Dynamics of skeletal muscle oxygenation during sequential bouts of moderate exercise

In rat muscle, faster dynamics of microvascular PO2 (approximately blood flow((Q) over dot(m)) to O-2 uptake (. (V) over dot O-2) ratio) after prior contractions that did not alter blood [ lactate] have been considered to be a consequence of faster. (V) over dot O-2 kinetics. However, in humans, prior exercise below the lactate threshold does not affect the pulmonary. (V) over dot O-2 kinetics. To clarify this apparent discrepancy, we examined the effects of prior moderate exercise on the kinetics of muscle oxygenation ( deoxyhaemoglobin, [HHb] alpha (V) over dot O-2m/(Q) over dot(m)) and pulmonary (V) over dot O-2 ((V) over dot O-2p) in humans. Eight subjects performed two bouts ( 6 min each) of moderate-intensity cycling separated by 6 min of baseline pedalling. Muscle ( vastus lateralis) oxygenation was evaluated by near-infrared spectroscopy and (V) over dot O-2p was measured breath-by-breath. The time constant (tau) of the primary component of (V) over dot O-2p was not significantly affected by prior exercise (21.5 +/- 9.2 versus 25.6 +/- 9.7 s; Bout 1 versus 2, P = 0.49). The time delay (TD) of [ HHb] decreased ( 11.6 +/- 2.6 versus 7.7 +/- 1.5 s; Bout 1 versus 2, P < 0.05) and tau [HHb] increased (7.0 +/- 3.5 versus 10.2 +/- 4.6 s; Bout 1 versus 2, P < 0.05), while the mean response time (TD+tau) did not change (18.6 +/- 2.7 versus 17.9 +/- 3.9 s) after priormoderate exercise. Thus, priormoderate exercise resulted in shorter onset and slower rate of increase in [ HHb] during subsequent exercise. These data suggest that prior exercise altered the dynamic interaction between (V) over dot O-2m and (Q) over dot(m) following the onset of exercise.