Background: A novel bi-exponential method has emerged to estimate critical speed (CS) and D-prime (D') from a 3-min all- out test (3MT). Objectives: To compare CS analysis methods to determine whether parameter estimations were interchangeable. Reference values and relationships with key soccer match- play variables were explored. Methods: Thirteen elite male youth (14-15 years old) players completed a 30 m shuttle run 3MT to estimate CS, D', rate of speed decline time constant, maximal speed (Smax), time to Smax (tmax), and fatigue index (FI), using the traditional method and bi-exponential model on average (Bi-ExpAverage) and max speed settings (Bi-ExpMax-Speed). High-speed running (HSR) and sprinting distances and counts, and the number of accelerations were collected from two matches. Magnitude- based inferences (p < 0.05) with smallest worthwhile change of 0.2 effect sizes were used to analyse differences. Pearson`s and Spearman`s correlation coefficients were used to measure associations between CS model variables and match-play parameters. Results: There were significant differences between the traditional method and both bi-exponential models for CS and D', as well as between the bi-exponential models for all variables except tmax. Using the Bi-ExpAverage model, strong correlations (r = 0.70-0.73; p < 0.05) were observed for D' and FI with the number of standardised and individualised HSRs, respectively. With the Bi-ExpMax-Speed model, there were strong correlations (r/p= 0.64-0.68; p < 0.05) between D' and the number of standardised HSRs and sprints, and the number of individualised sprints. Conclusion: There is a lack of interchangeability between analysis methods. It appears that D' and FI from the bi- exponential models could be associated with high-intensity actions in soccer match-play.
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