The effect of body posture (e.g., gravity) on circulatory responses to exercise remains to be clarified. This study was designed to examine cardiovascular dynamics during prone swim bench exercise in age-group swimmers and to compare these responses to those of matched non-athletic children. Fourteen trained swimmers (mean age 11.3+/-0.5 years) performed progressive exercise to exhaustion during simulated butterfly stroke exercise on a swim bench. Stroke volume was assessed by the Doppler ultrasound technique. Standard echocardiographic measures of left ventricular dimensions and function were recorded at rest prior to exercise. Swimmers were compared to a group of 11 non-athletic children matched for age, gender, and anthropometric measures. Compared to the nonathletes, the swimmers demonstrated larger resting left ventricular diastolic dimension and mass (adjusted for body size) but no differences in systolic or diastolic function. Mean peak VO(2) was 23.2+/-4.1mlkg(-1)min(-1) and 17.8+/-4.4mlkg(-1)min(-1) in the swimmers and nonathletes, respectively (p<0.05). No significant changes were seen in stroke index with increasing work in either group, with values consistently greater in the swimmers (peak 37+/-6mlm(-2) versus 31+/-5mlm(-2) in the untrained subjects). Failure of stroke volume to rise during a progressive simulated swim test is consistent with a model of peripheral facilitation of circulatory responses to exercise.