We examined the effect of differences in exercise intensity on the time constant (
t
c) of phosphocreatine (PCr) resynthesis after exercise and the relationships between
t
c and maximal oxygen uptake (VO
2max) in endurance-trained runners (
n = 5) and untrained controls (
n = 7) (average VO
2max = 66.2 and 52.0 ml · min
–1 · kg
–1, respectively). To measure the metabolism of the quadriceps muscle using phosphorus nuclear magnetic resonance spectroscopy, we developed a device which allowed knee extension exercise inside a magnet. All the subjects performed four types of exercise: light, moderate, severe and exhausting. The end-exercise PCr: [PCr + inorganic phosphate (P
i)] ratio decreased significantly with the increase in the exercise intensity (
P < 0.01). Although there was little difference in the end-exercise pH, adenosine diphosphate concentration ([ADP]) and the lowest intracellular pH during recovery between light and moderate exercise, significant changes were found at the two higher intensities (
P < 0.01). These changes for runners were smaller than those for the controls (
P < 0.05). The
c remained constant after light and moderate exercise and then lengthened in proportion to the increase in intensity (
P < 0.05). The runners had a lower
t
c at the same PCr and pH than the controls, particularly at the higher intensity (
P < 0.05). There was a significant correlation between
t
c and [ADP] in light exercise and between
t
c and both end-exercise PCr and pH in severe and exhausting exercise (
P < 0.05). The threshold of changes in pH and
t
c was a PCr: (PCr + P
i) ratio of 0.5. There was a significant negative correlation between the VO
2max and
t
c after all levels of exercise (P<0.05).However, in the controls a significant correlation was found in only light and moderate exercise (
P < 0.05). These findings suggest the validity of the use of
t
c at an end-exercise PCr:(PCr + P
i) ratio of more than 0.5 as a stable index of muscle oxidative capacity and the correlation between local and general aerobic capacity. Moreover, endurance-trained runners are characterized by the faster PCr resynthesis at the same PCr and intracellular pH.
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