*2.1. Participants*

Fourteen (n = 14) recreationally active preadolescent boys (age: 12.5 ± 0.5 years; body mass: 50.2 ± 10.5 kg; height: 159.4 ± 10.1 cm) volunteered to participate in the study. This sample size for the present experimental design corresponds to 0.8 power, for 0.65 e ffect size at a = 0.05 (G-power, v.3.1.9.4, University of Kiel, Kiel, Germany). Maturity o ffset from peak height velocity was calculated according to the prediction equation based on anthropometric measures, sex and age [33] and the participants

were characterized as pre-adolescents with a maturity o ffset of −2.11 ± 0.68 years. Body Mass Index (BMI) was calculated by the ratio of body mass to the body standing height squared (19.6 ± 2.8 kg/m2). All of them were healthy, with no musculoskeletal or neurological disease or lower limb injury. They joined two times a week for 90 min in a sports club, learning technical skills of team sports (soccer, handball, volleyball, basketball), in addition to the physical education class at school (according to school curriculum), two times per week for 45 min. Boys were asked to refrain from intense training 24 h prior to the testing days. Subjects' parents/legal guardians were informed about the experimental process and signed informed consent for the participation of their son/legal ward. The study was conducted according to the Declaration of Helsinki and was approved by the institutional research review board (EC-1/2020).

#### *2.2. Experimental Design*/*Procedures*

A randomized controlled design was used to investigate the acute e ffect of three warm-up protocols on 20-m sprint performance in preadolescent boys. The intervention protocols consisted of (a) a typical warm-up (control: CON), (b) 3 × 10-m maximal BwR bouts in addition to the typical warm-up, and (c) 3 × 10-m maximal FwR bouts added to the typical warm-up. Each of these protocols was assessed in random order, at three sessions carried out on non-consecutive days, separated by 72 h, at an indoor gym (wood parquet flooring), at a regular time of the day (14:00–16:00) in order to minimize any possible impact of testing time [34]. Each protocol lasted approximately 8–9 min. The participants wore light clothing and the same footwear during each session.

Rating of perceived exertion (RPE) was acquired immediately after the execution of each warm–up. The participants were tested in a 20-m sprint 4 min after the completion of each warm-up, in order to avoid fatigue [12,13,19]. Only one trial was performed since consecutive assessments could affect the performance of each subsequent sprint. The same investigator supervised all procedures and measurements.

One week before the first session, all participants were familiarized with the 20-m sprint and BwR [35]. Special attention was focused on the correct BwR technique, by means of demonstration and verbal feedback, following the guidelines of Utho ff et al. [32]. During the first session, anthropometric data of all participants were collected. A digital scale (BC-543, TANITA, Tokyo, Japan) and a stadiometer (Bodymeter 206, Seca, Ningbo, China) was used to measure body mass to the nearest 0.1 kg and body height (standing and seated) to the nearest 0.1 cm, respectively.
