Leg Stiffness and Quasi-Stiffness
Human locomotion is similar to the motion of a bouncing ball. Therefore, the term “bouncing gait” has been used to describe locomotion in which the lower limbs perform the role of “springs” responsible for the movement of the general centre of mass. “Spring-mass model” is used to describe a bouncing gait and contains a material point that represents a total body mass and massless lower limb used as a “spring”, which performs the supporting function. An estimation of the stiffness of this spring determines the value of leg stiffness; this stiffness is a ratio of changes in the ground reaction force to the respective change in the “spring length” representing both lower limbs. Leg stiffness is a quantitative measure of elastic properties and determines the ability to accumulate potential elastic energy. The conceptual and methodological confusion regarding the concept of leg stiffness have made it difficult to organize and compare the results obtained by different authors. Due to the substantial roles of inertia and damping on force-displacement relationships, especially during transient states, leg stiffness in terms of human continuous movement should be considered as “quasi-stiffness”.
KeywordsApparent stiffness Asymmetry Bouncing gait Centre of body mass Countermovement Joint stiffness Leg stiffness Locomotion Motion system Potential elastic energy Quasi-stiffness Spring-mass model Take-off Vertical jump Vertical stiffness
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