# Precision and Reliability of a Dynamometer for Trunk Extension Strength and Steadiness Assessment
## 4 Discussion  
This study aimed to evaluate the reliability and precision of a customized dynamometer in measuring both maximal and submaximal isometric strength, as well as muscle steadiness, during trunk extension. The results demonstrated high reliability across all maximal and submaximal tests for peak force, mean force, and force steadiness under both test and retest conditions, thus confirming our initial hypothesis. The calibration results validated that the single-point load cell meets research standards, ensuring that the sensor does not deform within our measurement range. This guarantees accuracy for both current and future studies involving loads up to 1000 N. The perfect fit of the linear regression model (R\({}^{2}\) = 1) confirms precise calibration, while the low root mean square error (0.55 N) and sum of squared errors underscore data consistency. These findings indicate that the dynamometer operates within an acceptable margin of error, comparable to commercial reference devices with an accuracy of \(\pm\)1% [44], as well as previous studies that have successfully employed similar devices [14; 15; 34; 43; 45; 46]. Additionally, the low heteroscedasticity observed in the analysis suggests that when a constant mass is applied, the voltage readings from the device remain stable without significant variations, further supporting the device's reliability. The Q-Q plot of the residuals confirmed that the errors are normally distributed, further validating the model's assumptions. Consequently, this transducer can be considered a calibrated tool for assessing isometric trunk strength in practical and research settings. The reliability analysis showed high agreement between sessions for maximum strength, average strength, and force steadiness, with no significant differences between  
Figure 5: Bland–Altman plots comparing test and retest conditions for peak force, mean force, and steadiness during maximum voluntary contraction (MVC) and submaximal contraction. Legend: * Multiplication between the critical value of the normal distribution (1.96) and the standard deviation of the differences between the measurements. the two assessment days (\(p>0.05\)), indicating consistent performance across sessions. ICC values ranged from 0.81 to 0.93, reflecting good to excellent reliability, with SMVC showing the highest reliability (ICC = 0.93). This suggests that lower-intensity efforts produce more consistent results than maximal efforts. Compared to previous studies, our results demonstrate superior reliability in maximum trunk extension strength. For example, [PERSON] et al. (1997) [18] reported low reliability (ICC = 0.24), likely due to improper dynamometer placement, while [PERSON] et al. (2014) [28] achieved an ICC of 0.90 with a fixed dynamometer similar to our study. This confirms that fixing the dynamometer to a stable structure yields reliable and consistent results, consistent with previous findings. The values of MDC, SEM, and R\({}^{2}\) further confirm the system's capability to detect differences that exceed random variability in peak and mean force variables, enhancing its sensitivity and utility in performance assessment. The reliability of the measurements, reflected by an SEM of 0.85% for force steadiness, indicates high precision with a reduced margin of error. However, the MDC value of 2.36% indicates some variability, suggesting the potential influence of external factors. This can be explained by the sample composition, which includes both men and women, whose natural differences in body composition and force production may affect effort reproducibility, fluctuations in muscle activation, and motor unit recruitment [32; 47; 48]. Bland-Altman analyses confirmed that most measurements fell within the limits of agreement between test and retest, indicating acceptable reproducibility without significant systematic bias.