Free-fall normalised peak braking velocity for countermovement jump monitoring

Peak braking velocity (PBV) during the countermovement jump (CMJ) is used to describe descent strategy, but its interpretation is affected by countermovement depth. This study introduced free-fall normalised peak braking velocity (PBVFF), a dimensionless index expressing PBV relative to the displacement-specific theoretical free-fall velocity limit, and evaluated its reliability, performance associations and ranking consequences.

Seventy injury-free male academy footballers (18.7 ± 1.6 years) performed three maximal hands-on-hips CMJs on dual force plates. Using retrospective observational routine monitoring data, one representative testing day per athlete was analysed. Two PBVFF variants were examined: PBVFF normalised to countermovement depth (PBVFF_CMD) and PBVFF normalised to displacement at PBV (PBVFF_{PBV_d}). Reliability was assessed using coefficient of variation and two-way mixed-effects intraclass correlation coefficients. Associations with modified reactive strength index (mRSI) and jump height (JH) were assessed using Pearson correlations, Benjamini-Hochberg false discovery rate correction and partial correlations controlling for JH. Ranking effects were examined using Spearman rank correlations, Kendall's coefficient of concordance and absolute rank shifts.

PBVFF_CMD showed reliability comparable to raw PBV, with a pooled coefficient of variation of 5.81% versus 6.75% and three-trial intraclass correlation coefficients of 0.865 versus 0.888, respectively. PBVFF_CMD was more strongly associated with mRSI than raw PBV (r = 0.573 versus r = 0.231). After controlling for JH, the association between raw PBV and mRSI reversed direction (partial r = - 0.266), whereas the association for PBVFF_CMD remained positive (partial r = 0.531). PBVFF_{PBV_d} showed similar but weaker associations with mRSI (r = 0.430; partial r = 0.438). Switching from raw PBV to PBVFF_CMD reclassified 46% of athletes by at least 10 ranking positions.

PBVFF provides a depth-contextualised interpretation of CMJ descent strategy without compromising within-session reliability. PBVFF_CMD appears particularly practical because it can be derived from commonly exported variables and may improve athlete monitoring by reducing misinterpretation driven by countermovement-depth and anthropometric differences. It should complement, rather than replace, established force-time and performance metrics.

Not applicable.