Influence of countermovement depth on the countermovement jump-derived reactive strength index modified

This study aimed to investigate the effect of countermovement depth on the magnitude of the countermovement jump (CMJ) derived reactive strength index modified (RSI_mod), and to compare the RSI_mod between the CMJ performed with a self-preferred knee flexion angle (CMJ_pref) and the CMJ performed from a pre-determined knee flexion angle (CMJ_refer) with the countermovement depth more similar to the CMJ_pref. Sixteen subjects (11 males and 5 females; age 25.1 ± 6.3 years, body mass 69.7 ± 10.2 kg, body height 172.9 ± 8.1 m) randomly performed in a single session the CMJ_pref and CMJs from five pre-determined knee flexion angles (60°, 75°, 90°, 105°, and 120°). Our results showed that lower knee flexion angles were generally associated with greater RSI_mod values with the CMJ performed at 60° showing the greatest RSI_mod (P ≤ 0.049; effect size [ES] range = 0.19-0.63). The greatest RSI_mod for the CMJ performed at 60° was caused by the proportionally lower values of the time to take-off (ES range = 0.65-1.91) compared to the decrease observed in jump height (ES range = 0.11-0.25). The RSI_mod was higher for the CMJ_pref compared to the CMJ_refer (P < 0.001; ES = 0.34) due to a higher jump height (P = 0.021; ES = 0.14) and reduced time to take-off (P < 0.001; ES = 0.85). These results indicate that practitioners should be careful when interpreting an individual's changes in RSI_mod when the countermovement depth is not similar across the testing sessions. However, since the use of pre-determined knee flexion angles negatively impacts the RSI_mod, we encourage practitioners to use the CMJ_pref but only compare the RSI_mod when CMJs are performed using consistent countermovement depths.