Agreement and Reliability of the G-Force System: Force Plate and Load Cell for the Isometric Mid-Thigh Pull in Physically Active Adults: A Repeated-Measures Method-Comparison Study

Objectives: We aimed to compare the agreement and reliability of the G-Force system (force plate and load cell) for the mid-thigh isometric pull test in physically active adults. Methods: Eighteen participants (age: 27.6 years; BMI: 27.8 kg/m2) performed three maximal IMTP trials under standardized conditions. Force-time data were collected simultaneously from both systems. Peak force and rate of force development (RFD) were analyzed. Agreement was assessed using intraclass correlation coefficients (ICC) and Bland–Altman analysis. Paired t-tests and Cohen’s d evaluated systematic differences. Linear regression analyses were conducted to assess proportional bias. Results: Peak force showed excellent agreement (ICC = 0.999; 95% CI: 0.998–1.000) but with a positive fixed bias (84.48 N) and narrow limits of agreement (42.10 to 126.90 N), indicating consistently higher values from the force plate. A statistically significant difference was observed (p < 0.001; d = 3.90). Although the standardized effect size was large, the absolute bias between devices was relatively small (84.48 N; ~6% of mean peak force). The elevated d value reflects the low variability of the inter-system differences rather than a substantial absolute discrepancy. Accordingly, the results indicate a consistent systematic bias that may affect direct interchangeability of absolute values despite the excellent agreement observed between systems. No proportional bias was detected (p = 0.159). In contrast, RFD max showed lower agreement (ICC = 0.887; 95% CI: 0.699–0.958), with a negative bias (−1361 N/s) and wide limits of agreement (−6168 to 3445 N/s). Differences were significant (p = 0.031; d = 0.55), with no proportional bias (p = 0.268). Conclusions: The G-Force system demonstrates acceptable agreement for peak force but cannot be considered interchangeable with force plate measurements due to the presence of substantial systematic bias. In contrast, agreement for RFD is reduced, indicating greater sensitivity of early-phase force-time measures to methodological differences between systems.