Purpose: The aims of this study were to (1) investigate the health-related physical fitness profile of untrained adolescent boys in comparison to adolescent soccer players, (2) determine the intensity and enjoyment of 6v6 and 4v4 small-sided games, and (3) evaluate the health-related effects of a short-period of soccer training in the untrained group. Methods: Forty-one adolescent boys (untrained, n = 24: age = 15.9 § 0.6 years; trained, n = 17: age = 15.7 § 0.7 years) were recruited. For Purpose 1, the players (n = 17) and the untrained (n = 24) boys were tested for speed, jumping power, balance, flexibility, and aerobic capacity. After baseline testing, Purposes 2 and 3 were addressed by randomly assigning the untrained boys to either a soccer-training group (small-sided games, 2 sessions per week for 8 weeks) or to a control group, followed by identical retesting. Results: At baseline, physical fitness was higher (p < 0.001) in trained players than in untrained for aerobic fitness, sprinting, jumping power, and balance. Small-sided games using 6v6 or 4v4 elicited similar heart rate (HR) (mean: ~ 85% peak heart rate, HRpeak), rate of perceived exertion, and enjoyment responses. Over 8 weeks, the between-group analysis revealed that soccer training had a large beneficial effect on balance (45%) when compared with control group with unclear effects on other fitness parameters. Conclusion: Adolescent soccer players had markedly higher physical fitness compared with untrained adolescents. Small-sided soccer games practiced by untrained adolescents elicited high exercise intensity. While 8 weeks of twice-weekly soccer training sessions induced significant improvement in balance, the short duration of the study was not sufficient to result in between-group differences in sprint and jump performance or aerobic fitness.
