Changes in the force-time curve during a repeat power ability assessment using loaded countermovement jumps

Background. Repeat power ability (RPA) assessments traditionally use discrete variables, such as peak power output, to quantify the change in performance across a series of jumps. Rather than using a discrete variable, the analysis of the entire force-time curve may provide additional insight into RPA performance. The aims of this study were to (1) analyse changes in the force-time curve recorded during an RPA assessment using statistical parametric mapping (SPM) and (2) compare the differences in the force-time curve between participants with low and high RPA scores, as quantified by traditional analysis. Materials and Methods. Eleven well-trained field hockey players performed an RPA assessment consisting of 20 loaded countermovement jumps with a 30% one repetition maximum half squat load (LCMJ20). Mean force-time series data was normalized to 100% of the movement duration and analysed using SPM. Peak power output for each jump was also derived from the force-time data and a percent decrement score calculated for jumps 2 to 19 (RPA(%dec)). An SPM one-way ANOVA with significance accepted at alpha = 0.05, was used to identify the change in the force-time curve over three distinct series of jumps across the LCMJ20 (series 1 = jumps 2-5, series 2 = jumps 9-12 and series 3 = jumps 16-19). A secondary analysis, using an independent T-test with significance accepted at p < 0.001, was also used to identify differences in the force-time curve between participants with low and high RPA(%dec). Results. Propulsive forces were significantly lower (p < 0.001) between 74-98% of the movement compared to 0-73% for changes recorded during the LCMJ20. Post hoc analysis identified the greatest differences to occur between jump series 1 and jump series 2 (p < 0.001) at 70-98% of the movement and between jump series 1 and jump series 3 (p < 0.001) at 86-99% of the movement. No significant differences were found between jump series 2 and jump series 3. Significant differences (p < 0.001) in both the braking phase at 44-48% of the jump and the propulsive phase at 74-94% of the jump were identified when participants were classified based on low or high RPA(%dec) scores (with low scores representing an enhanced ability to maintain peak power output than high scores). Conclusion. A reduction in force during the late propulsive phase is evident as the LCMJ20 progresses. SPM analysis provides refined insight into where changes in the force-time curve occur during performance of the LCMJ20. Participants with the lower RPA(%dec) scores displayed both larger braking and propulsive forces across the LCMJ20 assessment.