Reliability of measurements during countermovement jump assessments: Analysis of performance across subphases

The aim of this study was to examine the reliability of performance measures in the different phases and sub-phases of the countermovement jump (CMJ). Seventeen male athletes competing at a regional level completed 204 valid trails consisting of the execution of maximal hands-on-waist CMJs performed on a force platform. The vertical ground reaction force (VGRF) data obtained at a sampling frequency of 1000 Hz were used to calculate performance-derived variables and determine key points and jump sub-phases. The relative reliability of 92 variables was assessed using the intraclass correlation coefficient (ICC) from a two-way mixed effects model analyzing the time, force, power, velocity, and displacement variables across the different phases and sub-phases of the jump. The contact phase serves as a consistent approach (range ICC = .806 to .987), providing reliable information about the jump as a unit. Mean force was the only measure that presented high reliable values throughout all sub-phases (ICC = .931 to .963). The power and velocity variables were reliable from the point where force turned into an increasing value while displacement from minimum velocity was reached. Net impulse and ratios of force development showed high and acceptable values, respectively. Time-related variables were the least reliable measures (ICC = .587 to .720), showing acceptable reliability only in the contact phase and propulsive sub-phase. In conclusion, VGRF may be used to determine specific parameters during CMJ execution, although the appropriate variables should be selected based on acceptable levels of reliability.