Stroke volume (SV) response to exercise depends on changes in cardiac filling, intrinsic myocardial contractility and left ventricular afterload. The aim of the present study was to identify whether these variables are influenced by endurance training in pre-pubertal children during a maximal cycle test. SV, cardiac output (Doppler echocardiography), left ventricular dimensions (time-movement echocardiography) as well as arterial pressure and systemic vascular resistances were assessed in 10 child cyclists (VO2max: 58.5 +/- 4.4 mL min-1 kg-1) and 13 untrained children (UTC) (VO2max: 45.9 +/- 6.7 mL min-1 kg-1). All variables were measured at the end of the resting period, during the final minute of each workload and during the last minute of the progressive maximal aerobic test. At rest and during exercise, stroke index was significantly higher in the child cyclists than in UTC. However, the SV patterns were strictly similar for both groups. Moreover, the patterns of diastolic and systolic left ventricular dimensions, and the pattern of systemic vascular resistance of the child cyclists mimicked those of the UTC. SV patterns, as well as their underlying mechanisms, were not altered by endurance training in children. This result implied that the higher maximal SV obtained in child cyclists depended on factors influencing resting SV, such as cardiac hypertrophy, augmented myocardium relaxation properties or expanded blood volume.

These findings indicate that cardiac morphological adaptations can occur in prepubertal children after several months of aerobic training. These alterations differ however, in some areas, to those classically reported in adults following endurance training programs where both an increase in LV size and mass exist. Our data likewise demonstrate that endurance training is able to induce favourable LV diastolic filling modifications, directed principally towards an enhancement in the early rapid filling inflow and a corresponding reduction in the atrial contribution to the total diastolic inflow.

In children, the fact that cardiac anatomy and function, particularly during the diastolic phase, can adapt to endurance training is still uncertain. Therefore, this study was undertaken to evaluate the effect of a long-term intensive endurance swimming program on the cardiac structure and function of 10-11 year old children. The population consisted of 9 children who belonged to a local youth swimming team (S) and 11 recruited from a primary school to serve as a control group (C). The swimmers had been training on average 10 to 12 h x wk(-1) for at least 2 years. All the subjects were examined by M-mode, 2-dimensional and pulsed-wave Doppler analyses according to standard procedures recommended by the American Society of Echocardiography. Investigations were carried out at rest with the subjects in a supine position. The results showed that highly trained children exhibited significantly higher left ventricular (LV) internal diameter (S: 41.6+/-1.6, C: 39.0+/-2.2 mm/m(1/3) surface area, p<0.01) and LV mass (S: 68+/-7, C: 59+/-5 g/m2 SA, p<0.01). There were, however, no differences between S and C for chamber wall thickness (posterior wall S: 5.2+/-0.6, C: 5.3+/-0.6 mm/m(1/3) SA; septum S: 5.8+/-0.3, C: 5.8+/-0.4 mm/m(1/3) SA), LV systolic function parameters (ejection fraction S: 77.1+/-0.3, C: 77.7+/-0.4%; shortening fraction S: 38.9+/-3.0, C: 39.7+/-4.1%) and the diastolic function parameters, estimated from LV inflow velocitometry (E wave S: 1.04+/-0.12, C: 1.07+/-0.16 m/s; A wave S: 0.45+/-0.10, C: 0.55+/-0.11 m/s). Finally, transaortic Doppler examinations demonstrated similar resting cardiac output (Qc) between both groups (S: 3.76+/-0.81, C: 3.90+/-0.67 l x min(-1)). However, Qc were obtained with significantly lower heart rates (S: 69+/-7, C: 83+/-14 beat x min(-1), P<0.01) and higher stroke volumes (S: 55.2+/-8.0, C: 47.5+/-8.5 ml, P<0.05) in S when compared to C. Thus, these findings strongly suggest that, as has been shown before in adults, several cardiac adaptations (including resting bradycardia, increased stroke volume and enlarged left ventricular internal dimensions) can occur in prepubertal children as a result of intensive endurance training. However, our results did not demonstrate any effects of such training during prepuberty on both diastolic and systolic functions parameters.

The patterns of stroke volume, as well as its underlying mechanisms, were not age-related during an upright maximal exercise test. However, other studies are required to understand further the effect of pubertal status on the peripheral cardiovascular system.