The goal of these experiments was to investigate the relationship of ATP, phosphocreatinine (PCr), inorganic phosphate (Pi), monobasic phosphate (H2PO4-), and pH to human muscle fatigue. Phosphates and pH were measured in adductor pollicis using 31P nuclear magnetic resonance at 2.0 Tesla. The force of muscle contraction was simultaneously measured with a force transducer. The effects of aerobic and anaerobic exercise were compared using two exercise protocols: 4 min sustained maximal voluntary contraction (MVC) and 40 min of repeated intermittent contractions (75% MVC). The sustained maximal contraction produced a rapid decline of MVC and PCr, and was accompanied by a rapid rise of Pi, H+, and H2PO4-. Intermittent exercise produced steady state changes of MVC, pH, and phosphates. No significant changes of ATP were found in either protocol. During fatiguing exercises, PCr and Pi had a nonlinear relationship with MVC. H+ showed a more linear correlation, while H2PO4- showed the best correlation with MVC. Furthermore, the correlations between MVC and H2PO4- were similar in sustained (r = 0.70) and intermittent (r = 0.73) exercise. The highly significant linear relationship between increases of H+ and H2PO4- and the decline of MVC strongly suggests that both H+ and H2PO4- are important determinants of human muscle fatigue.
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