Purpose: To determine the reliability and variability of the Dynamic Strength Index (DSI) calculated from squat-jump (SJ) vs countermovement-jump (CMJ) peak force (PF) and to compare DSI values between methods. Methods: Male youth soccer and rugby league players (N=27; age 17.2±?0.7 y, height 173.9±5.7 cm, body mass 71.1±7.2 kg) performed 3 trials of the SJ, CMJ, and isometric midthigh pull (IMTP) on 2 separate days. DSI was calculated by dividing the PF during each jump by the IMTP PF. Results: DSI-SJ exhibited moderate (intraclass correlation coefficient [ICC]=.419) within-session reliability and high variability (percentage coefficient of variation [%CV]=15.91) during session 1; however, this improved noticeably during session 2 (ICC=.948, %CV=?4.03). In contrast, DSI-CMJ showed nearly perfect within-session reliability (ICC=.920-.952) and low variability (%CV=3.80-4.57) for both sessions. Moreover, DSI-SJ values demonstrated a small yet significant increase between sessions (P=.01, d=0.37), whereas only a trivial and nonsignificant increase was observed for DSI-CMJ between sessions (P=.796, d=0.07). Between-sessions reliability was very high for the DSI-SJ (ICC=.741) and nearly perfect for the DSI-CMJ (ICC=.924). There was no significant or meaningful difference (P=.261, d=0.12) between DSI-SJ (0.82±0.18) and DSI-CMJ (0.84±0.15). Conclusions: Practitioners should use DSI-CMJ, as it is a more reliable measure than DSI-SJ, although it produces similar ratios.
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