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 (thetaL) performed as a single step (S3) and in two equal steps (S1, 3 W to approximately 45% thetaL; S2, approximately 45% thetaL to approximately 90% thetaL). The time constants (tau) of pulmonary oxygen uptake (Vo2), leg blood flow (LBF), heart rate (HR), and muscle deoxygenation (HHb) were greater (P < 0.05) in S2 (tauVo2, approximately 52 s; tauLBF, approximately 39 s; tauHR, approximately 42 s; tauHHb, approximately 33 s) compared with S1 (tauVo2, approximately 24 s; tauLBF, approximately 21 s; tauHR, approximately 21 s; tauHHb, approximately 16 s), while the delay before an increase in HHb was reduced (P < 0.05) in S2 (approximately 14 s) compared with S1 (approximately 20 s). The Vo2 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 Vo2 response in S2 compared with S1 is consistent with a mechanism whereby Vo2 kinetics is limited, in part, by a slowed adaptation of blood flow and/or O2 transport when exercise was initiated from a baseline of moderate-intensity exercise.