Slow component of O₂ uptake during heavy exercise: Adaptation to endurance training

Seven untrained male subjects [age 25.6 ± 1.5 (SE) yr, peak O₂ uptake (V̇O₂) 3.20 ± 0.19 l/min] trained on a cycle ergometer 4 days/wk for 6 wk, with the absolute training workload held constant for the duration of training. Before and at the end of each week of training, the subjects performed 20 min of constant-power exercise at a power designed to elicit a pronounced slow component of V̇O₂ (end-exercise V̇O₂ - V̇O₂ at minute 3 of exercise) in the pretraining session. An additional 20-min exercise bout was performed after training at this same absolute power output during which epinephrine (Epi) was infused at a rate of 100 ng · kg^- · min^-1 between minutes 10 and 20. After 2 wk of training, significant decreases in V̇O₂ slow component, end-exercise V̇O₂, blood lactate ([La^-] and glucose concentrations, plasma Epi ([Epi]) and norepinephrine concentrations, ventilation (V̇E), and heart rate (HR) were observed (P < 0.05). Although the rapid attenuation of the V̇O₂ slow component coincided temporally with reductions in plasma [Epi], blood [La^-], and V̇E, the infusion of Epi after training significantly increased plasma [Epi] (Δ2.22 ng/ml), blood [La^-] (Δ2.4 mmol/l) and V̇E (Δ10.0 l/min) without any change in exercise V̇O₂. We therefore conclude that diminution of the V̇O₂ slow component with training is attributable to factors other than the reduction in plasma [Epi], blood [La^-] and V̇E.