## 1 Introduction  
Traditional resistance training is the most effective stimulus for increasing muscle mass and strength in individuals of all ages and training background [1, 2]. Traditionally, resistance exercise is performed by lifting an external resistance for a number of repetitions which are completed consecutively, with no rest between each repetition, while a rest period of a few seconds/minutes follows after this set of repetitions. This training method results in a temporal decrease in muscle power output and movement velocity mainly due to phosphocreatine (PCr) depletion and lactate accumulation in exercised muscle cells [3, 4], while other factors also contribute to muscle fatigue, such as high ammonia concentration and reduced neural activation [5, 6]. In contrast to this traditional resistance exercise paradigm, introduction of short rest periods implemented between single repetitions (inter-repetition rest: IRR), or between groups of repetitions (cluster sets) has been proposed as an alternative effective way to increase muscle strength and mass [7]. The rest between repetitions may be between 15 and 40 s, depending on the training goal, i.e., hypertrophy, maximum strength, or power [8], while cluster set structure may include double or triple repetitions, either increasing the total training rest of re-distributing the total rest period between repetitions and sets [7]. Acute and long-term training studies revealed that inter-repetition rest may result in faster resynthesis of adenosine triphosphate (ATP), lower blood lactate concentration, and an increase in the repetition velocity compared to traditional training [4; 9; 10]. In addition, inter-repetition rest has been reported to increase the total training volume and intensity compared to traditional resistance training [11; 12; 13], resulting in greater improvement in muscle strength [4; 14]. However, other studies favored traditional resistance training to increase muscle strength [15; 16], while in one study, similar increase in maximum strength was reported after inter-repetition rest and traditional resistance training with equated rest periods between training programs [17]. These ambiguous results may be due to the different training background of the participants, the acquired training experience and/or the concurrent resistance training of other body parts leading to divergent neural adaptations and as a consequence different strength outcomes [18]. Thus, whether inter-repetition rest training is superior in strength improvements or not to traditional resistance training remains largely unresolved. The rate of force development (RFD) characterizes the ability of the neuromuscular system to produce force rapidly in limited time frames, usually during the initial 250 ms after contraction initiation [19]. Traditional resistance training increases RFD, even after short training periods (<6 weeks) [20]. Moreover, the intention to perform each repetition with high velocity, regardless of the external load used, is of great importance to increase RFD with traditional resistance training [20; 21; 22]. Some data support that cluster training enhances power and velocity to a greater extent than traditional strength training [3; 23]. Since inter-repetition rest training allows an increased repetition velocity compared to traditional training [9; 10], it might result in increased explosive performance; however, to the best of the authors' knowledge, the effect of inter-repetition rest sets configuration on RFD has not been investigated yet. The architectural characteristics of the muscles, namely muscle thickness, fascicle angle and fascicle length, have been associated with muscle strength, muscle power and RFD [24; 21; 25; 26]. Traditional resistance training induces significant changes in muscle architecture by increasing mainly the muscle thickness and fascicle angle [27; 28].