Repetion jump testing & training sport-specific Reactivity Jump, (stiffness jump) Specifically determining the repeatant plyometric strength in the plantair flexors, on the basis of the contact times, jump heights and frequency made, in this case registered. Calculation of the Reactive Strength Index Rebound Jump (RJ), Determining the general repeatant plyometric strength on the basis of contact times and jump heights and time registered. Calculation of the Reactive Strength Index         Rep-Jump in the Topjump-system, is a very powerful function.     By means of interactive feedback, sportsperson as well as physiotherapist     or coach get insight in the quality and quantity of the repetitionjump.     A matrix offers insight into whether or not the targets have been reached.     In this way sportspersons quickly develop a sense of jumping that can     be checked.; Reactive Strength Index