Inter- and Intra-individual Variation in Acute Responses to Two Different Warm-up Types. A Randomized Crossover Trial With U16 and U17 Football Players

Trial design We are following the Consolidated Standards of Reporting Trials (CONSORT) 2010 statement: extension to randomized crossover trials (Dwan, Li, Altman, & Elbourne, 2019). Randomized crossover designs increasing statistical power, as participants act as their own controls (Dwan et al., 2019). Here, a randomized crossover design with two arms is elaborated. A two-week familiarization period with the two interventions and with testing will be performed, during which the participants will make four visits, in all of which they will go through pre-testing, protocols A and B (on alternate days), followed by post-testing. This period will be followed by two-weeks of performing each intervention, separated by a two-week wash-out period to avoid a carryover effect (Higgins et al., 2019), which is superior to that used in similar research (Barbosa et al., 2019). In each period, the groups will perform four visits in terms of intervention, and in two of those visits they will be tested with the goal of assessing within-group, within-intervention consistency. Allocation ratio will be ~1:1. The protocol will be sent to approved by the Ethics Committee of the Faculty of Sport of the University Porto, and then registered at http://www.clinicaltrials.gov. Participants will be informed of the goals and methods of the research and their legal tutors will sign informed consent forms (as they will be <18 years-old), following the updated version (2013) of the Declaration of Helsinki.

Participants Participants will be 40 to 50 healthy football players from the U16 and U17 teams of a club from the Oporto city, engaged in three weekly training sessions with an average weekly training volume of 270 minutes. Following similar research (Beato, De Keijzer, et al., 2019), participants will have to be free from injuries or illness at the start of the study. Data will be collected in the facilities of the club where the participants play, and therefore no familiarization to the environment will be required.

Interventions In line with previous research (Beato, De Keijzer, et al., 2019; Faude, Roth, Di Giovine, Zahner, & Donath, 2013), a 10-minute general warm-up will precede the implementation of the specific warm-up protocols. The general warm-up will consist of three minutes running at low to moderated intensity, followed by three minutes of active, dynamic mobility work, followed by approximately four minutes of 20-m accelerations at 70% of maximum perceived intensity, where each player will perform 5 repetitions of increasing distances (i.e., 10 m, 15 m, 20 m, 25 m, 30 m). During the pre-tests, general warm-up, interventions and post-tests, players will wear Global Positioning System (GPS, VX Sport, Model VXLOG 175).

The interventions will consist of two different warm-up strategies. In the first two weeks of implementation, one group will perform intervention A while the other will perform intervention B. After the wash-out period, the groups will crossover (AB/BA design). In each two-week period of the interventions, the participants will make two weekly visits, in a total of four visits per period. In the first visit of each week, the participants will only perform the interventions, while in the second visit of each week, the participants will also engage in pre- and post-testing. Therefore, two tests per period will be applied.

Intervention A is based on football-specific small-sided games (SSGs), consisting of 4 versus 4 matches (with mini-goals), performed in a 30 m by 24 m court (Giménez et al., 2018), using five sets of 90 seconds of work interspersed with 30 seconds of pause, in a total duration of 10 minutes. Intervention B is based on speed, COD and plyometrics (i.e., a "potentiation" protocol), inspired by ideas presented by Howe, Coward, and Price (2017). Participants will perform five repetitions of the following exercises, in this order: unilateral hurdle hops and lateral hurdle hops with each lower limb; countermovement jump; broad jump; forward in and out; slalom jumps; lateral scissor jumps; single leg linear hops. Each intervention will last ~10 minutes and will be applied in the football field. Two experienced football coaches with a degree in Sports Sciences will deliver and monitor all the sessions. The interventions were designed to be easily applicable by coaches without the need for sophisticated facilities or material. All interventions will be implemented at the same time of the day and in the same days of the week for both groups. Due to the nature of the interventions, participants and implementers will not be blinded.