| Summary: | Purpose: The main purpose of this study was to determine the reliability and variability of the dynamic strength index (DSI) calculated from the bilateral and unilateral isometric and dynamic peak force (PF) obtained from two different approaches: (1) the conventional approach: isometric mid-thigh pull (IMTP) and dynamic countermovement jump (CMJ) on a force platform (DSI-IMTP) versus (2) the novel approach: isometric squat (ISQT) and CMJ on a sledge machine (DSI-ISQT). The second aim of this study was to compare and to correlate DSI values between methods. Methods: 13 young and healthy subjects (eight males: age 23.2 ± 1.4 years; height 176.3 ± 12.7 cm; body mass 72.0 ± 14.8 kg and five females: age 23.4 ± 3.5 years; height 165.0 ± 7.5 cm; body mass 61.4 ± 6.1 kg) performed 3 trials of a bilateral and unilateral IMTP and CMJ on a force platform to determine vertical peak force, on 2 different days plus 3 trials of a bilateral and unilateral isometric squat and CMJ on a sledge machine in 2 other days. The unilateral DSI was calculated using both left vs. right and dominant vs. nondominant limbs. Reliability and variability were assessed by intraclass correlation coefficient (ICC) and coefficient of variation (%CV). Results: Both approaches to calculate either the bilateral or unilateral DSI showed excellent within-session reliability (ICC > 0.9). The DSI-IMTP approach had a good consistency (12.99% ≤ CV ≤18.46%) whereas the DSI-ISQT approach had a consistency that varied from acceptable to not acceptable (22.22% ≤ CV ≤ 33.33%). Between-session reliability was higher for the DSI-IMTP when using the average PF of the 3 trials (0.903 ≤ ICC ≤ 0.913) except for the left limb (ICC = 0.895) and non-dominant lower limb (ICC = 0.898) which had a higher reliability when using only the PF of the best trial of each test. This approach had a good consistency of the results (14.10% ≤ Pooled CV ≤ 18.75%). On the other hand, the DSI-ISQT’s results had an acceptable consistency (21.9% ≤ Pooled CV ≤ 29.82%) and, whereas for the bilateral DSI, using the average PF of the 2 best trials had the greatest reliability (ICC = 0.850), for the unilateral DSI there wasn’t a method of calculating the DSI which showed to have greater reliability than the others. There was a statistically significant difference between both approaches in the isometric strength tests, dynamic strength tests and DSI values and the association Abstract iv between both the DSI-IMTP and DSI-ISQT varied from trivial to moderate (0.031 ≤ r ≤ 0.390). Conclusions: This study provides normative data for the unilateral DSI trough two methods and indicates acceptable levels of within- and between-session reliability, whereas the consistency of individual measures of the DSI can be considered good when quantified unilaterally with the DSI-IMTP approach and acceptable when using the DSIISQT approach. In order to calculate the bilateral DSI-IMTP using the average PF of the mean of 3 trials is the most reliable method, whereas for the unilateral DSI-IMTP either using the average PF of the 3 trials or the PF of the best trial seems to be the most reliable method. In order to calculate the bilateral DSI-ISQT, using the average PF of the 2 best trials seems to have the highest reliability whereas more studies are needed to determine which approach is best for calculating the unilateral DSI-ISQT.
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