Time series of ground reaction forces following a single leg drop jump landing in elite youth soccer players consist of four distinct phases

- We studied singe leg drop jump landings in elite young soccer players.
- We analyzed center of pressure speed, sway and horizontal ground reaction force in landing.
- Factor analysis distinguished four phases in the time series of these outcome measure.
- Center of pressure speed in landing was the more reliable outcome measure.
- Correlations were weak between phases but high between outcome measures.

The single leg drop jump landing test may assess dynamic and static balance abilities in different phases of the landing. However objective definitions of different phases following landing and associated reliability are lacking.Therefore, we determined the existence of possible distinct phases of single leg drop jump landing on a force plate in 82 elite youth soccer players. Three outcome measures were calculated over moving windows of five sizes: center of pressure (COP) speed, COP sway and horizontal ground reaction force (GRF).Per outcome measure, a Factor Analysis was employed with all windows as input variables. It showed that four factors (patterns of variance) largely (>75%) explained the variance across subjects/trials along the 12 s time series. Each factor was highly associated with a distinct phase of the time series signal: dynamic (0.4-2.7 s), late dynamic (2.5-5.0 s), static 1 (5.0-8.3 s) and static 2 (8.1-11.7 s).Intra-class correlations (ICC) between trials were lower for the dynamic phases (0.45-0.68) than for the static phases (0.60-0.86). The COP speed showed higher ICC's (0.63-0.86) than COP sway (0.45-0.61) and GRF (0.57-0.71) for all four phases.In conclusion, following a drop jump landing unique information is available in four distinct phases. The COP speed is most reliable, with higher reliability in the static phases compared to the dynamic phases. Future studies should assess the sensitivity of information from dynamic, late dynamic and static phases.