## 1 Introduction  
Long-term adaptation to training is due to the cumulative effects of repeated single bouts of exercise. These single bouts of resistance exercise lead to acute physiological responses, which may be significant mediators of adaptation to the imposed stimuli [1]. The magnitude of these acute responses may be affected by several factors, such as the training variables (i.e., volume and intensity, exercise order) [2; 3], as well as exercise type [4; 5]. One of the basic exercises used to develop upper body strength and power is the bench press [6; 7]. Seeking optimal power output adaptations, a range of 30-60% one-repetition maximum (1 RM) has been suggested for the bench press exercise [8; 9; 10]. Nevertheless, power-oriented training should combine a wide range of loads to target the entire force-velocity curve properly. In addition, repetitions should be executed at maximum velocity until slight velocity losses of 10-20% to maintain the maximum power output production and to develop explosive performance optimally [11]. Hence, proper management of fatigue levels is necessary to avoid undesirable adaptive changes, such as those related to muscular endurance [12]. Although many studies have analyzed muscle electromyography [13; 14; 15], metabolic and endocrine changes [16; 17; 18] as indicators of fatigue arising during and after resistance exercise, there is less research investigating the impact of resistance exercise on the mechanical properties of muscle [19; 20; 21]. Non-invasive methods, such as tensiomyography and myotonometry, assess muscles' contractile and mechanical properties, respectively. However, it seems that tensiomyography has been used more often [19; 20; 22; 23; 24; 25] than myotonometry [21; 26], despite the fact that the device for myotonometry measurement (the Myoton) is more practical, being faster due to no electrical stimulation being needed, as is the case with tensiomyography (after electrode placement, the protocol consists of increasing intensity, i.e., five steps of 10 mA with a 10 s interval in between to avoid potentiation and fatigue [27]), and the device is much smaller, making it easier to transport. Moreover, the Myoton device has been validated for measurement of muscle tone and stiffness in healthy participants with \"good\" to \"excellent\" (intra- and inter-rater) reliability [28] and shows a high level of agreement compared with measurements from shear-wave ultrasound elastography [29; 30]. Furthermore, [PERSON] et al. [31] indicate much higher reliability measures for myotonometry in repeated testing than tensiomyography. Acute changes in muscle mechanical properties (measured by Myoton) due to exercise were indicated as fatigue or potentiation (acute muscle function improvement due to previous muscle activity) [21; 26; 27]. Studies in which the Myoton device was used show mainly increases in muscle stiffness due to the fatiguing protocols ([PERSON] et al., 2016, 2017; [PERSON] et al., 2019; 2020). For instance, [PERSON] et al. [32] reported an acute increase in vastus lateralis stiffness after all-out 200 m and 4000 m track cycling, with a higher increase after 200 m. An exception is the study by [PERSON] et al. [21], which indicated an acute decrease in vastus lateralis and gastrocnemius muscle stiffness induced by a fatiguing protocol consisting of five sets of 20 drop jumps. Furthermore, a study by [PERSON] et al. [26] found an acute increase in Achilles' tendon stiffness and elasticity after back squats with increasing external load (from 60 to 100% one-repetition maximum (1 RM), with 10% steps). Nonetheless, the impact of particular resistance exercise intensity (load) on muscle mechanical properties is unknown. Additionally, [PERSON] et al. [26] did not assess physical performance level (i.e., countermovement jump); thus, it is unestablished whether the changes in muscle mechanical properties could be related to performance outcomes.