## 4 Discussion
The main difference is that the FW shaft continues to rotate and does not stop at the transition from the concentric to the eccentric part of the squat, whereas the linear motion recorded by the linear encoder stops (note the \(v\) drop represented by the black curve at the transition from the concentric to the eccentric part of the squat in Figure 3, lower part). Therefore, the average of the calculated \(v\) (and consequently _GRF_) from each repetition is higher. One of the general reasons for the higher values of linear \(v\) could also be a multi-step variables calculation (see data processing). Despite the theoretical integrity according to [PERSON]'s laws, there are also some mechanical factors that could have altered the results, such as the vertical angle of the rope pull, the compliance of the harness, and the rope tension in the initial position (standing) of the squat which, can cause the differences in the measured squat vertical distance between the systems. It is clear that lower inertias require faster transitions from the concentric to the eccentric phase and consequently less control over the timing of the transitions. Consequently, the initial setting of the rope is crucial. While too tight an adjustment will limit smooth transitions at low inertias, the same tight adjustment is desirable at higher inertias because the transitions are slower and more controlled. Consequently, a longer relaxed phase (when the rope is loose) at low inertias/high velocities could have increased inter-measure differences. The results could also stem from the fact that light inertia/high velocity conditions could influence squat trajectory, especially by intensifying non-vertical movement, as discussed by [PERSON] et al. (2015) [54], when using linear encoder data during squats with additional weights. Additionally, it has already been mentioned in the literature that the mean variables from position time data with the linear position encoder [56; 57], especially when calculating \(F\)-\(v\) outcomes [58], may be compromised because they cannot accurately determine the end of the concentric phase of the jump. In the case of FW squat without flight phase, we believe that absolute linear encoder gave us valid results defining the lowest and the highest squat amplitude. In contrast, it is only arbitrarily possible to define the end of the concentric phase from the incremental rotary encoder signal. In our case the concentric phase was calculated as half of the amplitude of the squat for each repetition, which could have caused minor errors. We believe they were minimized by averaging the results over six consecutive repetitions. Overestimation of _GRF_ results from direct measure could be a consequence of the mechanism of information acquisition by the rotary encoder sensor, which can be explained in the following few steps. **First**, these differences could have occurred due to the fact that the rotary encoder evaluates the harness-rope attachment displacement (Figure 1F), whereas the force plate measures the body center of mass displacement. Therefore, the differences should mainly originate from the recorded harness-rope attachment acceleration that is inevitably higher than the acceleration of the center of mass recorded by the force plates, similarly to the previous study [59] on using jumps with additional weights. We can assume that a considerable portion of the center of mass is displaced at a vertical distance that is significantly shorter than the vertical distance covered by the harness-rope attachment during FW squat performance. **Second**, it was found in jumps with additional weights that higher weight inevitably moves the system's center of mass closer to the bar, thereby minimizing the gap distance between the center of mass and the bar. Therefore, the difference in predicted _GRF_ using linear encoder and measured _GRF_ using force plates decreases with an increase in the added weight [60]. In our case, no additional weight was added, and the center of mass position has not changed.