Does task experience moderate habituation effects in drop jumps? - An intra- and interday reliability and measurement error analysis

Background: Drop jumps (DJs) are common tests for determining stretch-shortening cycle efficiency. Despite their wide implementation, numerous studies did not adhere to validity guidelines, and participants reached ground contact times (GCTs) of > 250ms. The DJ is a coordinatively-demanding task and it could be hypothesized that a lack of adherence to GCT thresholds could be attributed to insufficient test familiarization. Accordingly, this study evaluated the impact of extensive task habituation on test-retest reliability and GCT adherence. Methods: Eighty-nine healthy participants were allocated to an experienced and inexperienced subgroup. All subjects performed DJs on five days from three different drop heights, with three trials per height (5 × 3 × 3 DJs = 45 jumps). Reliability coefficients and test-retest systematic and random errors were quantified between testing days. Results: The inexperienced subgroup showed systematic performance increases between test day 1 and the subsequent days in most evaluated parameters with largest effects obtained for GCTs (up to d = 0.83, p < 0.001). With up to 30%, the MAPEs peaked for reactive strength index (RSI) in the day 1 to day 5 comparison. No systematic test-retest increases occurred in the experienced group, and the random percentage error was meaningfully smaller (16–17% for the RSI). Conclusion: The systematic bias observed in the inexperienced subgroup might indicate significant task habituation, while the experienced group was already familiar with the testing procedure and showed no test-retest improvements. Since subgroup measurement errors differed meaningfully (e.g. 16% versus 30% RSI MAPE), future DJ research should provide their own reliability quantifications and sufficient habituation.