## 4 Discussion  
The main findings of this study indicate that while correlations between the two systems are consistently high, significant systematic and random biases are present, varying according to the parameter assessed, the resistance configuration, and the phase of the movement [48]. A certain level of bias and TEE is inherent in systems that evaluate mechanical parameters in isoinertial exercises based on rotational speed, as this speed does not perfectly correlate with the mechanical parameters being assessed [25, 27, 33]. In the present study, the highest (RedWP) and lowest (Yellow) resistance levels offered by the system were used, along with three intermediate loads, to cover the full force-velocity spectrum provided by HandyGym [43]. Despite its compact size compared to other isoinertial systems, the average force levels produced during exercise (Table 1) were substantial, though lower than those reported in studies involving squat and half-squat exercises with other isoinertial systems [15, 53, 44, 50, 51, 52, 45, 53]. It is important to note that participants were not instructed to attempt eccentric overload (i.e., by delaying the braking action) [14]. As a result, the E/C ratio was 0.7 for average force and 0.6 for average power, indicating no eccentric overload [26]. These values are expected for this type of system, as the measured values represent averages of force and power, and ratios tend to be higher when peak values are assessed [33]. In terms of force measurements, correlations between HandyGym and MuscleLab were predominantly \"very large\" to \"almost perfect\" across different loading conditions and in the pooled data, suggesting that the device is capable of tracking general trends in force measurements (Table 2). This is consistent with findings for other previously validated rotary encoders [29, 32, 39, 40]. However, Bland-Altman plots reveal a negative bias across all tested loading configurations, indicating that HandyGym systematically underestimates force compared to the reference system (Figure 3). This bias is particularly pronounced in the lower load configurations (Yellow, Blue, and Red; large bias) and diminishes in configurations with load-multiplying pulleys (BlueWP and RedWP; moderate  
Figure 4: Scatterplots and Bland–Altman plots illustrating the agreement between the criterion and practical measures of power (W) across the five tested load configurations, as well as for the pooled data. The scales are consistent across all conditions. In the scatterplots, R\({}^{2}\) values and regression equations for estimating criterion measures from practical measures are provided. Regression lines are included to assess the homoscedasticity of errors. In the Bland–Altman plots, the difference between the two methods is plotted on the y-axis, with the average of the two methods on the x-axis. The solid horizontal line represents the mean bias, while the dashed horizontal lines indicate the upper and lower 95% limits of agreement. bias). Additionally, random error, as assessed by TEE, was generally moderate but tended to be higher in the lower load configurations (Yellow, Blue, and Red; large TEE). These findings are significant as they suggest that the device may be more accurate when higher resistances are used but less reliable with lower loads. This discrepancy may be due to the fact that lower inertias require quicker transitions from the concentric to the eccentric phase, resulting in less precise control over transition timing and increased variability [29]. When analyzing the exercise phases, bias and TEE were similar between the concentric and eccentric phases, indicating no greater accuracy in estimating one phase over the other. Overall, based on the pooled data in Table 1, the bias and TEE for force measurements can be considered moderate. Regarding power measurements, the results show high correlations between HandyGym and MuscleLab but with a different bias pattern.