Grassi, Bruno, L. Bruce Gladden, Michele Samaja, Creed M. Stary, and Michael C. Hogan. Faster adjustment of O 2 delivery does not affect V O 2 on-kinetics in isolated in situ canine muscle. J. Appl. Physiol. 85(4): 1394-1403, 1998.-The mechanism(s) limiting muscle O 2 uptake (V O 2 ) kinetics was investigated in isolated canine gastrocnemius muscles (n ϭ 7) during transitions from rest to 3 min of electrically stimulated isometric tetanic contractions (200-ms trains, 50 Hz; 1 contraction/2 s; 60-70% of peak V O 2 ). Two conditions were mainly compared: 1) spontaneous adjustment of blood flow (Q ) [control, spontaneous Q (C Spont)]; and 2) pump-perfused Q , adjusted ϳ15 s before contractions at a constant level corresponding to the steady-state value during contractions in C Spont [faster adjustment of O 2 delivery (Fast O 2 Delivery)]. During Fast O 2 Delivery, 1-2 ml/min of 10 Ϫ2 M adenosine were infused intra-arterially to prevent inordinate pressure increases with the elevated Q . The purpose of the study was to determine whether a faster adjustment of O 2 delivery would affect V O 2 kinetics. Q was measured continuously; arterial (Ca O 2 ) and popliteal venous (Cv O 2 ) O 2 contents were determined at rest and at 5-to 7-s intervals during contractions; O 2 delivery was calculated as Q ·Ca O 2 , and V O 2 was calculated as Q · arteriovenous O 2 content difference. Times to reach 63% of the difference between baseline and steady-state V O 2 during contractions were 23.8 Ϯ 2.0 (SE) s in C Spont and 21.8 Ϯ 0.9 s in Fast O 2 Delivery (not significant). In the present experimental model, elimination of any delay in O 2 delivery during the rest-tocontraction transition did not affect muscle V O 2 kinetics, which suggests that this kinetics was mainly set by an intrinsic inertia of oxidative metabolism. gas exchange kinetics; muscle oxidative metabolism; submaximal exercise IT HAS BEEN KNOWN FOR DECADES that on a step transition from rest to exercise, or from a lower to a higher workload, O 2 uptake (V O 2 ) lags behind the power output increase (12), following a time course usually termed V O 2 on-kinetics. The mechanism(s) determining this kinetics has been a matter of considerable debate, mainly between those who consider it mainly related to the rate of adjustment of O 2 delivery to the exercising muscles (13-15) and those who support the concept that V O 2 on-kinetics is mainly set by an inertia of intramuscular oxidative metabolism (3, 32).An experimental approach to discriminate between the two conflicting hypotheses would be to increase the rate of adjustment of O 2 delivery to muscles and then determine whether the V O 2 on-kinetics becomes faster or not. Unfortunately, previous studies conducted following this approach yielded conflicting results. Hughson and co-workers (14), for example, described a significantly faster V O 2 on-kinetics when their subjects cycled in a supine position during the application of lower body negative pressure, which presumably enhanced the rate of O 2 delivery to the exercis...

During conventional cycle ergometry, as work rate (WR) is increased toward maximum, O2 extraction increases hyperbolically, typically achieving values of 80-90% at peak O2 uptake (VO2). In contrast, studies using isolated knee-extensor exercise report much higher mass-specific blood flows (Q) and lower maximal O2 extractions (approximately 70%), which have been interpreted as transit time limitation to O2 movement out of the muscle capillary. However, maximal achievable WR levels during conventional cycle ergometry are generally reached (over 10-15 min) after rapid increases in WR, whereas the reported knee-extensor studies have used only more lengthy protocols (45 min). The duration of these protocols may have prevented the attainment of high WR levels and thus high O2 extraction ratios. Accordingly, this investigation examined leg Q and O2 extraction responses during single-leg knee-extensor exercise incremented rapidly (steps of 15-25 W per 2- to 3-min interval), which produced fatigue in 13-15 min. Q and muscle VO2 increased linearly with WR to fatigue with Q-WR and VO2-WR slopes similar to those reported in previous knee-extensor studies. However, with the use of this protocol, very high maximal achievable WR [99 +/- 6 (SE) W] and muscle Q (385 +/- 26 ml.min-1 x 100 g-1) levels were attained, some 80% greater than previously reported. An O2 extraction of 84.6 +/- 2.1% was reached, giving a maximal VO2 of 60.2 +/- 5.8 ml.min-1 x 100 g-1. We conclude that, even under the high Q conditions of single-leg knee-extensor exercise, O2 extraction does not reach a plateau on the basis of short transit times and that previous conclusions to the contrary reflect failure to attain sufficiently high WR levels. Maximal VO2, Q, and O2 extraction in this model have yet to be defined.