## 2 Materials and Methods
### Procedure  
Before performing the tests, athletes performed typical pre-competition 45 min warm-ups, including low-pace running, specific running exercises, active stretching and some sprints. After the warm-up, athletes performed the following assessments: a vertical drop jump (VDJ) and a horizontal drop jump (HDJ) from a 30 cm high box [16; 27], a 20 m straight-leg running drill (SLR) and a 60 m sprint [14]. Before data collection, the investigators gave verbal instructions about the tests' execution; all the athletes, however, were familiar with the tests. Athletes were instructed to minimize the foot contact time and maximize the vertical or horizontal displacement in the VDJ and HDJ, respectively [22]. For SLR, the instructions were to minimize the foot contact time and maximize the speed on the 20 m. For the sprint, the athletes were asked to run at their maximum speed. Athletes performedthree attempts in each jump (VDJ and HDJ) with a 2 min recovery time. Attempts not meeting a good level of technical execution were discarded. The RSI for the VDJ (RSI VDJ) was calculated as the jump height divided by the contact time. The RSI for the HDJ (RSI HDJ) was calculated as the jump distance divided by the contact time. Following the jump tests, athletes performed three trials of the SLR test, with 1 min of rest between the trials. The RSI (RSI SLR) was then calculated as the average length of eight steps divided by the average contact time of the same steps. Finally, athletes ran two full-speed 60 m sprints (a stop line was set 5 m after the 60 m mark in order to induce athletes to run at top speed until the end of the trial), with a 5 min recovery time. Intermediate times were measured at 30 m (start to 30 m and 30 m to 60 m times were then calculated). All the times were registered using photocells (Smart Speed Micrograte, Bolzano, Italy) positioned at 0, 30 m and 60 m. The photocells positioned at the starting point were positioned at a height corresponding to the athlete's knee. The height of the other photocells (30 and 60 m) corresponded to the athlete's chest level. For jumps and sprint tests, an Optojump Next system (13 m in length, Microgate, Bolzano, Italy) was used. The Optojump was placed on a portion of the track at 47 m and 60 m from the starting point to measure contact times, flight times, step lengths and speed in the maximum speed phase. The RSI for the sprint (RSI RUN) was calculated as the average length of five steps divided by the average contact time of the five steps analyzed. To avoid the influence of the athlete's anthropometric characteristics on stride lengths, RSI HDJ, RSI SLR and RSI RUN were normalized to the athlete's height [22] using the equation (step length/athletes height)/contact time, and the NRSI HDJ, NRSI SLR and NRSI RUN indices were obtained. The fastest 60 m sprint was selected for subsequent analyses. Skeletal muscle ultrasound images were collected from the participant's right side after the anatomical locations of interest were identified using specific landmarks for the vastus lateralis muscle (VL). The VLMT (vastus lateralis muscle thickness) was measured along its longitudinal distance midway between the superior margin of the patella and the most prominent point of the great trochanter of the femur, with the knee bent 10\({}^{\circ}\)[28]. The probe was positioned on the skin surface without depressing the dermal layer (gain = 50 dB; image depth = 5 cm). Athletes were asked to lie on a physical therapy table for a minimum of 10 min before images were collected. The same qualified investigator performed all landmark measurements for each participant. A 12 MHz linear probe scanning head (Mindray MD20, Mindray Bio-Medical Electronics Co., Ltd., Shenzhen, China) was coated with water-soluble transmission gel to optimize spatial resolution and used to collect all ultrasound images. All ultrasound images were taken and analyzed by the same investigator.