Effect of endurance training on possible determinants of V̇O₂ during heavy exercise

When moderate exercise begins, O₂ uptake (V̇O₂) reaches a steady state within 3 min. However, with heavy exercise, V̇O₂ continues to rise beyond 3 min (V̇O₂ drift). We sought to identify factors contributing to V̇O₂ drift. Ten young subjects performed cycle ergometer tests of 15 min duration for each of four constant work rates, corresponding to 90% of the anaerobic threshold (AT) and 25, 50, and 75% of the difference between maximum V̇O₂ (V̇O₂ (max)) and AT for that subject. Time courses of V̇O₂, minute ventilation (V̇E), and rectal temperature were recorded. Blood lactate, norepinephrine, and epinephrine were measured at the end of exercise. Eight weeks of cycle ergometer endurance training improved average V̇O₂(max) by 15%. Subjects then performed four tests identical to pretraining studies. For the above AT tests, training reduced V̇O₂ drift substantially; reduction in each of the possible mediators we measured was also demonstrated. The training-induced decrease in V̇O₂ drift was well correlated with decreases in end exercise lactate and less well correlated with the drift in V̇E seen at above AT work rates. The training-induced reduction in V̇O₂ drift was not significantly correlated with attenuation of rectal temperature rise or decrease in end-exercise level of the catecholamines. Thus the slow rise in V̇O₂ during heavy exercise seems linked to lactate, though a component dictated by the work of breathing cannot be ruled out.