JOINT STIFFNESS - MYTH OR REALITY

The notion of joint stiffness as commonly studied in biomechanics and motor control is compared with the physical definition of stiffness. The importance of elastic deformation and storage of elastic energy is stressed. Different terms are suggested in order to differentiate between experimentally observed relations between joint angle and torque that are likely to have different nature. A review of studies measuring stiffness of joint subcomponents and intact joints is presented. We suggest to either abandon the term 'joint stiffness' as misleading or to state up front stiffness of which of the joint components or subsystems is analyzed in each particular study. We also suggest that each study of 'joint stiffness' should clearly state to what extent the results are defined by the system's properties and to what extent they are reflections of the particular experimental procedure.