Oxygen uptake kinetics during two bouts of heavy cycling separated by fatiguing sprint exercise in humans

We tested the hypothesis that O-2 uptake ((V)over dotO(2)) kinetics at the onset of heavy exercise would be altered in a state of muscle fatigue and prior metabolic acidosis. Eight well- trained cyclists completed two identical bouts of 6- min cycling exercise at >85% of peak (V)over dotO(2) separated by three successive bouts of 30 s of sprint cycling. Not only was baseline (V)over dotO(2) elevated after prior sprint exercises but also the time constant of phase II (V)over dotO(2) kinetics was faster (28.9+/-2.4 vs. 22.2+/-1.7 s; P<0.05). CO2 output ((V)over dotCO(2)) was significantly reduced throughout the second exercise bout. Subsequently (V)over dotO(2) was greater at 3 min and increased less after this after prior sprint exercise. Cardiac output, estimated by impedance cardiography, was significantly higher in the first 2 min of the second heavy exercise bout. Normalized integrated surface electromyography of four leg muscles and normalized mean power frequency were not different between exercise bouts. (V)over dotO(2) and (V)over dotCO(2) kinetic responses to heavy exercise were markedly altered by prior multiple sprint exercises.