Exploring Peak Concentric Force and Zero Velocity Synchronization in the Drop Jump

The application of force is a key aspect of performance during athletic activities. In jumping, the timing and magnitude of force application are important performance factors. The relative timing of forces has only been investigated in the countermovement jump. This study aimed to explore if the synchronization of peak concentric force with the instance of zero velocity during a drop jump impacted performance and examine the relationship of force–time curve shape to performance. Sixty-six state-level athletes (24 males and 42 females) completed drop jumps from a 30 cm box onto dual force plates. The jump with the highest reactive strength index (RSI) score was taken for analysis and classified as synchronous or asynchronous based on the relative timing of peak concentric force and zero velocity. RSI and other force–time variables were compared between groups, and functional principal component analysis (fPCA) was performed on the force–time curves, which were used to perform functional principal component regression (fPCR). Synchronous jumpers exhibited greater RSI scores and shorter contact times compared to asynchronous jumpers. Performance differences were largely driven by improved concentric kinetics, which the fPCR model revealed to have the greatest influence on RSI. Sex was also found to be a significant factor for RSI in the fPCR model. fPCA revealed that greater force application preceding and throughout the amortization phase was positively associated with RSI. The timing of peak concentric force and the shape of the force–time curve are key factors for drop jump performance, and these concepts should be investigated further in other athletic activities.