WHAT LIMITS ENDURANCE IN NORMAL-CHILDREN

We have compared the results of a standard progressive maximal exercise test to those of an endurance exercise test in 22 healthy school children (13 girls, 9 boys, mean age 14.8 years) in order to examine whether it is possible to extrapolate results from a maximal test to predict their endurance capacity. All children performed a standard progressive maximal exercise test (15 W increments every minute until exhaustion) and an endurance test (individually calculated loads to mimic cycling at 20 km/h against a windforce 5 of Beaufort for 30 minutes) on 2 separate days. In both tests metabolic [oxygen uptake (V(O2)), CO2 production, blood lactate accumulation], ventilatory [minute ventilation (V(E))], and circulatory variables were measured. From the maximal test the threshold of lactate accumulation (LT) was determined. Thirteen children were capable of enduring the 30 minute exercise (Group 1), and 9 could not complete the endurance test (Group 2). These two groups were comparable with respect to age, height, and baseline lung function. Children in Group 2 had a higher mean weight (P < 0.005) than those in Group 1. Eight of the 9 children in Group 2 were girls, whereas Group 1 consisted of 5 girls and 8 boys. There was no significant difference between Group 1 and 2 in the mean values of V(O2) max, maximal respiratory exchange ratio (R max), V(E)max, LT, oxygen pulse, and other variables obtained during the maximal exercise tests. Lactate accumulation during the endurance test in Group 2 was larger than in Group 1 (P < 0.005). This confirms that lactate metabolism is a key factor that determines endurance capacity. The endurance time also correlated significantly (P < 0.01) with the ratio between V(E) and maximal voluntary ventilation (MVV). We conclude that exercise endurance in healthy children correlates with variables related to lactate metabolism. It appears that a load at or lower than the LT can be sustained for 30 minutes in the great majority of healthy children.