Research design and participants
The present pilot study assessed BB based on practical considerations in a paired crossover design. In total, n = 33 U13 soccer players (32 male, 1 female; body height: 151 ± 8 cm, body mass: 38 ± 6 kg, age: 12.2 ± 0.3 years, maturity offset: -1.6 ± 0.8 years) and n = 29 U14 soccer players (28 male, 1 female, 157 ± 6 cm, 45 ± 5 kg, 13.2 ± 0.4 years, -0.8 ± 0.6 years) were evaluated. All players were recruited from two elite soccer clubs participating in the Swiss talent development programme . The programme represents the national top 2,000 U13 and top 1,200 U14 players. All players from the present study participated for at least 4.3 ± 0.7 years in structured and regular soccer practice and match play. After ethical approval (Swiss Federal Institute of Sport; Nr. 2018/057), all participants and their legal guardians signed written informed consent to participate. The study was conducted in accordance with the guidelines of the Declaration of Helsinki.
Maturity offset (MO), age at peak height velocity (APHV) and percentage of adult height were assessed by measuring body weight, standing height and sitting height using a body scale (Seca 876, Seca, Hamburg, Germany) and a stadiometer (Seca 217, Seca, Hamburg, Germany; Mirwald, 2002; Sherar, 2005). Subsequently, all male players were ranked based on their MO. The ranking was split at its median and the biologically younger half assigned to team BA1 (MO: -2.5 to -1.2 years) and biological older half to BA2 (MO: -1.2 to 0.0 years). Both female players were on-time maturing and therefore stayed with their team. With respect to practical considerations and previous recommendations [1, 7], team coaches evaluated each player’s technique, game intelligence, personality and playing speed [10, 11]. Based on the coaches’ evaluation, two players from BA1 were assigned to BA2 due to their superior technical and tactical abilities. Two players from BA2 still had to catch up with their technical and tactical skills and were assigned to BA1.
After a two-month familiarisation period with BB training sessions and matches, eight 20-min matches were carried out on two natural grass pitches (size: 55 × 58 m). The first and fourth 20-min matches were played with BB, while the second and third 20-min matches were played with conventional CA groups (U13 and U14). The matches were video-captured ball-oriented (focused on the ball) for a subsequent analysis of technical and tactical parameters. The camera was positioned near the middle line of the pitch at a height of 3.5 meters above the ground. Video data were coded and analysed for difference in ball possession between teams (% p.), mean time of ball possession per action (s), changes in ball possession (n), shots (n), successful goal attempts (%), passes (n) (successful % vs unsuccessful %), duels (n) and fouls (n) using DartFish Note (Dartfish, Fribourg, Switzerland). Goal kicks, throw-ins, free kicks and penalty kicks were summarised as set pieces (n). Technical and tactical parameters were defined beforehand by an expert group of the Swiss Soccer Federation. Physical parameters were measured using global positioning data (FieldWiz, Sensitech AG, Goldach, Switzerland) collected with 18-Hz. Because some players were substituted between matches, 49 of 72 possible paired subjects playing both BB and CA matches were used for the analysis. Based on previous recommendations, running distances were analysed for walking, jogging, running, high-speed running and sprinting at 0–5.8 km/h, 5.8–11.5 km/h, 11.5–15.8 km/h, 15.8–20.0 km/h and > 20.0 km/h, respectively .
All data are presented as the mean ± standard deviation (SD). All matches were divided into 5-min intervals, from which mean values were used to compare differences between BB and CA using a t-test. Wilcoxon’s signed-rank test was applied for all non-normally distributed data (Shapiro–Wilk test). Cohen’s d effect sizes were calculated  and regarded as small (< 0.2), medium (< 0.6) and large (< 1.2) . Statistically significant differences were analysed with SPSS 25.0 (IBM Corp., Armonk, NY, USA) and an alpha level of 0.05.