## 2 Materials and Methods
### Procedures  
The timeline of the experimental procedures is shown in Figure 1.  
Figure 1: Timeline of the experimental procedures. Testing Protocols: Data collection took place at the Sport and Exercise Physiology Laboratory of the University of Cassino and Lazio Meridionale. The entire testing was conducted in a single session. Prior to the baseline evaluations, all subjects were familiarized with the experimental protocols. At the start of the official testing procedures, following a 10-min warmup of low-intensity running (1-2 AU), which was monitored using the rating of perceived exertion (RPE) on the category-ratio (CR-10) scale [21; 34], subjects underwent a familiarization jumping session involving a total of 50 free jumps on a mini trampoline. To assess exercise session intensity, the session RPE was recorded using the CR-10 scale 30 min after the completion of the Superjump(r) training session, which is consistent with similar studies [21; 34]. Anthropometric Measurements: Before starting the testing sessions, anthropometrical parameters, including height (in meters) and body mass (in kilograms), were collected for each subject. The measurements were obtained using a scale with an integrated stadiometer (Seca, model 709; Vogel & Halke, Hamburg, Germany) with an accuracy of 0.1 kg for body mass and 0.1 cm for height. The body mass index (BMI) of each subject was calculated as weight (kg) divided by height squared (m\({}^{2}\)). The anthropometric characteristics of the participants are presented in Table 1.  
Repetitive Jump Test: Following a familiarization jumping session, the subjects performed a repetitive jump test to assess lower limb stiffness and muscle R\({}_{\text{P}}\) before (PRE) and after (POST) the Superjump(r) training session. The RJs testing session comprised three trials, with each trial consisting of five continuous jump repetitions (RJs) at the subject's preferred jumping frequency. A 1-min recovery period was provided between each trial. During the RJs, subjects were instructed to begin from a standing position, jump upwards without bending their knees, and keep their hands on their hips. The goal was to achieve maximum height and speed in each jump while minimizing contact times between jumps. Data collection during the RJs was performed by using a force plate (Kistler 9290 AD, Kistler, Winterthur, Switzerland) with a measurement range from 0 N to 10,000 N, and linearity of \(\leq\)\(\pm\)0.5% FSO. The data acquisition system (Tektronix TBS 1202B, Tektronix, Beaverton, OR, USA) and charge amplifier (Kistler 5001, Kistler, Winterthur, Switzerland) were used to record the data. The leg stiffness mean (K\({}_{\text{MEAN}}\)) and R\({}_{\text{P}}\) mean of the five repetitive jumps (RJs) in each of the three trials along with the mean of the best RJs (one per trial) (K\({}_{\text{BEST}}\)) over the five jumps for each subject were utilized for the subsequent analysis. Superjump(r) Training: The Superjump(r) training used in this study was an elastic mini trampoline (CoalSport, Rome, Italy) with the following specifications: diameter of 122 cm, height of 26 cm, weight of 15 kg, equipped with 40 springs, 8 folding feet, and capable of supporting up to 150 kg. For this study, the Superjump(r) Original was employed, which is designed for untrained and/or beginner subjects. Participants were instructed to follow the workout instructions that were provided via a DVD disc [21]. The video workout was displayed on a computer screen that was positioned in front of the mini trampoline. The Superjump(r) training session consisted of a period of free practice on the mini trampoline followed by a 30-min workout. The session included a 5-min warmup and a 5-min cooldown phase, during which subjects were required to engage in continuous jumping and perform upper limb exercises (e.g., alternating