Basketball is a high-intensity team sport requiring jump, sprint, and change of direction abilities, and demanding technical and tactical skills (Stojanovic et al., 2018; Taylor et al., 2017). Therefore, designing optimal training programs aimed at improving these qualities is of paramount importance for coaches and sport scientists. In this context, plyometric jump training (PJT) has been shown to induce meaningful improvements in jumping, sprinting, change of direction speed, and technical abilities (Shallaby, 2010). Among youth basketball players, PJT has also been shown to be effective in improving the aforementioned physical characteristics (Matavulj et al., 2001; Shallaby, 2010). However, the optimization of PJT programs and a better understanding about different plyometric training schemes and their possible influence on performance deserve further investigation. Some studies have demonstrated the importance of PJT specificity (Ramirez-Campillo et al., 2015a; 2015b) and volume-overload (Ramirez-Campillo et al., 2015c) among other relevant training factors (de Villarreal et al., 2009). However, it is still unknown if "training variability" (i.e., changing the order of plyometric drills within the session) may affect the adaptations provided by a PJT program.
For example, in resistance training, a programmed variation in training schemes (i.e., varying training loads and exercises) seems to exert an important stimulus, especially during long-term training interventions (Hartmann et al., 2015). Indeed, when the order of resistance training workouts was compared, variable acute responses relating to neuromuscular fatigue, lactate, and rating of perceived exertion (RPE) (Soares et al., 2016) were observed in addition to differences in long-term adaptation (Assumpcao et al., 2013; Simao et al., 2010). Regarding PJT studies, some training interventions have used a randomization approach, suggesting that this strategy could significantly optimize chronic adaptations (Ramirez-Campillo et al., 2016b; Rosas et al., 2016). On the contrary, other works have implemented only PJT interventions without drill randomization or modification throughout the interventional period (Kobal et al., 2017). Since the variation in training loads and stimulus seems to be very important for the effectiveness of a given training program (Assumpcao et al., 2013; Simao et al., 2010; Soares et al., 2016), it needs to be established whether randomization in training protocols during PJT would induce different adaptations when compared to a traditional pre-programmed PJT. This is especially important for youth athletes, who need to progressively develop their physical and technical abilities from the early stages of development.
To address the described issue, the aim of this single-blind randomized controlled trial was to compare the effects of PJT, with and without between-session drill randomization, on specific performance (i.e., jumping, sprint time, change of direction speed, and technical performance) of youth male basketball players. It was hypothesized that both PJT with and without drill randomization would improve youth basketball players' performance, although the improvement would be greater with a between-session drill randomization approach.
After parents or legal guardians providing a written informed consent form, nineteen male youth basketball players participated in this study. Due to the age of the participants (10.2 [+ or -] 1.7 years), they had no specific positions in the team. Participants underwent no regular strength training or PJT during the three-month period prior to the intervention, although they regularly performed basketball training. The sample size was determined according to changes in vertical jump performance in a group of team-sport players submitted to control ([DELTA] = 0.5 cm; SD = 1.1) or short-term PJT ([DELTA] = 2.6 cm; SD = 1.6) conditions (Ramirez-Campillo et al., 2015a) comparable with those adopted in this study. Six participants per group would yield a power of 80% and [alpha]
This is a randomized single-blind controlled (i.e., active controls) study. Participants were familiar with the testing procedures, as they were a regular aspect of their training schedule. Measurements were taken one week before and after intervention and were completed in one day. All assessments were administered in the same order, at the same period of the day, and by the same experienced researchers, blinded to each participant's group assignment.
After height and body mass measurements, athletes completed ten minutes of a standard warm-up (Andrade et al., 2015) before countermovement jump, 20-cm drop jump, 30-m sprint, with and without ball dribbling, and change-of-direction speed (i.e., T-test) tests. Three maximal trials were allowed for all tests. At least two minutes of rest were permitted between each maximal trial to reduce possible effects of fatigue. Anthropometric measurements were taken using a stadiometer (Bodymeter 206, SECA, Hamburg, Germany) and weighing scales (InBody120, model BPM040S12F07, Biospace, Inc., USA, to 0.1 kg). The protocols for the jumps, 30-m sprint, change-of-direction speed (Asadi et al., 2017), and 30-m sprint with ball dribbling (Shallaby, 2010) tests were performed as previously described.
Briefly, for the jumps, players executed maximal effort jumps with arms akimbo on a contact mat (Ergojump; Globus, Codogne, Italy), with the obtained flight time (t) being used to estimate the height of the rise of the body's centre of gravity (h) during the vertical jump (i.e., h = [gt.sup.2]/8, where g = 9.81 m x [s.sup.-2]). Take-off and landing were standardized to full knee and ankle extension on the same spot. In addition, for the 20-cm drop jump, players were instructed to minimize ground contact time after dropping down from a 20 cm drop box.
The sprint time was assessed to the nearest 0.01 s using timing gates (Brower Timing System, Salt Lake City, UT). For the 30-m sprints with and without ball dribbling, participants performed from a standing start with the toe of the preferred foot forward just behind the starting line. Timing began when athletes voluntarily initiated movement, triggering the timing apparatus. The timing gates were positioned at the beginning (0.3 m in front of the starting line) and at 30-m and were set ~0.7 m above the floor (i.e., hip level), to ensure the capturing of trunk movement rather than a premature trigger from a limb. The fastest sprint was considered for analysis.
The change-of-direction test was performed over a T-shaped course (Figure 1), by starting 0.3-m behind the timing gates (Brower Timing System, Salt Lake City, UT). The athletes started running forward 9.14-m, touched their hand on a cone and moved 4.57-m to the left in lateral shuffling and touched a cone. Next, they moved 9.14-m to the right in lateral shuffling and touched another cone. Finally, the athletes moved to the left 4.57-m, still in lateral shuffling, touched a cone and ran backwards a...