## 2 Materials and Methods
### Statistical Analyses  
The six repetitions averaged values are presented as means (standard deviations). Normal distribution of the data was confirmed using the Shapiro-Wilk test (\(p>0.05\)). Figure 2: Technical drawing of the shaft-based sensor. (**A**) flywheel (FW); (**B**) a custom-made sprocket-wheel profiled with 50 teeth that interact with optocoupler photocell sensor (**C**); (**D**) block bearings; (**F**) rotating shaft with 30 mm diameter (**E**) standing platform; the dashed line represents a pulling rope wrapped twice longitudinally around the shaft, enabling a constant wrapping radius. In the concentric part of the squat, the athlete accelerates the FW into a spinning motion by unwinding the rope and, in contrast, the spinning motion of the FW is slowed down and stopped by winding the rope around the shaft in the eccentric part of the squat. The inter-set reliability of the results was assessed using mean bias and limits of agreement (LoA) [40], the intra-class correlation coefficient (ICC\({}_{2,k}\)) [41] with 95% confidence intervals (CI), the coefficient of variation (CV) [42] and paired-samples _t_-test statistics. Heteroscedasticity was visually inspected by plotting the absolute differences between devices against the mean of both for each individual. CV values < 10% were deemed acceptable [43]. The values of the ICC were interpreted according to recent guidelines (ranges): ICC < 0.5 indicates poor reliability, ICC 0.5-0.75 indicates moderate reliability, 0.75 < ICC \(\leq\) 0.9 indicates good reliability, and ICC > 0.90 indicates excellent reliability [41]. Concurrent validity bias with LoA was calculated as mean difference between direct and criterion measures values. Agreement between the _GRF_, \(v\), and between _F-v-P_ outcomes was assessed with Pearson correlation coefficient (r), typical error of estimate (TE; SD\({}_{\mathrm{diff}}\)/r\({}^{-2}\)) and two-way repeated measures analysis of variance (ANOVA) with factors inertia (0.025, 0.075, 0.225, and 0.25 kg\(\cdot\)m\({}^{2}\)) and measure (criterion, direct), separately for peak and mean variables. Each ANOVA was performed after checking equality of variance with [PERSON]'s test. In the case of violation of the assumption of sphericity the Greenhouse-Geisser's correction was applied. Simple main effects with Bonferroni correction were used to highlight significant differences between measures at each inertia. The TE was additionally standardized (TEs) using the SD of the criterion to allow for qualitative rating. The standardized TE was rated as trivial < 0.10, 0.10 \(\leq\) small \(\leq\) 0.29, 0.30 \(\leq\) moderate \(\leq\) 0.59, or 0.59 < large [42]. The magnitude of correlation was rated as trivial < 0.10, 0.10 \(\leq\) small \(\leq\) 0.29, moderate 0.30 \(\leq\) moderate \(\leq\) 0.49, 0.50 \(\leq\) large \(\leq\) 0.69, 0.70 \(\leq\) very large \(\leq\) 0.89, or nearly perfect > 0.90. Coefficient of determination (R\({}^{2}\)) values assessed goodness of fit of the _GRF_-_v_ relationship and were interpreted as: very high \(\geq\) 0.81, 0.81 > high \(\geq\) 0.49, 0.49 > moderate \(\geq\) 0.25, 0.25 \(\geq\) low 0.09, or negligible < 0.09 [44]. Data were visually inspected using Orange: Data Mining Toolbox in Python [45]. Statistical analyses were performed in SPSS (Version 26, IBM, Armonk, NY, USA) and RStudio (Version 1.3.1073; RStudio, Inc., Boston, MA, USA) using the repeated measures correlation (rmcorr) package [46]. For all analyses, the level of significance was set at \(p\) < 0.05. Figure 3: Representative ground reaction force (**upper part**) and linear velocity data (**lower part**) during flywheel squats. Filtered data are presented for six consecutive squat repetitions at inertia 0.225 kg\(\cdot\)m\({}^{2}\). Concentric and eccentric parts of the repetitions were determined from the vertical position data: from the lowest squat position (approximately 90\({}^{\circ}\) knee flexion) to the standing position (approximately 0\({}^{\circ}\) knee flexion). The shaded area represents the eccentric phase of the squat.