The effect of three different start thresholds on the kinematics and kinetics of a countermovement jump.

Force plates are used to quantify kinematic and kinetic variables of countermovement jump (CMJ). The jump is initiated by an unloading phase resulting in the force-time curve to drop below body weight (BW). However, there is no consensus in the literature to determine the start of the analysis of the jump (i.e., start threshold). To determine the effects and reliability of 3 different start thresholds on the kinematics and kinetics of a CMJ, 10 soccer players performed 3 CMJs for maximal jump height 30 seconds apart. Initiation of the jump movement was defined as the point where the force-time curve dropped below a threshold of 2.5, 5, or 10% BW. Significant differences were found in all variables except eccentric and concentric peak force and force at zero velocity between 2.5 and 10%, and 5 and 10% thresholds. Similar results were found for the 2.5 and 5% threshold variables except that no significant difference was found in the concentric ground contact time. The different thresholds did not appear to influence the variability of the measurement and only concentric time to peak power was found to have high variability. These data suggest that the use of any of the start thresholds can be used to determine the starting point of a CMJ considering their reliability. However, the 2.5% BW threshold is preferable because most of the force-time signal can be considered for analysis, and significant differences in eccentric and concentric kinematic and kinetic variables were found when a higher threshold was used.