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 6 v 6 and 4 v 4 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,postural 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 postural balance.Small-sided games using 6 v 6 or 4 v 4 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 pos-tural 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 postural balance,the short duration of the study was not sufficient to result in between-group differences in sprint and jump per-formance or aerobic fitness.