Relationship between Mechanics of the Negative and Positive Phases of the Hopping; Identify the Effect of Stiffness and Stretch Shortening Cycle on the Optimal Performance of the Cyclic Movements

 The Stiffness of human could have the substantial effect on motion efficiency by the influence in Stretch-Shortening cycle. The purpose of this study was to investigate the relationship between mechanics of the negative and positive phases of hopping; to identify the effect of stiffness and stretch shortening cycle on the optimal performance of the cyclic movements.  To calculate the leg stiffness, knee and ankle joint stiffness and also angular velocity of the 30 young male with the age range of 18-33 and weight range of 60-88 kg during hopping, we used motion analysis and force platform system.  the results of the Principle component analysis showed that all of the ankles related parameters were classified on the first PC.  Pearson correlation coefficient also showed that there are a significant relationship between joint stiffness, negative and positive mechanical energy and angular velocity (P<0.05, P<0.01).  Due to results of this study, it seems that during cyclic motions, the common mechanism for performing strength and power activity is not useful for having the maximum advantage from stretch shortening cycle, though the ankle joint stiffness reduction is an efficient way to decrease required energy for performing these activities.