## 4 Discussion  
The aim of our study was to compare the inter-set reliability and agreement of _GRF_, \(v\) and _F-v-P_ outcome variables between the force plates and the linear encoder (used as a criterion measure), and the low-cost FW-shaft integrated rotary encoder (used as a direct measure) in FW squats. To our knowledge, this was the first validation study in FW squats analyzing _GRF_, \(v\), and especially _F-v-P_ relationship outcome variables. Contrary to our hypothesis, the results show similar intra-session reliability of _GRF_ and \(v\) (good to excellent) obtained from criterion and direct measures, regardless of the inertia and type of variable (mean or peak). In contrast, lower reliability of _F-v-P_ variables (V0, Pmax, and _Slope_) (moderate to good) was obtained from the direct measure. In agreement with our hypothesis, statistically significant differences and very high to nearly perfect intra-measures correlations were found for the _GRF_, \(v\), and _F-v-P_ outcome variables. The main finding of our study is that _GRF_ and \(v\) variables at different inertias and _F-v-P_ relationship variables can be calculated from the rotational data of the FW shaft. The predicted variables have similar intra-session reliability as variables obtained with force plates and linear encoder (assumed to be the gold standard). Therefore, they can be used for longitudinal monitoring of athletic performance, but not interchangeably. Due to significant overestimation of the direct measure variables as compared to the criterion, caution should be taken when using the calculated kinetic measures for research and training purposes and especially when generalizing the results to different FW devices. It has been previously explored that the optimal external load that maximizes the mechanical \(P\) output of a neuromusculoskeletal system during jumping corresponds to a certain percentage of the maximum strength [22]. Furthermore, it has been illustrated that the optimal load during maximal vertical jumps is the athlete's own \(m\), regardless of the strength of the lower limb muscles [22]. The phenomenon could be a consequence of the long-term adaptation of the muscular _F-v_ relationship ratio to body weight and body weight inertia, changes in jumping technique, load-specific muscle activation, and jumping skills [47]. In addition, the optimal contribution of _GRF_ and \(v\) components to a common peak \(P\) generated under body mass conditions also influences jumping and sprinting performance [21; 23; 24]. Therefore, we believed that adding mass component to a common \(P\) production under FW conditions (in our case FW squat) is crucial for further research on _F-v-P_ variables, especially when collecting reference values and ensuring external validity of results. Furthermore, it is of practical importance to determine the possible differences between a gold standard laboratory method (criterion measure set-up) and a low-cost everyday use sensory system (direct measure set-up), in particular to obtain a valid and representative insight into _GRF_ exerted during squatting with the FW device when, for example, only the encoder sensory system is used. Based on previous research [17], four different inertias were used at different parts of the _F-v_ curve, which fit well with our study purpose while allowing us to compare results from a wide range of velocities in FW squats. The results show excellent inter-set reliability of _GRF_ and \(v\) variables, which is in line with the results of previous research [15; 34; 48]. Moreover, only minor inter-measures reliability differences in _GRF_ and \(v\) variables were observed, regardless of the type of variable (mean or peak) and phase of the squat. In contrast, the calculated _F-v-P_ output variables showed lower reliability than the variables calculated in the first step of the analysis (_GRF_ and _v_). The lowest ICC and the highest CV values were found for V0 and _Slope_ in the case of direct measure.