The dynamics of pulmonary O₂uptake (V˙o₂) during the on-transient of high-intensity exercise depart from monoexponentiality as a result of a “slow component” whose mechanisms remain conjectural. Progressive recruitment of glycolytic muscle fibers, with slow O₂utilization kinetics and low efficiency, has, however, been suggested as a mechanism. The demonstration of high- and low-pH components of the exercising skeletal muscle³¹P magnetic resonance (MR) spectrum [inorganic phosphate (P_i) peak] at high work rates (thought to be reflective of differences between oxidative and glycolytic muscle fibers) is also consistent with this conjecture. We therefore investigated the dynamics ofV˙o₂(using a turbine and mass spectrometry) and intramuscular ATP, phosphocreatine (PCr), and P_iconcentrations and pH, estimated from the³¹P MR spectrum. Eleven healthy men performed prone square-wave high-intensity knee extensor exercise in the bore of a whole body MR spectrometer. A V˙o₂slow component of magnitude 15.9 ± 6.9% of the phase II amplitude was accompanied by a similar response (11.9 ± 7.1%) in PCr concentration. Only five subjects demonstrated a discernable splitting of the P_ipeak, however, which began from between 35 and 235 s after exercise onset and continued until cessation. As such, the dynamics of the pH distribution in intramuscular compartments did not consistently reflect the temporal features of theV˙o₂slow component, suggesting that P_isplitting does not uniquely reflect the activity of oxidative or glycolytic muscle fibers per se.