Muscular and pulmonary O2 uptake kinetics during moderate- and high-intensity sub-maximal knee-extensor exercise in humans

The purpose of this investigation was to determine the contribution of muscle O-2 consumption (m(V) over dot(O2)) to pulmonary O-2 uptake (p(V) over dot(O2)) during both low-intensity (LI) and high-intensity (HI) knee-extension exercise, and during subsequent recovery, in humans. Seven healthy male subjects (age 20-25 years) completed a series of LI and HI square-wave exercise tests in which m(V) over dot(O2) (direct Fick technique) and p(V) over dot(O2) (indirect calorimetry) were measured simultaneously. The mean blood transit time from the muscle capillaries to the lung (MTTc-l) was also estimated (based on measured blood transit times from femoral artery to vein and vein to artery). The kinetics of m(V) over dot(O2) and p(V) over dot(O2) were modelled using non-linear regression. The time constant (tau) describing the phase II p(V) over dot(O2) kinetics following the onset of exercise was not significantly different from the mean response time (initial time delay + tau) for m(V) over dot(O2) kinetics for LI (30 +/- 3 vs 30 +/- 3 s) but was slightly higher (P < 0.05) for HI (32 +/- 3 vs 29 +/- 4 s); the responses were closely correlated (r = 0.95 and r = 0.95; P < 0.01) for both intensities. In recovery, agreement between the responses was more limited both for LI (36 +/- 4 vs 18 +/- 4 s, P < 0.05; r = -0.01) and HI (33 +/- 3 vs 27 +/- 3 s, P > 0.05; r =-0.40). MTTc-l was similar to 17 s just before exercise and decreased to 12 and 10 s after 5 s of exercise for LI and HI, respectively. These data indicate that the phase II p(V) over dot(O2) kinetics reflect m(V) over dot(O2) kinetics during exercise but not during recovery where caution in data interpretation is advised. Increased m(V) over dot(O2) probably makes a small contribution to p(V) over dot(O2) during the first 15-20 s of exercise.