PURPOSE: The extent that enhanced ventricular function contributes to superior aerobic fitness of trained athletes is unclear. This study compared cardiovascular responses to progressive cycle exercise in 12 adolescent soccer players and 10 untrained boys with assessment of ventricular inotropic and relaxation properties by Doppler ultrasound techniques. METHODS: Resting cardiac dimensions were measured by M-mode echocardiography. Stroke volume was estimated by the Doppler technique, and systolic function by peak aortic flow velocity and ejection flow rate. Diastolic transmitral pressure gradient was assessed by pulse wave peak E-wave velocity, ventricular relaxation properties by tissue Doppler imaging (E' velocity, adjusted for ventricular size), and ventricular filling pressure by E/E' ratio. RESULTS: Size-adjusted cardiac dimensions were significantly greater in the athletes. Peak V O2 values for the athletes and nonathletes were 57.4 4.8 and 44.4 6.6 mL.kg.min, respectively. Maximal cardiac index and stroke index were greater in the athletes (11.10 1.52 vs 9.02 2.05 L.min.m; 59 8 vs 46 10 mL.m). Athletes and nonathletes demonstrated similar maximal peak aortic velocity (231 20 and 208 45 cm.s, respectively) and ejection rate (13.3 1.0 and 12.5 2.8 mL.s.cm x 10, respectively). No significant group differences were observed in Emax (155 17 and 149 23 cm.s for athletes and nonathletes, respectively), adjusted E'max (5.9 1.2 and 5.8 1.2 cm.s.mm for athletes and nonathletes, respectively), and E/E'max (265 40 and 262 56 for athletes and nonathletes, respectively). CONCLUSIONS: This study revealed no differences between young trained athletes and nonathletes in myocardial functional responses to progressive exercise, implying that greater aerobic fitness in these athletes reflected volume expansion of the cardiovascular system without contribution of enhanced systolic or diastolic ventricular function. Such findings should be considered limited to the context of young athletes with limited duration of athletic training.