Metabolic consequences of repeated exercise in long distance runners.

To assess the rates of change in muscle metabolites such as phosphocreatine (PCr) and inorganic phosphate (Pi) during repeated exercise sessions with rest periods, 31-phosphorus nuclear magnetic resonance spectroscopy was used for continuous and noninvasive measurements. Five long-distance runners and six healthy male subjects as controls performed a 2-min femoral flexion exercise at 20 kg.m.min-1 in a 2.1 T superconducting magnet with a 67-cm bore; they repeated this exercise four times with a 2-min rest period. At the beginning of exercise, PCr decreased exponentially; at the end, it increased. During exercise and in the early phase of the recovery in every exercise session, the PCr values were significantly higher in the long-distance runners than in the control subjects (P < 0.05). The Pi increases and decreases involved with exercise also revealed exponential changes. The Pi values did not significantly differ during exercise; however, Pi recovery was faster in the long-distance runners than in the control subjects (P < 0.05). The Pi:PCr ratio during exercise increased linearly with exercise; and Pi:PCr during recovery was smaller in the long-distance runners than in the control subjects (P < 0.05). In conclusion, the long-distance runners revealed faster PCr and Pi kinetics after exercise and a smaller Pi:PCr during exercise than did the control subjects. It is suggested that these results were attributable to a greater oxidative capacity of muscles in the long-distance runners.