Stiffness--an unknown world of mechanical science?

"Stiffness" is a term used to describe the force needed to achieve a certain deformation of a structure. In the biomechanical world, several different definitions of stiffness are used, but not all of them are explained adequately to those readers who are less familiar with biomechanical terminology. This paper gives examples for specific definitions which are based on the basic definition of stiffness of a loaded structure