## 4 Discussion
The lower accuracy of the %Rep-%VL equations observed in our study could be attributed to the fact that the pattern of repetition velocity loss is not always linear due to improper technique, lack of concentration, or other sources of biological variability [25; 26]. Furthermore, as observed in the present study, this problem could be magnified by the fatigue induced when performing multiple RT sets [17]. Considering all the above, the findings of the current study suggest that, independently of whether individualized or generalized Rep-%VL relationships are applied, the %Rep cannot be estimated with a high precision using %VL during a parallel back-squat training session consisting of multiple sets. In line with previous studies, the fatigue induced by executing multiple sets was manifested not only by a decrease in the number of repetitions completed to failure, but also by a decrease in the MVfastest of the set [17; 18]. Consequently, another objective in the present study was to elucidate whether the reference repetition (i.e., using MVfastest specific to each set vs. MVfastest of the first set) used for computing the %VL may affect the magnitude of the errors when estimating %Rep. Our findings suggest that, regardless of whether the individualized or generalized %Rep-%VL equations are considered, the %Rep estimated when reaching a 20%VL was generally overestimated when using the MVfastest of the first set but not MVfastest specific to each set. Therefore, it seems that fatigue induced by performing multiple RT sets compromises the utility of the %Rep-%VL relationship when using the MVfastest of the first set. However, it should still be noted that individuals might stop the sets closer to failure (i.e., leaving a lower number of repetitions in reserve) if sets are initiated in a fatigue condition when a fixed %VL is prescribed using the MVfastest specific to each set, because the final absolute velocities of RT sets are expected to be lower. While our study has revealed valuable and novel insights regarding the effect of fatigue on the precision of %Rep-%VL equations for predicting %Rep during a standard training session consisting of multiple sets of the parallel back-squat exercise, we should note a number of limitations and potential areas for future research. Similar to the majority of studies that have examined the %Rep-%VL relationships [5; 6; 15; 16; 19; 25; 27], our sample size consisted exclusively of resistance-training males and, therefore, the present findings might not transfer to female individuals or untrained individuals. Therefore, future studies should investigate the goodness-of-fit and precision of %Rep-%VL equations in female individuals. Second, we have explored the effect of fatigue on the accuracy of %Rep-%VL equations obtained during the parallel back-squat exercise against the 75%1 RM load. Since the %Rep-%VL relationship is exercise- and load-dependent [6], more studies are needed to elucidate whether the present findings can be extrapolated to other loads and RT exercises. Finally, similar to previous studies [5; 6; 15; 19], the use of a Smith machine could compromise the ecological validity of our findings. Although the accuracy of the %Rep-%VL equations is unlikely to improve when using free-weights, it is important that future studies compare the behavior of %VL and %Rep between the same exercises performed with free-weights and in a Smith machine.