Kinetics of O 2 uptake, leg blood flow, and muscle deoxygenation are slowed in the upper compared with lower region of the moderate-intensity exercise domain

Six male subjects [23 yr (SD 4)] performed repetitions (6–8) of two-legged, moderate-intensity, knee-extension exercise during two separate protocols that included step transitions from 3 W to 90% estimated lactate threshold (θ L ) performed as a single step (S3) and in two equal steps (S1, 3 W to ∼45% θ L ; S2, ∼45% θ L to ∼90% θ L ). The time constants (τ) of pulmonary oxygen uptake (V̇o 2 ), leg blood flow (LBF), heart rate (HR), and muscle deoxygenation (HHb) were greater ( P < 0.05) in S2 (τV̇o 2 , ∼52 s; τLBF, ∼ 39 s; τHR, ∼42 s; τHHb, ∼33 s) compared with S1 (τV̇o 2 , ∼24 s; τLBF, ∼21 s; τHR, ∼21 s; τHHb, ∼16 s), while the delay before an increase in HHb was reduced ( P < 0.05) in S2 (∼14 s) compared with S1 (∼20 s). The V̇o 2 and HHb amplitudes were greater ( P < 0.05) in S2 compared with S1, whereas the LBF amplitude was similar in S2 and S1. Thus the slowed V̇o 2 response in S2 compared with S1 is consistent with a mechanism whereby V̇o 2 kinetics is limited, in part, by a slowed adaptation of blood flow and/or O 2 transport when exercise was initiated from a baseline of moderate-intensity exercise.