Introduction of dynamic rate-of-force development scaling factor in progressive drop jumps

Rapid force generation across submaximal levels has been evaluated with the rate of force development scaling factor (RFD-SF) in different isometric tasks, while such measurement was still not verified in dynamic tasks. Our study was designed to evaluate the feasibility of the RFD-SF in dynamic drop jump (DJ) task (RFD-SF_DJ). A total of 55 young athletes performed isometric plantarflexion at different submaximal intensities and 60 DJs (6 different drop heights). For each participant we calculated linearity (r²) and slope in isometric task (RFD-SF_PF), eccentric part of DJ (RFD-SF_DJ-ECC) and concentric part of DJ (RFD-SF_DJ-CON), as well as average jump height (DJ_H) from each drop height. Our results revealed strong linear force-RFD relationship for isometric plantarflexion (r² = 0.90 ± 0.06), eccentric (r² = 0.87 ± 0.09) and concentric phase of DJ (r² = 0.80 ± 0.18). Significant moderate positive correlations were calculated between RFD-SF_PF and RFD-SF_DJ-ECC (r = 0.311, p < 0.05) and small negative correlations between RFD-SF_DJ-CON and RFD-SF (r = -0.276, p < 0.05)_. Significant positive moderate correlations were seen only between RFD-SF_DJ-ECC and DJ_H from 10 cm (r = 0.459, p < 0.001) and 15 cm (r = 0.423, p < 0.01). This is the first study to introduce and confirm that RFD-SF_DJ can be obtained from the multi-joint tasks (60 jumps) and still provide acceptable reliability and linear relationship. Furthermore, RFD-SF_DJ may have greater practical application than RFD-SF assessed under the isometric conditions. This verification of RFD-SF_DJ opens opportunities for further research regarding its practical application.