How does postural stability following a single leg drop jump landing task relate to postural stability during a single leg stance balance task?

Twenty-five healthy participants performed two sessions of five valid trials for both tasks in a reproducibility-agreement design. Three postural stability outcome measures ('COP speed', 'COP sway' and 'Horizontal GRF') were calculated for DJ (5-20 s after landing) and for SLS (15 s), and were averaged per session. Paired T-tests revealed a learning effect of SLS for postural stability (4.6-6.1%; P-values <0.03), in contrast to DJ (P-values >0.27). Only session 2 resulted in superior postural stability for SLS compared to DJ for 'COP speed' (5.0%; P=0.017) and 'Horizontal GRF' (8.2%; P=0.001). Bland and Altman methods demonstrated inter-session SD's of difference for DJ of 11-12% and for SLS of 10-12%, while inter-task SD's of difference ranged 10-17%. Precision ('SD within') was better for SLS concerning 'COP speed' (14-15% vs 13%) and 'Horizontal GRF' (18-20% vs 14-15%). In conclusion, postural stability during DJ and SLS cannot be considered interchangeable, due to a learning effect for SLS and inferior precision for DJ. However, a DJ task may be used as a proxy for static postural stability, although more than three trials are needed to achieve individual errors similar to SLS for 'COP speed' (4) and 'Horizontal GRF' (5).