Countermovement Jump and Isometric Strength Test-Retest Reliability in English Premier League Academy Football Players.

Purpose: To examine the test-retest reliability of countermovement jump (CMJ) and isometric strength testing measures in elite-level U-18 and U-23 academy football players. Methods: 36 players performed three maximal CMJ’s and isometric abductor (IABS), adductor (IADS) and posterior chain (IPCS) strength tests on two separate test days using dual force plates (CMJ and IPCS) and a portable strength testing device (IABS and IADS). Relative (ICC) and absolute (CV, SEM and MDC%) reliability for 34 CMJ, 10 IABS, 10 IADS and 11 IPCS measures were analysed using between-session Best, Mean and within-session methods. Results: For all methods, relative reliability was good to excellent for all CMJ and all IADS measures, and poor to good for all IABS and IPCS measures. Absolute reliability was good (i.e., CV <10%) for 27 (Best) and 28 (Mean) CMJ variables and for 6 (IABS and IADS) and 2 (IPCS) isometric measures. Commonly used CMJ measures (jump height, eccentric duration and FT:CT) had good to excellent relative reliability and an MDC% range of 14.6% to 23.7%. Likewise, commonly used isometric peak force measures for IABS, IADS and IPCS had good to excellent relative reliability and an MDC% range of 22.2% to 26.4%. Conclusions: Commonly used CMJ and isometric strength measures had good test- retest reliability but might be limited by their MDC%. Rate of force development measures (for all isometric tests) and impulse measures (IPCS) are limited by poor relative and absolute reliability and high MDC%. MDC% statistics should be considered in the context of typical responsiveness.