Field-Based Concurrent Validity and Test-Retest Reliability of a Portable Force Platform During IMTP and...

Portable force platforms are increasingly used for neuromuscular performance assessment in field-based environments; however, their validity may vary depending on the analyzed variable and the experimental configuration. The present study investigated the concurrent validity and test-retest reliability of a novel portable force platform (Fitforce) during commonly used static and dynamic performance assessments under field-based conditions. Thirty recreationally active male university students (age: 24.5 ± 4.1 years; height: 177.1 ± 6.18 cm; body mass: 75.38 ± 4.62 kg) performed the isometric mid-thigh pull (IMTP) and countermovement jump (CMJ) on two force platforms positioned in a stacked configuration, with the Fitforce system placed on top of a laboratory-grade reference platform (ForceDecks). Concurrent validity was evaluated using paired comparisons, intraclass correlation coefficients (ICC), coefficients of determination (R²), and Bland-Altman analyses. Test-retest reliability of the Fitforce system was assessed across two testing sessions conducted 24 h apart. Very high agreement was observed between systems for IMTP-derived variables (ICC = 0.95-0.98) and for CMJ propulsion-related variables, including jump height, flight time, and peak take-off force (ICC = 0.92-0.96). In contrast, peak landing force showed poor agreement across systems (ICC = -0.88, R² = 0.19), with substantial systematic bias, whereas braking phase duration showed only moderate agreement (ICC = 0.50). Excellent test-retest reliability was observed across all IMTP (ICC > 0.96; CV% < 3.59) and CMJ (ICC > 0.97; CV% < 3.42) variables. Bland-Altman analyses demonstrated narrow limits of agreement for IMTP and propulsion-related CMJ variables but wide limits for landing-related force measurements. The Fitforce platform demonstrates strong concurrent agreement and excellent between-day reliability for selected IMTP and CMJ propulsion-related force-time variables under field-based conditions. However, landing-related variables should be interpreted cautiously under stacked measurement configurations due to their sensitivity to rapid impact transients and force transmission characteristics.