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 (CMJAEL). 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 (CMJBW) followed by CMJAEL conditions at 20% (CMJAEL20) and 30% (CMJAEL30) 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 CMJAEL conditions compared with CMJBW, with the greatest improvement during CMJAEL20. 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 CMJAEL20 but changes were uncertain for CMJAEL30. Braking responses were inconsistent, as higher braking vGRF were not accompanied by meaningful changes in braking velocity or power. These findings suggest CMJAEL 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.