Inter-Day Reliability of Countermovement Jump Metrics in Elite Academy Soccer Players

The purpose of this study was to determine the inter-day reliability of countermovement jump (CMJ) force plate metrics in elite male academy soccer players. Fourteen players performed 3 CMJ´s on 3 separate days over an 8-day period during a typical in-season competition and training period. Absolute (coefficient of variation (CV)) and relative reliability (interclass coefficient (ICC)) were calculated using two inter-day combinations; condition 1: Monday-Tuesday, condition 2: Monday-Monday and using two data treatment methods; mean of trials (“mean3”) and single trial with the highest Flight Time:Contraction Time (“BestFT:CT”). In the mean3, Monday-Tuesday condition, all CV´s except for peak landing force were < 10%, with most < 5%, while all ICC’s were > 0.75 (good), and most ICC’s > 0.9 (excellent). Several metrics had lower CV´s and higher ICC´s in condition 1 than condition 2 and in mean3 than BestFT:CT. Importantly, in the context of using downward “eccentric” phase metrics in monitoring, eccentric deceleration rate of force development, durations, power and displacement within this phase demonstrated good-excellent absolute reliability (CVs between 2.9% and 7.03%). Overall, CV´s were substantially lower than most previous studies, but similar to investigations involving elite team sport athletes who also perform the CMJ regularly. Our findings suggest that metric reliability is enhanced by this exposure and by a competitive environment, and that reliability data obtained in populations without these characteristics is not generalizable to the elite setting. Practitioners should endeavor to assess inter-day reliability within their team. In-season implementation represents an ecologically valid option.