Investigations of muscle bioenergetics with 31P NMR

Many factors have been proposed to contribute to the development of muscle fatigue, but as yet a definitive assessment of their relative contributions has not been possible. To determine whether changes in bioenergetic factors are determinants of muscle force during the development of fatigue, simultaneous measurements were made during fatiguing exercise of the decline in muscle force and changes in phosphorus (31P) nuclear magnetic resonance (NMR) spectra. The results showed that the decline in force strongly correlated with a rise in inorganic phosphate (Pi) concentration; in contrast, fatigue correlated less strongly with intracellular pH. Similar results were found in different muscles and with different exercise protocols. They suggest that the rise in [Pi] is a mechanism that produces progressive inhibition of the force-generating reaction, while energy availability is not limiting. Down-regulation of force and energy utilization are consistent with fatigue being not simply a disruptive process, but an adaptive response that establishes an equilibrium between energy supply and demand.