High muscle blood flow in man: is maximal O2 extraction compromised?

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.