Review of the Concept of Stiffness in the Research on Mechanical Properties and Behavior of Human Body and Its Measurement Methods in Lower Extremity

The material's resistance to deformation under loading is named “Stiffness”. This concept is considered as an important factor in movement generation and neuromechanical control of human body and is related to musculoskeletal performance and injury risk. Various methods are presented to in vivo measurement of stiffness that may partially justify the mechanical properties and behavior of structures, joints, or overall body. The purpose of the present review article was to state the basic principles of stiffness in biomechanics, classification of various types of stiffness measured in lower extremity, and presentation of the methods introduced to in vivo measuring of them.
Seven types of stiffness including vertical stiffness, leg stiffness, joint stiffness, musculo-articular stiffness, musculotendinous stiffness, tendon stiffness, and muscle stiffness were recognized. Mass-spring model, dynamometry, free oscillation technique, quick-release method, ultrasonography, and mayotonometery are used to measure these variables. In the assessment of vertical and leg stiffness, overall lower body is considered as a unit component. Also, joint stiffness, musculo-articular stiffness, and musculotendinous stiffness are measured in the level of one joint. As for muscle stiffness and tendon stiffness, these structures are separately studied.