The aim of this study was to look at differences in countermovement jump (CMJ) performance and selected kinetic parameters in athletes of different training backgrounds and to examine the relationships between these parameters. The subjects were 14 endurance athletes, 12 sprinters, and 13 fire-fighter aspirants (controls); each performed two CMJ on a force plate. The best jump of two attempts was selected and the following parameters were calculated: CMJ height (h), peak power (PP), normalized vertical stiffness (Kvert), rate of force development (RFD), peak RFD (pRFD) during concentric phase, and the ratio between pRFD and the time of its occurrence (iRFD). Sprinters exhibited greater h, PP, Kvert and RFD values than the other groups. A strong correlation was revealed between PP and h, and between pRFD and Kvert in all groups. The magnitude of correlations improved with iRFD when compared to pRFD (.5-.6 vs. .7-.9). There were strong correlations (r>.7) between PP, Kvert, and both pRFD and iRFD only for the endurance athletes group. From these results, it would be recommended to record different RFD calculations during CMJ evaluations, including the new RFD index (iRFD), in athletes of different training backgrounds
