No abstract

No abstract

Previous investigations have indicated that children demonstrate a lower cardiac output at a given oxygen uptake during exercise compared with adults. This study compared cardiac responses with maximal upright cycle exercise in 15 boys (mean age 10.9 yr) and 16 men (mean age 30.7 yr) to determine whether this observation reflects differences in size or age-related influences on myocardial function. Stroke volume, aortic peak velocity, and systolic ejection time were measured to peak exercise in all subjects using Doppler ultrasound techniques. No significant differences were observed in resting, submaximal, or peak mean values for these variables relative to body size between the boys and men. Average values for peak stroke index, cardiac index, and peak aortic velocity were 59 (+/-11) mL.m-2, 11.33 (+/-2.32) L.min-1.m-2, and 152 (+/-30) cm.s-1, respectively, for the boys. Respective values for the men were 61 (+/-14) mL.m-2, 11.08 (+/-2.52) L.min-1.m-2, and 144 (+/-24) cm.s-1 (P > 0.05). This study failed to demonstrate evidence of impaired cardiac responses to maximal exercise in prepubertal boys compared with that in adult males.

The observations in these two studies also suggest 1) peripheral vasodilatation plays an important role in the early rise in stroke volume, 2) increasing heart rate acts to maintain a stable stroke volume and left ventricular diastolic dimension at high workloads, and 3) improvements in contractility serve to maintain stroke volume as the systolic ejection period shortens.

There is still considerable debate and some confusion as to the most appropriate method of scaling or normalizing maximum oxygen uptake ( _ V O 2max ) for differences in body mass (m) in both adults and children. Previous studies on adult populations have demonstrated that although the traditional ratio standard _ V O 2max (ml kg )1 min )1 ) fails to render _ V O 2max independent of body mass, the ratio standard is still the best predictor of running performance. However, no such evidence exists in children. Hence, the purpose of the present study was to investigate whether the ratio standard is still the most appropriate method of normalising _ V O 2max to predict 1-mile run speed in a group of 12-year-old children (n=36). Using a power function model and log-linear regression, the best predictor of 1-mile run speed was given by: speed (m s . With both the _ V O 2max and body mass exponents being close to unity but with opposite signs, the model suggest the best predictor of 1-mile run speed is almost exactly the traditional ratio standard recorded in the units (ml kg )1 min )1 ). Clearly, reporting the traditional ratio standard _ V O 2max , recorded in the units (ml kg )1 min )1), still has an important place in publishing the results of studies investigating cardiovascular fitness of both children and adults.

Cardiovascular responses to exercise in trained endurance child athletes have been largely unexplored. Doppler echocardiography was utilized to compare cardiac variables during progressive upright cycle exercise to exhaustion in trained male prepubertal distance runners (n = 8) and untrained control boys (n = 14). Athletes demonstrated a greater maximal stroke index and cardiac index. Stroke volume rose progressively with increasing workloads in the runners but remained stable beyond low intensities in the non-athletes. No significant differences in stroke volume were observed between the two groups at rest. This contrasts with the greater resting stroke volumes typically seen in adult endurance athletes compared to non-athletes. Likewise, values for maximal stroke index were less in the child runners than those typically seen in trained adults. This study demonstrated that the stroke volume response to exercise differs in boy runners compared to non-athletes. The findings also suggest quantitative differences in such responses between prepubertal and young adult athletes.

Maximal oxygen uptake (VO2max) in females, expressed as ml x kg(-1) x min(-1), declines steadily during the first three decades of life. The contribution of diminished cardiovascular function to this apparent fall in aerobic fitness is unknown. Cardiac responses to maximal cycle exercise were compared in 24 premenarcheal females (mean age 11.7 years) and 17 young adult women (mean age 27.4 years) using Doppler echocardiography. Mean VO2max was 40 ml x kg(-1) x min(-1) and 34.7 ml x kg(-1) x min(-1) in the girls and women, respectively (p < 0.05). When VO2max was expressed relative to allometrically-derived mass(0.52), however, no significant difference was observed in aerobic fitness between the two groups. Similar allometric analyses revealed no significant differences in average maximal cardiac output (10.50 vs 10.07 L x min(-1) BSA(-1.11) for girls and women, respectively) nor maximal stroke volume (53 vs 56 ml BSA(-1.13) respectively). These findings suggest that 1) allometric scaling is important in eliminating the effects of body size on VO2max, 2) body dimension differences can account for variations in VO2 in young females, and 3) cardiac functional reserve is similar in premenarcheal girls and young adult women.

Background: Endurance run tests are administered in schools to assess cardiovascular fitness, defined in the laboratory as maximum oxygen consumption. Objective: To examine the validity of this concept, assessing the influences of body fat and maximum values of oxygen consumption per unit time, stroke volume, heart rate, and arteriovenous oxygen difference on 1-mile (1.6km) run time in healthy sixth-grade boys. Study Design: Subjects were 36 boys with a mean (SD) age of 12.2 (0.5) years. The relationship was examined between body fat content (estimated by skinfold measurements) and maximum oxygen consumption per kilogram and cardiac variables (during maximum cycle testing) with 1-mile run velocity. Results: Body fat content and maximum oxygen consumption per kilogram (independent of body fat) accounted for 31% and 28% of the variance in run velocity, respectively. Stroke volume was the only component of maximum oxygen consumption that related to run performance. Conclusions: These findings suggest that cardiovascular fitness and body fat content contribute equally to 1-mile run time in healthy boys and together account for only 60% of the variance in performance on this endurance fitness test. Consequently, 1-mile run performance in children may not serve as a strong indicator of cardiovascular fitness.