Dynamics of intramuscular 31P-MRS Pi peak splitting and the slow components of PCr and O2 uptake during exercise

The dynamics of pulmonary O-2 uptake ((V)over dotO(2)) 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 O-2 utilization kinetics and low efficiency, has, however, been suggested as a mechanism. The demonstration of high- and low-pH components of the exercising skeletal muscle P-31 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 Of (V)over dotO(2) (using a turbine and mass spectrometry) and intramuscular ATP, phosphocreatine (PCr), and P-i concentrations and pH, estimated from the P-31 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)over dotO(2) slow 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-i peak, 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 the (V)over dotO(2) slow component, suggesting that P-i splitting does not uniquely reflect the activity of oxidative or glycolytic muscle fibers per se.