Estimation of muscle fiber recruitment pattern during physical exercise in inorganic phosphate kinetics by ^<31>P-NMR

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The studies using ^<31>P-NMR reported that an inorganic phosphate (Pi) splitted into high and low pH peak during a high intensity exercise. However, it was not clear which factor induces this splitting. We hypothesized that a muscle fiber recruitment pattern explains this splitting. It was essential to improve the resolution time of ^<31>P-NMR to clarify the Pi kinetics. We adjusted the angular and interval of the pulse in ^<31>P-NMR to allow a 5 seconds interval data acquisition. To test the hypothesis, we observed Pi patterns under sufficient and insufficient oxygen conditions, i.e., during active and passive recovery from exercise, and during exercise with and without circulatory occlusion.High pH Pi disappeared just after the end of calf flexion exercise. Low pH Pi during the active recovery decreased significantly faster than during the passive recovery. This would resulted from insufficient oxygen supply to the fast twitch muscle fiber during the passive recovery. Simultaneously measured near infrared spectroscopy data indicated that more existing blood volume during the active recovery than the passive recovery, revealing more oxygen supplement to muscle during active recoverty. Pi peak splitting was observed during circulatory occlusion on leg by 280 mmHg at lower exercise intensity than non occlusion trial. This means that Pi metabolism covers the energy loss resulted from oxygen deficit. It is suggested that fast twitch muscle fiber recruitment causes the Pi splitting.