Kinetics of O2 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 (theta(L)) performed as a single step (S3) and in two equal steps (S1, 3 W to similar to 45% theta(L); S2, similar to 45% theta(L) to similar to 90% theta(L)). The time constants (tau) of pulmonary oxygen uptake ((V) over dot O-2), leg blood flow (LBF), heart rate (HR), and muscle deoxygenation (HHb) were greater ( P < 0.05) in S2 (tau(V) over dot O-2, similar to 52 s; tau LBF, similar to 39 s; tau HR, similar to 42 s; tau HHb, similar to 33 s) compared with S1 (tau(V) over dot O-2, similar to 24 s; tau LBF, similar to 21 s; tau HR, similar to 21 s; tau HHb, similar to 16 s), while the delay before an increase in HHb was reduced ( P < 0.05) in S2 (similar to 14 s) compared with S1 (similar to 20 s). The (V) over dot O-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 (V) over dot O-2 response in S2 compared with S1 is consistent with a mechanism whereby (V) over dot O-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.