movement. Taking this concept into account, we further pursue the calculation of the curves about the human ankle along which there is maximum, minimum and zero stretching with body movement. These lines can be used in the development of a novel biomechanical device, a soft second skin ankle-foot orthotic, kinematically compatible with the biological joint. The main purpose of this work is to obtain the strain field maps of the foot's skin for the ankle's large movements and find the directions of the extension lines. The investigation of the extension lines relies on digital technology to acquire a cloud of points that define the anatomical surface of the ankle-foot and that will be used in the process of finding the lines of maximum and zero stretch.