Matching of blood flow to metabolic rate during recovery from moderate exercise in humans

It is unclear whether measurement of limb or conduit artery blood flow during recovery from exercise provides an accurate representation of flow to the muscle capillaries where gas exchange occurs. To investigate this, we: (a) examined the kinetic responses of femoral artery blood flow ((Q) over dot(FA)), estimated muscle capillary blood flow ((Q) over dot (cap)) and estimated muscle oxygen uptake ((V) over dot (O2m)) following cessation of exercise; and (b) compared these responses to verify the adequacy of O(2) delivery during recovery. Pulmonary (V) over dot (O2) ((V) over dot (O2p)) was measured breath by breath, (Q) over dot (FA) was measured using Doppler ultrasonography, and deoxy-haemoglobin/myoglobin (deoxy-[Hb/Mb]) was estimated by near-infrared spectroscopy over the rectus femoris in nine healthy subjects during a series of transitions from moderate knee-extension exercise to rest. The time course of (Q) over dot (cap) was estimated by rearranging the Fick equation [i.e. (Q) over dot (cap)(t) alpha (V) over dot (O2m)(t)/deoxy-[Hb/Mb](t)], using the primary component of (V) over dot (O2p) to represent (V) over dot (O2m) and deoxy-[Hb/Mb] as a surrogate for arteriovenous O(2) difference. There were no significant differences among the overall kinetics of (V) over dot (O2m) (tau, 31.4 +/- 8.2 s), (Q) over dot (FA) [mean response time (MRT), 34.5 +/- 20.4 s] and (Q) over dot (cap) (MRT, 31.7 +/- 14.7 s). The (V) over dot (O2m) kinetics were also significantly correlated (P < 0.05) with those of both (Q) over dot (FA) and (Q) over dot (cap). Both (Q) over dot (FA) and (Q) over dot (cap) appear to be coupled with (V) over dot (O2m) during recovery from moderate knee-extension exercise, such that extraction falls (thus cellular energetic state is not further compromised) throughout recovery.