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 (RSImod), and to compare the RSImod 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 degrees, 75 degrees, 90 degrees, 105 degrees, and 120 degrees). Our results showed that lower knee flexion angles were generally associated with greater RSImod values with the CMJ performed at 60 degrees showing the greatest RSImod (P <= 0.049; effect size [ES] range = 0.19-0.63). The greatest RSImod for the CMJ performed at 60 degrees 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 RSImod 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 RSImod when the countermovement depth is not similar across the testing sessions. However, since the use of pre-determined knee flexion angles negatively impacts the RSImod, we encourage practitioners to use the CMJ(pref) but only compare the RSImod when CMJs are performed using consistent countermovement depths.

Automated summary

This study investigated the effect of countermovement depth on CMJ performance and found that lower knee flexion angles were associated with higher RSImod values. Additionally, the RSImod was higher for CMJ(pref) compared to CMJ(refer), indicating the importance of consistent countermovement depths when comparing RSImod values.