Relationship of muscular fatigue to pH and diprotonated Pi 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.
Wilson
J R
JR
Department of Medicine, University of Pennsylvania, Philadelphia 19104.
McCully
K K
KK
Mancini
D M
DM
Boden
B
B
Chance
B
B
D000328:Adult; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008297:Male; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D010758:Phosphorus; D005082:Physical Exertion
D010710:Phosphates; D010758:Phosphorus