## 4 Discussion  
The aim of the present study was to compare inter-repetition rest vs. traditional resistance training on lower body strength, RFD, VL muscle architecture, VI muscle thickness, and quadriceps CSA in active participants. The acute and chronic benefits of the cluster training method on muscle strength, power and velocity [9; 11; 12; 13; 14] has led to the hypothesis that inter-repetition rest training would induce greater increases in RFD compared to traditional strength training. The main finding of the present study was that inter-repetition rest training lead to greater percentage increase of lower body RFD in contrast to traditional resistance training. Additionally, this finding was accompanied by a significant increase in VL fascicle length which was previously reported to increase after eccentric and strength-power training [21; 25; 31]. At the same time, inter-repetition rest resistance training increased strength, VL and VI thickness, and total quadriceps CSA, similarly to traditional resistance training. Consequently, increased VL fascicle length and lower body muscle hypertrophy after inter-repetition rest training may partially explain the greater percentage increases in RFD compared to traditional resistance training. These results suggest that 20 s of rest between single repetitions during heavy-load resistance training may enhance RFD and VL fascicle length besides increasing muscle mass. Inter-repetition rest training induced significant percentage increases in 0-30 ms, 0-50 ms, and 0-80 ms RFD time frames compared to traditional training. Recently, [PERSON] et al. [20], reported that RFD can be increased after resistance strength training when there is an intent for fast force production during training, even if the movement velocity lowers by the continued number of heavy load repetitions. In the present study, there was no specific direction to the participants to move the load fast. However, several studies have shown that inter-repetition rest resistance training increase the velocity of each repetition leading to a lower metabolic demand regardless of the directions provided to the participants [4; 9; 10]. RFD depends on several biological attributes, including type IIx fibres [19]. Traditional resistance training with several repetitions increases the metabolic demand of muscle fibers, which has been suggested to contribute to the type IIx to IIa muscle fibers transformation [42]. Training with inter-repetition rest induces lower metabolic demands on muscle fibers, which may suggest a smaller change of type IIx to IIa muscle fibers, which combined to increased type IIx fibers CSA would contribute to increased RFD. Unfortunately, it was not possible to obtain muscle biopsies in this study, which would have provided a better insight into the possible fiber type alterations in response to the two different training protocols used. Therefore, adding to the previous studies, the present results suggest that performing heavy load resistance training with 20 s of rest between repetitions favors the early phase of RFD compared to no-rest between repetitions and this may be achieved without the intention of fast force production. However, these results should be interpreted with caution due to the rarity of data on RFD changes in response to inter-repetition rest training. Both 1-RM leg press and isometric PF were similarly increased after inter-repetition rest and traditional resistance training. This finding is in agreement with the findings of [PERSON] et al. [17], where five weeks of either traditional resistance training or inter-repetition rest training re-distributing the total rest period between repetitions and sets, induced the same increases in 1-RM strength. However, there is a controversy among studies on this issue. Some studies revealed a greater increase in muscle strength after cluster resistance training [4; 13]. Additionally, other studies showed a greater improvement in muscle strength following traditional resistance training [15; 16].