A 31P study of fatigue and metabolism in human skeletal muscle with voluntary, intermittent contractions at different forces.

Normal subjects performed voluntary, isometric exercise 1 s contraction, 1 s rest for 10 min) of the first dorsal interosseous (FDI) muscle with a target force of 25, 50 and 100% of the maximal voluntary contraction (MVC) force. 31P NMR spectra were collected continuously before, during and after exercise. Data were also taken from the resting muscles 2-28 h after the studies at 50% MVC. Calculations were made of the intracellular pH and concentrations of PCr, Pi, ATP, ADP and H2PO4-. The 25% MVC contractions did not affect the MVC, but those at 50 and 100% MVC reduced the force by 20% and 60%, respectively (p less than 0.005). During the highest force contractions, the MVC declined from the first minute but the target forces of 25 and 50% were maintained throughout. All protocols caused significant changes in pH, PCr, Pi, ADP and H2PO4-. Exercise at 50% MVC caused greater metabolic changes than that at 25%, but there was no overall difference in the pH and phosphorus metabolites between the two higher forces. In parallel studies, electrical stimulation of the muscle indicated that during the voluntary contractions with a target force of 100% MVC in the magnet: (a), additional muscles were being used to generate the recorded force; and (b), the subjects were not fully activating the FDI. There was no obvious causal relationship between any one metabolite and the decline of force. Resting muscle showed an increase in the Pi peak 2-28 h after exercise at 50% MVC force, despite the muscles being of full strength and pain free.