Dynamics of skeletal muscle oxygenation during sequential bouts of moderate exercise

In rat muscle, faster dynamics of microvascular P(O2) (approximately blood flow (Q(m) to O2 uptake (V(O2) ratio) after prior contractions that did not alter blood [lactate] have been considered to be a consequence of faster V(O2) kinetics. However, in humans, prior exercise below the lactate threshold does not affect the pulmonary V(O2) kinetics. To clarify this apparent discrepancy, we examined the effects of prior moderate exercise on the kinetics of muscle oxygenation (deoxyhaemoglobin, [HHb] alpha V(O2m)/Q(m)) and pulmonary V(O2) (V(O2p) 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(O2p) was measured breath-by-breath. The time constant (tau) of the primary component of V(O2p) 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 prior moderate exercise. Thus, prior moderate 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(O2m)and Q(m) following the onset of exercise.