Effects of prior exercise on oxygen uptake and phosphocreatine kinetics during high-intensity knee-extension exercise in humans

1. A prior bout of high-intensity square-wave exercise can increase the tem poral adaptation of pulmonary oxygen uptake to a subsequent bout of high-in tensity exercise. The mechanisms controlling this adaptation, however, are poorly understood. 2. We therefore determined the dynamics of intramuscular [phosphocreatine] ([PCr]) simultaneously with those of in seven males who performed two conse cutive bouts of high-intensity square-wave, knee-extensor exercise in the p rone position for 6 min with a 6 min rest interval. A magnetic resonance sp ectroscopy (MRS) transmit-receive surface coil under the quadriceps muscle allowed estimation of [PCr]; V-O2 was measured breath-by-breath using a cus tom-designed turbine and a mass spectrometer system. 3. The V-O2 kinetics of the second exercise bout were altered compared with the first such that (a) not only was the instantaneous rate of V-O2 change (at a given level of V-O2) greater but the phase II tau was also reduced-a veraging 46.6 +/- 6.0 s (bout 1) and 40.7 +/- 8.4 s (bout 2) (mean +/- S.D. ) and (b) the magnitude of the later slow component was reduced. 4. This was associated with a reduction of, on average, 16.1% in the total exercise-induced [PCr] decrement over the 6 min of the exercise, of which 4 .0% was due to a reduction in the slow component of [PCr]. There was no dis cernable alteration in the initial rate of [PCr] change. The prior exercise , therefore, changed the multi-compartment behaviour towards that of functi onally first-order dynamics. 5. These observations demonstrate that the V-O2, responses relative to the work rate input for high-intensity exercise are non-linear, as are, it appe ars, the putative phosphate-linked controllers for which [PCr] serves as a surrogate.