Field-Based Concurrent Validity and Test–Retest Reliability of a Portable Force Platform During IMTP and Countermovement Jump Assessments

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.