## 4 Discussion
These conflicting results may be due to the different training background of the participants and/or the simultaneous training of other body parts which may have resulted in different neural adaptations [18] and as a consequence different strength adaptations. In the present study, training volume and RPE were similar between IRRG and TG, suggesting that both groups experienced similar subjective exertion. This may partially explain the similar increases in muscle strength after both training programs. Inter-repetition rest resistance training induced greater increases in VL fascicle length compared to traditional resistance training. It has been suggested that increases in fascicle length indicate increases in the number of sarcomeres in series which in turn contribute to higher shortening velocity and intrinsic muscle power, especially in response to eccentric and strength-power training [25; 43]. The key difference between the inter-repetition rest and traditional resistance training is the rest between repetitions [7]. This training parameter may result in lower metabolic stress during a cluster set compared to a traditional set [8]. To our knowledge no data has linked metabolic stress and fascicle length, therefore the current higher response of the VL fascicle length in response to inter-repetition rest training is difficult to interpret. Thus, changes in fascicle length after inter-repetition rest resistance training needs further investigation. Increases in lower body 1-RM strength and isometric PF were accompanied by increases in quadriceps CSA and thickness of VL and VI, in both groups. In line with these results, [PERSON] et al. [13], showed that cluster training induced similar hypertrophic adaptations to traditional training after 12 weeks. Muscle hypertrophy, as measured here with muscle ultrasonography, is mainly depending on training volume [44]. In the current study, both groups trained with similar training volumes performing the same number of sets and repetitions. Thus, the inclusion of 20 s inter-repetition rest did not result in greater training volume or muscle hypertrophy which could have increased the hypertrophic result [7] compared to traditional resistance training. To our knowledge, this is the first study evaluating changes in RFD and muscle architecture following inter-repetition rest compared to traditional resistance training; thus, the results of this study should be interpreted with a certain degree of caution. A longer training period might have induced stronger changes in RFD and muscle architecture which would probably illustrate better the degree of these adaptations in response to inter-repetition rest training. Moreover, increases in early RFD are linked with increased neural input and the percentage CSA of type II muscle fibers. Unfortunately, neither electromyographic data nor muscle biopsies were obtained in the current study which would have increased our understanding of the underlying physiological adaptations to this type of training. Another limitation of the current study was the relatively small number of participants in both training groups. As a final point, resistance training was performed without the intention to produce maximum velocity during the concentric phase. Future studies may address these issues and provide a more detailed assessment of the effectiveness of the inter-repetition rest resistance training using maximum velocity during the concentric phase.