Relationship of muscular fatigue to pH and diprotonated P(i) in humans: A ³¹P-NMR study

Seventeen normal subjects performed maximal wrist flexion exercise with continuous monitoring of forearm muscle pH and H₂PO₄^-, measured with ³¹P nuclear magnetic resonance, and muscle fatigue, expressed as a percentage of decline in maximal developed force. Four minutes of exercise (flexion duration = 1 s) reduced maximal developed force from 100 to 74 ± 9% and pH from 6.99 ± 0.04 to 6.17 ± 0.33 and increased H₂PO₄^- to 927 ± 401% of resting levels. In all subjects, linear relationships were noted between developed force and pH (r = 0.90 ± 0.08) and between developed force and H₂PO₄^- (r = -0.89 ± 0.08). Doubling the contraction duration to 2 s produced more rapid changes in developed force, pH, and H₂PO₄^- but no change in the relationship of force to pH and H₂PO₄^-. Two minutes of submaximal exercise before maximal exercise significantly reduced pH and increased H₂PO₄^-. During subsequent maximal exercise, the relationship between developed force and H₂PO₄^- remained unchanged. In contrast, the relationship between developed force and pH was shifted leftward; muscle pH remained lower throughout maximal exercise, and developed force remained comparable to that noted during control exercise. These observations suggest that muscle fatigue during intense short-term exercise is primarily caused by an increase in intramuscular H₂PO₄^- rather than by a decrease in intramuscular pH.