Effects of handheld dumbbell load on force-time characteristics during countermovement jumps with accentuated eccentric loading in youth athletes

The purpose of this study was to examine the effects of handheld dumbbell load on force-time characteristics during countermovement jumps with accentuated eccentric loading (CMJ_AEL). Nineteen youth athletes (9 males and 10 females; age 15 ± 2 years; stature 1.66 ± 0.09 m; body mass 54.8 ± 8.4 kg) performed bodyweight CMJs (CMJ_BW) followed by CMJ_AEL conditions at 20% (CMJ_AEL20) and 30% (CMJ_AEL30) of body mass. Vertical ground reaction force (vGRF) data were analysed using a combined forward and backward integration method to account for changes in system mass. Jump height increased in both CMJ_AEL conditions compared with CMJ_BW, with the greatest improvement during CMJ_AEL20. Propulsion time increased with load, while propulsion mean vGRF decreased, suggesting participants produced force over a longer duration to attain a greater jump height. Propulsion mean velocity and power increased under CMJ_AEL20 but changes were uncertain for CMJ_AEL30. Braking responses were inconsistent, as higher braking vGRF were not accompanied by meaningful changes in braking velocity or power. These findings suggest CMJ_AEL can acutely increase jump height; however, associated changes in force-time characteristics, particularly phase durations and velocities, should be considered, as they provide insight into how jump performance is achieved in response to AEL.